JP2019164619A - Ray casting program, search control data, search control data generation method, and ray casting device - Google Patents

Abstract

To minimize an operation load when generating a volume rendering image using a ray casting method.SOLUTION: A ray casting program according to the present invention can adopt a method of: previously calculating a coordinate conversion parameter to be used for subjecting coordinates of each voxel in a viewpoint coordinate system in which a voxel image is viewed from a viewpoint to coordinate conversion into a voxel coordinate system in which the voxel is defined; carrying out coordinate conversion only for a voxel required when previously searching for origin coordinates of virtual light for performing ray casting processing using the coordinate conversion parameter; and carrying out coordinate conversion only for a voxel required for calculating a pixel value of a volume rendering image in the ray casting processing. The number of times of coordinate conversion can be reduced, and an operation load can be suppressed, as compared to well-known methods of, upon previously converting all voxels of a voxel image into the viewpoint coordinate system to generate a voxel data structure converted into the viewpoint coordinate system, searching for origin coordinates and ray casting processing are performed.SELECTED DRAWING: Figure 2

Description

  The present disclosure relates to a technique for generating a volume rendering image using a ray casting method.

  In the medical field, a three-dimensional image of an object may be generated from a slice image of the object acquired by a CT apparatus or an MRI apparatus. Such an image is called a volume rendering image. As a method for generating a volume rendering image, there is a method called a ray casting method.

  In the ray casting method, voxels having color values (specifically, RGB values) and opacity (specifically, α values) defined based on the voxel values are three-dimensionally arranged. By irradiating virtual light (ray) from the viewpoint to the voxel space, the color value defined based on the voxel value in each voxel, the light intensity attenuated according to the opacity, and the luminance value of the pixel of the volume rendering image (Specifically, RGB values) are calculated along the line-of-sight direction. This makes it possible to create a volume rendering image when the three-dimensional structure of the object is viewed from the viewpoint. The volume rendering image can be generated at an arbitrary depth viewed from the viewpoint. Furthermore, by mapping an array (referred to as a color map, opacity curve, etc.) that defines color values and opacity based on the voxel values for each voxel, a color image in which a part of the tissue is made transparent is generated. Can do.

  Patent Document 1 below discloses a technique for generating a volume rendering image using a ray casting method. The document “Efficiently performs ray casting calculation processing from an arbitrary viewpoint direction for a three-dimensional voxel space. "In the three-dimensional image display method for obtaining a three-dimensional image of the internal tissue of the object, the three-dimensional voxel space 1 is a hexahedron, and the eight vertices constituting each surface are defined by the viewpoint direction 4". Three-dimensional coordinate transformation is performed to obtain the plane coordinates of the eight vertices on the projection plane perpendicular to the viewpoint direction, and each plane 8 on the projection plane when the three-dimensional voxel space is projected onto the projection plane from the plane coordinates of the eight vertices. A rectangular projection region 7 including each projected surface on the projection surface is defined, each surface in the projection region is filled with an arbitrary non-zero luminance value, and a ray is projected from the viewpoint direction to the three-dimensional voxel space. When performing the casting process, the ray casting process is performed only for each position in the projection area where the brightness value is not zero. Is disclosed (see summary).

Japanese Patent Laid-Open No. 11-175743

  In principle, the ray casting method projects the virtual light from the coordinates of each pixel toward the voxel image for all the pixels of the volume rendering image, and the color values defined for each voxel through which the virtual light has passed while performing coordinate conversion. Based on the opacity, the attenuation of the light intensity of the virtual light is calculated, and the luminance value of the pixel is calculated. Therefore, the calculation load tends to be large, and it is generally difficult to carry out only by software processing on a computer having a relatively small calculation capability such as a personal computer.

  Japanese Patent Application Laid-Open No. 2004-228688 aims to suppress the calculation load by omitting ray casting processing for a transparent voxel region where there is no voxel effective for generating a volume rendering image whose opacity is not zero. However, even if the ratio of the transparent voxel area to the entire image is large, the effect of suppressing the calculation load is considered to be limited to the volume ratio of the transparent voxel area. Therefore, even if the technique described in the document is implemented, it is still difficult to implement it on a general computer only by software processing.

  The present disclosure has been made in view of the above-described problems, and an object of the present disclosure is to suppress the calculation load when generating a volume rendering image using the ray casting method as much as possible.

  The present application includes a plurality of means for solving the above-described problems. For example, a process for generating a volume rendering image using a ray casting method on a tomographic image acquired by tomographic imaging of an object. Is a ray casting program that causes the computer to create a voxel image of the object having color values and opacity set based on the voxel values of the tomographic image from the tomographic image. A parameter for converting a three-dimensional (x, y, z) viewpoint coordinate system based on a viewpoint (x, y, 0) with respect to the object into a coordinate system of the voxel image; A coordinate conversion parameter calculation step for calculating a coordinate conversion parameter used in common for the viewpoint; By referencing the voxel image while performing coordinate conversion using the coordinate conversion parameter along the z direction of the viewpoint coordinate system, the origin coordinates are coordinates whose opacity is not zero among the z coordinates of the viewpoint coordinate system. A starting point coordinate search step that is set as: obtaining the color value and opacity of the voxel of the voxel image while performing coordinate transformation using the coordinate transformation parameter along the z direction of the viewpoint coordinate system starting from the starting point coordinate A ray casting program for executing a ray casting step of calculating a pixel value of a pixel (x, y) of the volume rendering image.

  According to the ray casting program according to the present disclosure, it is possible to suppress the calculation load of the coordinate conversion process performed in the course of the ray casting process, and to suppress the overall calculation load of the ray casting process.

1 is a configuration diagram of a ray casting apparatus 100 according to Embodiment 1. FIG. It is a flowchart explaining the procedure in which the ray casting apparatus 100 produces | generates a volume rendering image. It is a conceptual diagram of coordinate transformation. It is a figure explaining the procedure in which the origin coordinate search part 123 produces a search control table in step S230. It is a figure explaining the procedure in which the starting point coordinate search part 123 searches a starting point coordinate in each pixel in step S230. It is a flowchart explaining the procedure in which the starting point coordinate search part 123 searches a starting point coordinate in each pixel. It is a figure explaining the procedure in which the ray casting process part 124 implements a ray casting process in step S240. It is an example of the volume rendering image which the ray casting apparatus 100 produces | generates, when a user changes a viewpoint by dragging a mouse | mouth. It is a structural example at the time of comprising each function part as a program.

<Embodiment 1>
FIG. 1 is a configuration diagram of a ray casting apparatus 100 according to Embodiment 1 of the present invention. The ray casting apparatus 100 is an apparatus that generates a three-dimensional image (volume rendering image) of a target object from a slice image of the target object acquired by a CT apparatus or an MRI apparatus. The ray casting apparatus 100 includes a CPU (Central Processing Unit) 110, a voxel image creation unit 121, a coordinate conversion unit 122, a starting point coordinate search unit 123, a ray casting processing unit 124, a display unit 125, and a storage unit 130. The CPU 110 is an arithmetic device. The display unit 125 is a display device that outputs and displays created information. The storage unit 130 is a storage device that stores data used by the CPU 110.

  The voxel image creation unit 121, the coordinate conversion unit 122, the starting point coordinate search unit 123, the ray casting processing unit 124, and a hardware device such as a circuit device on which these functions are mounted, or software on which these functions are mounted Can also be configured by the CPU 110 executing. The following description is based on the assumption that the CPU 110 executes as shown in FIG. For convenience of description, these functional units may be described as the subject of operation, but it is added that the CPU 110 actually executes these functional units. Details of these functional units will be described later.

  FIG. 2 is a flowchart for explaining a procedure by which the ray casting apparatus 100 generates a volume rendering image. Assume that a slice image of an object is created in advance by a CT apparatus or an MRI apparatus. Hereinafter, each step of FIG. 2 will be described.

(FIG. 2: Step S210)
The voxel image creation unit 121 creates a voxel image from the slice image. Each voxel of a voxel image is given a pixel value of a slice image (signal value, also called a CT value in the case of a CT image) as a voxel value, and a color value (specifically, an RGB value) based on the voxel value. It has opacity (specifically, α value). The voxel value is, for example, a CT value acquired by a CT apparatus and is generally given by 16 bits, whereas the color value (RGB value) and the opacity (α value) each have an 8-bit value and are expressed by 32 bits per voxel. The When assigning a color map to each voxel, a color value (RGB value) and opacity (α value) are given based on the voxel value of each voxel. Even if no color map is assigned to each voxel, the 16-bit voxel value is converted into an 8-bit color value (RGB value) and opacity (α value) based on a predetermined calculation formula for the voxel value of each voxel. Convert.

(FIG. 2: Step S220)
The coordinate conversion unit 122 calculates coordinate conversion parameters used when performing coordinate conversion in subsequent steps. In the coordinate conversion parameter calculation S220, a voxel image is defined based on a viewpoint coordinate system based on the viewpoint in order to perform sequential coordinate conversion processing for each voxel when irradiating virtual light to the voxel. This is a process for converting a three-dimensional coordinate value of a voxel into a voxel coordinate system, and interpolating color values and / or opacity of a plurality of eight neighboring voxels in the voxel coordinate system. A conceptual diagram of coordinate transformation and specific examples of coordinate transformation parameters will be described later.

(FIG. 2: Step S230)
The origin coordinate search unit 123 calculates, for each pixel of the volume rendering image, the origin coordinates of the voxel that starts the process of integrating the light intensity of the virtual light and the luminance (RGB value) of the pixel for each pixel of the volume rendering image. To do. If all the voxels through which the virtual light passes are completely transparent (opacity is 0), the light intensity of the virtual light is maintained and the voxel image space is passed through, so ray casting processing is necessary Absent. Therefore, in the first embodiment, the coordinates of the first voxel whose opacity is not 0 when viewed from the viewpoint are searched along the line-of-sight direction. The coordinates of the voxel whose opacity that is first found by the search is not zero are called starting point coordinates. In this step, the starting point coordinates are searched for each pixel (x, y) of the volume rendering image. The search result is stored as a search control table in association with the coordinate value (x, y) of the pixel of the volume rendering image. An example of the search control table will be described later.

(FIG. 2: Step S230: Supplement 1)
Each pixel (x, y) of the volume rendering image is defined by a two-dimensional coordinate system. On the other hand, the voxel coordinate system is defined by a three-dimensional coordinate system, and a volume rendering image is calculated by projecting the three-dimensional voxel coordinate system into a two-dimensional coordinate system. Since there are a plurality of voxels projected and converted for each pixel (x, y) in the depth direction, a plurality of voxels continuous in the line-of-sight direction corresponding to the same coordinate (x, y) are arranged in the z direction. The arrangement is called the viewpoint coordinate system. In calculating the luminance value of each pixel, three-dimensional voxel coordinates where a voxel image is defined by projecting virtual light in the z direction in the viewpoint coordinate system, performing coordinate conversion on each voxel through which the virtual light passes You will get the color value and opacity of the corresponding voxel in the system. In this coordinate conversion process, the coordinate conversion parameter calculated in step S220 can be used. The coordinate conversion process is performed by the coordinate conversion unit 122 under the control of the origin coordinate search unit 123. The coordinate conversion process is performed every time the voxel of the viewpoint coordinate system is referred to in this step.

(FIG. 2: Step S230: Supplement 2)
In this step, since the voxel whose opacity is not 0 is searched, it is not necessary to refer to other values (color values) of the voxel. That is, the origin coordinate search unit 123 refers only to the opacity of each voxel in this step, and does not refer to other values (color values). In the coordinate transformation process, for a voxel having a single coordinate (x, y, z) in the viewpoint coordinate system, corresponding coordinates (x ′, y ′, z ′) is a process for acquiring the color value (RGB) and opacity of the voxel, but the coordinate value (x ′, y ′, z ′) is a real value and has a fraction, so the coordinate value (x ′, y An arithmetic process for interpolating the color value (RGB) and opacity of each voxel corresponding to an integer coordinate value in the vicinity of 8 of ', z') is required. By adopting a method of referring only to the opacity without referring to the color value, the calculation load of the coordinate conversion process is reduced to ¼.

(FIG. 2: Step S240)
The ray casting processing unit 124 projects, in each pixel (x, y) of the volume rendering image, virtual light in the z-direction in the viewpoint coordinate system, starting from the starting point coordinates searched in step S230, and each virtual light passes through. By integrating the light intensity of the virtual light and the luminance (RGB value) of the pixel using the color value and opacity of the corresponding voxel in the three-dimensional voxel coordinate system in which the voxel image is defined for the voxel, The pixel value of each pixel (x, y) of the volume rendering image is calculated. Also in this step, since the viewpoint coordinate system and the voxel coordinate system are different, coordinate conversion is performed using the coordinate conversion parameter calculated in step S220, and the color of the corresponding voxel in the voxel coordinate system is determined for each voxel in the viewpoint coordinate system. Both value and opacity need to be calculated. The coordinate conversion process is performed by the coordinate conversion unit 122 under the control of the ray casting processing unit 124. The coordinate conversion process is performed in this step every time a voxel in the viewpoint coordinate system is referred to.

(FIG. 2: Step S250)
The display unit 125 outputs a volume rendering image according to the output destination apparatus or device using the luminance value (RGB value) of each pixel (x, y) calculated in step S240.

  FIG. 3 is a conceptual diagram of coordinate transformation. Since the volume rendering image is an image viewed from the viewpoint, the viewpoint coordinate system serving as a reference for each pixel (x, y) of the volume rendering image does not necessarily match the voxel coordinate system. Therefore, the coordinate conversion unit 122 converts the voxel image defined in the voxel coordinate system into the viewpoint coordinate system. However, for example, when a voxel image composed of 256 slice images of 512 × 512 pixels is converted into the viewpoint coordinate system, x: 512 × y: 512 × z: It is necessary to create a voxel data structure that has been converted to the 512 voxel viewpoint coordinate system, and 512 coordinate transformations are required. On the other hand, when searching for the origin coordinates, if a voxel with an opacity of 0 is immediately found, there is no need to refer to the voxel that follows in the z direction, and when performing the ray casting process, If the light intensity of the virtual light is attenuated to a predetermined value or less, the integration process can be terminated, and there is no need to refer to the voxels that follow in the z direction. That is, most of the 512 cubed voxels converted to the viewpoint coordinate system are not referred to, and therefore it is not necessary to perform coordinate conversion on all the voxels of the voxel image and hold them. Therefore, the voxel image whose coordinates are converted into the viewpoint coordinate system as shown on the right in FIG. 3 is not generated, and limited to the voxel to which the virtual light indicated by the downward arrow in FIG. In each case, the coordinate conversion processing is executed in the voxel coordinate system in which the voxel image is defined. Since the coordinate conversion process is not executed for unreferenced voxels, all voxel data structures converted from the voxel coordinate system to the viewpoint coordinate system are created in advance, and the origin coordinate search and ray casting process are performed. Compared with the method, a memory for holding a three-dimensional voxel data structure is not required, the number of coordinate conversions can be reduced, and a memory access load and a calculation load can be suppressed.

  When coordinate conversion is performed, various coordinate conversion parameters are used. If the positional relationship between the viewpoint coordinate system and the voxel coordinate system is the same, the coordinate conversion parameters are also considered to be the same. That is, in the starting point coordinate search unit 123 and the ray casting processing unit 124, the coordinate conversion parameters for performing coordinate conversion on the voxel coordinate system are the same for all voxels referred to in the viewpoint coordinate system. Therefore, the coordinate conversion unit 122 calculates the coordinate conversion parameter in advance in step S220, and uses the coordinate conversion parameter as a common coordinate conversion parameter in subsequent steps. Examples of coordinate conversion parameters include the following.

When the rotation angle around the X axis is Rx (unit: radians), sin (Rx) and cos (Rx) are calculated in advance as coordinate conversion parameters. The YZ axis is similarly calculated in advance.
The offset Xoff (unit: voxel) in the X-axis direction is stored in advance as a coordinate conversion parameter. Similarly, the YZ axis is held in advance.
-The ROI (Region Of Interest) in the XYZ-axis directions is held in advance as a coordinate conversion parameter. The ROI indicates the display range in each direction of the XYZ axes, and has an effect of setting all opacity of voxels outside the range to 0 so that voxels outside the specified range are not displayed in the volume rendering image.
-The enlargement / reduction ratio Scale is stored in advance as a coordinate conversion parameter. The same value may be used for each axis of XYZ, or different values may be used.
-Z-direction magnification: In the CT or MRI scan process, the two-dimensional XY-direction pixel interval in the slice image and the slice interval in the Z-direction (body axis direction) of the slice image are generally different. The image scale in the XY direction and the image scale in the Z direction of the three-dimensional voxel image differ. Therefore, it is necessary to scale the image in the Z direction. In this case, a ratio between the resolution in the XY direction and the resolution in the Z direction is calculated in advance, and this is applied together with the enlargement / reduction ratio at the time of coordinate conversion.

  The ray casting process assumes that the voxel is irradiated with virtual light from the viewpoint, searches for the deepest point (the point where the light intensity of the virtual light is attenuated below a predetermined value), and determines the light locus from the deepest point. The basic method is to calculate the pixel value on the projection plane by retroactively integrating the luminance values on the projection plane. In the process of searching for the deepest point, the ray casting processing unit 124 accumulates the light intensity of the virtual light based on the opacity of all the passed voxels, and based on the color value and opacity of all the passed voxels. The process of integrating the luminance values on the projection surface is performed simultaneously. Since the color values and opacity of all the passed voxels are calculated according to the color values and opacity of the corresponding voxels in the voxel coordinate system by coordinate conversion from the viewpoint coordinate system, there are 8 neighboring voxels in the voxel coordinate system. It is necessary to interpolate the color value and opacity of.

  FIG. 4 is a diagram illustrating a procedure in which the origin coordinate search unit 123 creates a search control table in step S230. The starting point coordinate search unit 123 creates a search control table in which starting point coordinates are described for each pixel (x, y) of the volume rendering image. The origin coordinate is z in the viewpoint coordinate system of a voxel having a non-zero opacity value that is first found by searching a voxel from the viewpoint (z = 0) in the z-axis direction (z> 0) in the viewpoint coordinate system. It is a coordinate value (positive or zero value). As a result of the search, if no voxel having a non-zero opacity value is found, that is, if all voxels are transparent up to the end in the z direction, a negative value (−1) is set as the starting point coordinate. For the coordinates (x, y) of all the pixels of the volume rendering image, it is necessary to first search for the starting point coordinates for projecting the virtual light. This search process is performed for each coordinate (x, y) in the viewpoint coordinate system z. Need to refer to the opacity of multiple voxels in the axial direction, perform coordinate transformation to get the opacity of each voxel and interpolate the opacity values of multiple voxels in the corresponding voxel coordinate system Since the process is executed, the calculation load is relatively large. Therefore, by using a method of creating a two-dimensional search control table, for the start point coordinates for some coordinates, the start point coordinates searched for in the nearby coordinates can be used, and the search process can be omitted. In the first embodiment, a search control table is created according to the following procedure in order to suppress the calculation load of the search processing for the starting point coordinates executed for each pixel.

  The starting point coordinate search unit 123 first sets even-numbered pixels (2m, 2n) in the x-axis direction and the y-axis direction of the volume rendering image (m = 0, 1,..., N = 0, 1,...). Search for the origin coordinates. The shaded portion in the left figure of FIG. 4 corresponds to this. In general, the number of pixels of the volume rendering image is often set to an even number such as 512 × 512, and the starting point coordinates of the odd-numbered pixels at the end of the image cannot be set by the interpolation processing described later. The origin coordinates are also searched for the rightmost pixel in the x-axis direction and the lowermost pixel in the y-axis direction.

  The starting point coordinate search unit 123 for the odd-numbered pixels (2m + 1, 2n) in the x-direction among the even-numbered pixels in the y-axis direction of the volume rendering image, Check whether the origin coordinates of the pixels are the same. For example, for pixel (1, 0), it is checked whether the starting point coordinates of pixel (0, 0) that is adjacent to each other are the same as the starting point coordinates of pixel (2, 0). However, even if the pixel at the right end in the x direction is an odd-numbered pixel, the starting point coordinates are already set as shown in the left diagram of FIG. If the starting point coordinates on both sides are the same, the object is presumed to have the same shape in the vicinity of the pixel, so the starting point coordinates of the pixels sandwiched between them are the same as the starting point coordinates on both sides. Can be considered. In addition, in the case where the pixels of the background portion where the starting point coordinates on both sides are negative values, the pixels sandwiched between them can also be regarded as the background portion. Therefore, the starting point coordinate search unit 123 sets the same starting point coordinates for the pixel (2m + 1, 2n) when the starting point coordinates on both sides of the pixel (2m + 1, 2n) are the same. This corresponds to the vertical hatched portion in the center of FIG. On the other hand, when the start point coordinates on both sides are different, the start point coordinate search unit 123 executes a process of searching for the start point coordinates and sets the searched start point coordinates, as with the even-numbered pixels on both sides.

  The starting point coordinate search unit 123 determines whether or not the starting point coordinates of the even-numbered pixels on both sides in the y direction are the same for all pixels (x, 2n + 1) in the y direction in the y direction of the volume rendering image and in the x direction. To check. For example, for pixel (0, 1), it is checked whether the starting point coordinates of pixel (0, 0) that are adjacent to each other are the same as the starting point coordinates of pixel (0, 2). However, even if the number of pixels in the lower end row in the y direction is an odd number, the starting point coordinates are already set as shown in the center diagram of FIG. When the starting point coordinates on both sides are the same, the same starting point coordinates are set for pixels sandwiched between them as in the x direction. When the starting point coordinates on both sides are different, the starting point coordinate searching unit 123 executes a process of searching for the starting point coordinates and sets the searched starting point coordinates, similarly to the x direction. The horizontal hatched portion in the right side of FIG. 4 corresponds to this.

  FIG. 4 shows a method of searching for the origin coordinates every other vertical and horizontal pixels and interpolating one pixel sandwiched between them based on both adjacent pixels, but this is only an example, and vertical and horizontal M (M> 1) It is also possible to use a method in which the origin coordinates are searched every other pixel and the M pixels sandwiched between them are interpolated based on the adjacent pixels. In this case, the calculation load is further suppressed. it can.

  FIG. 5 is a diagram illustrating a procedure in which the starting point coordinate search unit 123 searches for starting point coordinates in each pixel in step S230. In FIG. 4, the procedure for speeding up the setting of the start point coordinates by omitting a part of the start point search process based on the start point coordinates searched in the adjacent pixels in the xy direction has been described. Instead of this or in combination, the calculation process of the starting point search in the z direction can be speeded up, so that the calculation load of the starting point search can be doubled.

  The starting point coordinate search unit 123 refers to the opacity of each voxel starting from the viewpoint and along the z direction of the line-of-sight coordinate system. First, the opacity of the voxel at the viewpoint position (z = 0) is confirmed. If the opacity is not 0, the object is the surface (cut surface) of the subject. Is small. However, when the opacity of the voxel at the viewpoint position (z = 0) is 0 (mostly in the background portion), there is a high possibility that transparent voxels continuously exist in the z direction, and the background portion has an end in the z direction. If the opacity of all voxels until the part becomes 0 and the method of referring to the adjacent voxels sequentially is taken, the coordinates for all the voxels up to the end part in the z direction are determined in order to determine that the starting point coordinate is a negative value. It is necessary to refer to the opacity of the voxel while performing the conversion, which entails a huge calculation load. Therefore, when the opacity of the voxel at the viewpoint position (z = 0) is 0, the opacity is referred to while skipping an arbitrary number of voxels instead of sequentially referring to the adjacent voxels (the left figure in FIG. 5). ). For example, opacity is referenced every 8 voxels in the z direction. If a voxel whose opacity is not 0 is not found even when reaching the terminal voxel in the z direction, a negative value is set as the starting point coordinate and the search is terminated. On the other hand, when the first voxel whose opacity is not 0 (not completely transparent) is found, the process proceeds to the process on the right side of FIG. 5 in order to accurately determine the starting point coordinates.

  The origin coordinate search unit 123 searches for the first voxel with an opacity of 0, going back from the first found opaque voxel toward the viewpoint (right diagram in FIG. 5). At this time, the opacity of the voxels adjacent in the z direction of the viewpoint coordinate system is sequentially referenced for each voxel with the skip width as the upper limit. When the first voxel with an opacity of 0 (white voxel in the right figure in FIG. 5) is found, search control is performed using the z coordinate in the viewpoint coordinate system of the previous voxel (direction away from the viewpoint) as the origin coordinate. Store in a table. Going back to the upper limit of the skip width, if the first voxel with an opacity of 0 is not found, the z coordinate in the viewpoint coordinate system of the first found opaque voxel is stored as a starting point coordinate in the search control table. With the above procedure, the search for the starting point in the z direction can be speeded up.

  In the procedure shown in FIGS. 4 and 5, the starting point coordinate search unit 123 refers to the opacity of the voxels arranged in the z direction with the viewpoint coordinate system (x, y, z) as a reference. However, since what is actually referred to is the opacity of the voxel in the voxel coordinate system, coordinate conversion from the viewpoint coordinate system to the voxel coordinate system is necessary to obtain the opacity of the actual voxel. Parameters for coordinate conversion may be calculated in advance in step S220. The coordinate values (x ′, y ′, z ′) of the voxel coordinate system calculated by the coordinate conversion are real values, but in order to refer to the voxels in the voxel coordinate system, it is necessary to convert them to integer values. At this time, it is known that if the method of referring to a single voxel in the voxel coordinate system by rounding off the decimal point is used as an integer, moire is generated in the volume rendering image, so the calculated voxel coordinate system By adding the weighted values according to the numerical value after the decimal point of the real coordinate value to the opacity of the voxel of the integer coordinate value near 8 adjacent to the real coordinate value (x ′, y ′, z ′), A method of calculating opacity is desirable. The same applies to the ray casting process described later. However, in the ray casting process, it is necessary to refer not only to the voxel opacity but also to the color value composed of RGB three values. Compared to the processing load of coordinate conversion in the casting process, it can be suppressed to ¼.

  FIG. 6 is a flowchart for explaining the procedure by which the origin coordinate search unit 123 searches for the origin coordinates in each pixel. The starting point coordinate search unit 123 executes this flowchart for each target pixel (x, y). Hereinafter, each step of FIG. 6 will be described.

(FIG. 6: Step S601)
The starting point coordinate search unit 123 sets the target pixel (x, y) and three-dimensional coordinates (x, y, z) for searching the starting point coordinate. The initial value of the z coordinate is z = Zs. Zs is the z coordinate of the viewpoint. In the description of FIG. 5, the initial value of the z coordinate in the viewpoint coordinate system is set to z = 0, and the method of searching in the positive direction for increasing the value of the z coordinate has been described. However, in FIG. Based on this, the initial value of the z coordinate in the viewpoint coordinate system is set to z = Zs (> 0), and a method of searching in the negative direction for decreasing the value of the z coordinate will be described.

(FIG. 6: Step S602)
The starting point coordinate search unit 123 calculates the opacity α corresponding to the voxel before the coordinate conversion is performed on the three-dimensional coordinates (x, y, z) and the coordinate conversion is performed on the viewpoint coordinate system. As the calculation procedure, for example, a method of weighting and adding the opacity of adjacent voxels as described above can be used. If α is 0, the process proceeds to step S603, and if α is not 0, the process proceeds to step S605.

(FIG. 6: Step S603)
The starting point coordinate search unit 123 skips a predetermined number of voxels in the z-axis direction. Specifically, m (for example, m = 8) is subtracted from the current z coordinate. If the z coordinate is less than 0 as a result of the subtraction, the process proceeds to step S604. If the z coordinate is greater than or equal to 0, the process returns to step S602 and the same processing is repeated.

(FIG. 6: Step S604)
The starting point coordinate search unit 123 has a value (for example, -1 or the like) indicating that the starting point coordinate is not found with respect to the value M (x, y) of the search control table corresponding to the current target pixel (x, y). (Negative value) is set, and this flowchart is terminated.

(FIG. 6: Steps S605 to S606)
The starting point coordinate search unit 123 initializes the variable i (S605), and increments it by 1 (S606). When i reaches m, or when i is added to the current z coordinate and the viewpoint z coordinate is reached, the process skips to step S608. If i has not reached m and the viewpoint z coordinate is not reached even if i is added to the current z coordinate, the process proceeds to step S607.

(FIG. 6: Step S607)
The starting point coordinate search unit 123 calculates the opacity α corresponding to the voxel before the coordinate conversion is performed on the three-dimensional coordinates (x, y, z + i) and the coordinate conversion to the viewpoint coordinate system is performed. The calculation procedure is the same as in step S602. If α is 0, the process proceeds to step S608. If α is not 0, the process returns to step S606, i is incremented, and the same processing is repeated.

(FIG. 6: Step S608)
The starting point coordinate search unit 123 sets the z coordinate (z + i−1) immediately before step S607 to the value M (x, y) in the search control table corresponding to the current target pixel (x, y). And this flowchart is complete | finished.

  FIG. 7 is a diagram illustrating a procedure in which the ray casting processing unit 124 performs the ray casting process in step S240. The ray casting processing unit 124 refers to the origin coordinates held in the search control table with respect to the (x, y) coordinates of each pixel of the obtained volume rendering image, and in the (x, y, z) viewpoint coordinate system. Virtual light is projected in the z direction from the origin coordinates, and for each voxel through which the virtual light passes, virtual light is used using the color value and opacity of the corresponding voxel in the (x ′, y ′, z ′) voxel coordinate system. And the luminance (RGB value) of the pixel is integrated for each voxel in the z direction, and the light intensity of the virtual light is attenuated to a predetermined value or less, or is integrated up to the stage reaching the terminal voxel in the z direction. The luminance (RGB value) of the pixel finally integrated is given as the pixel value of the pixel (x, y) of the volume rendering image.

  In the course of the ray casting process, the opacity of the voxel at a certain z coordinate may be zero (that is, transparent). The transparent voxel is skipped and the next opaque voxel is searched because the integration process of the light intensity of the virtual light and the luminance (RGB value) of the pixel is not performed on the transparent voxel. At this time, the ray casting processing unit 124 starts searching from a transparent voxel in the middle where z> 0 deviated from the viewpoint in the z direction of the viewpoint coordinate system, and uses the same method as in FIG. A voxel can be searched (FIG. 7 left figure-FIG. 7 right figure). In the ray casting process, not only the opacity of the voxel but also a color value composed of RGB three values is necessary. However, when searching for an opaque voxel, the color value of the voxel is unnecessary, so only the opacity is calculated in the coordinate conversion. What is necessary is just to suppress the calculation load of coordinate transformation to 1/4. When a voxel with an opacity of 0 is found, the ray casting process is restarted from that voxel, and the ray casting process is continued until the light intensity of the virtual light is attenuated below a predetermined value or reaches the terminal voxel in the z direction. . FIG. 7 <Summary> is a summary of the results obtained by classifying the opaque voxels to be subjected to the ray casting process and the transparent voxels to be skipped by the above procedure. By skipping the process of referring to color values and opacity while performing coordinate conversion for transparent voxels existing in the middle of the ray casting process, the speed of the ray casting process can be increased.

  When the ray casting processing unit 124 refers to the color value or opacity, the value of the voxel in the corresponding voxel coordinate system is referred to based on the viewpoint coordinate system (x, y, z). Therefore, coordinate conversion from the viewpoint coordinate system to the voxel coordinate system is required. The coordinate conversion parameters are the same as those used by the origin coordinate search unit 123. However, in the ray casting process, not only the opacity of the voxel but also a color value composed of RGB three values is required. Therefore, the decimal point of the real coordinate value with respect to the color value and opacity of the voxel corresponding to the integer coordinate value in the vicinity of 8 adjacent to the real coordinate value (x ′, y ′, z ′) of the coordinate-converted voxel coordinate system. A method of obtaining the color value and opacity of the voxel by adding the weighted values according to the following numerical values.

<Embodiment 1: Summary>
The ray casting apparatus 100 according to the first embodiment calculates the coordinate conversion parameters for the coordinate conversion used in the starting point coordinate search and the ray casting process in advance, and uses them for reference in each of the starting point coordinate search and the ray casting process. Coordinate transformation is limited to the minimum required voxels. As a result, all the voxel data structures converted from the voxel coordinate system to the viewpoint coordinate system in advance are created, and the three-dimensional voxel data structure is compared with the known method for performing the origin coordinate search and the ray casting process. A memory to be held becomes unnecessary, the number of coordinate transformations can be reduced, and a memory access load and a calculation load can be suppressed. For example, when a voxel image composed of 256 slice images of 512 × 512 pixels is converted to the viewpoint coordinate system, the viewpoint of x: 512 × y: 512 × z: 512 voxels is set to a Z-direction scaling factor of 2.0. It is necessary to create a voxel data structure converted into the coordinate system, and 512 cubed coordinate conversion is required. On the other hand, when a starting point coordinate search is performed, if a voxel with an opacity of 0 is found immediately, there is no need to refer to a voxel that follows in the z direction, and coordinate conversion can be omitted. In addition, when performing ray casting processing, if the light intensity of the virtual light is attenuated to a predetermined value or less, the integration processing can be terminated, and there is no need to refer to the voxel following in the z direction, and coordinate conversion is performed. Can be omitted. By omitting these, the number of coordinate transformations can be reduced by about 1/3. Therefore, even a general computer such as a personal computer can suppress the calculation load to such an extent that a volume rendering image can be generated using a ray casting method within a practical time.

  The ray casting apparatus 100 according to the first embodiment searches in advance for start point coordinates for starting the ray casting process for a part of the pixels of the volume rendering image, and stores the result as a search control table. Then, the remaining pixels of the volume rendering image are interpolated based on the starting point coordinates recorded in the search control table. This eliminates the need to search for the starting point coordinates corresponding to all the pixels of the volume rendering image, thereby reducing the number of times of searching for the starting point coordinates and executing coordinate transformation, and suppressing the calculation load.

  When creating the search control table, the ray casting apparatus 100 according to the first embodiment uses the starting point coordinates of the adjacent pixels as they are for an intermediate pixel sandwiched between two pixels having the same starting point coordinate. Thereby, the calculation load for searching for a starting point coordinate can be suppressed.

  When creating the search control table, the ray casting apparatus 100 according to the first embodiment searches for the first voxel with non-opacity of 0 while skipping along the z direction, and when this is found, it opposes the z direction. Go back in direction and search for the first voxel with an opacity of zero. As a result, the coordinates of the starting point are made efficient on the premise that the distribution of transparent voxels with an opacity of 0 in the background of the object and the distribution of opaque voxels with an opacity of the object within the object of 0 are continuous to some extent. Can be searched.

  When the ray casting apparatus 100 according to the first embodiment reaches a voxel having an opacity of 0 in the process of performing the ray casting process, the ray casting apparatus 100 searches for the first voxel having an opacity of 0 while skipping along the z direction. When this is found, the first voxel with an opacity of 0 going backward in the z direction is searched. Thereby, similarly to the process of searching for the starting point coordinates, it is possible to efficiently search for a voxel to be a target of the ray casting process.

<Embodiment 2>
In the ray casting process, in order to improve the image quality of the volume rendering image, specifically, to remove the step / jaggy (called aliasing) at the edge of the subject in the volume rendering image, when performing coordinate conversion, The coordinate value of the voxel is slightly offset by an intermediate coordinate between one voxel (referred to as sub-sampling) (referred to as sub-sampling), and the coordinate conversion parameter calculation S220, the origin coordinate search S230, and the ray casting process S240 are performed a plurality of times. In some cases, smoothing may be performed by averaging the pixel values of the obtained volume rendering image. This is called anti-aliasing. Specifically, when the coordinate transformation parameter calculation, the origin coordinate search, and the ray casting process are each performed twice and averaged, the three-dimensional voxel of the viewpoint coordinate system to be referred to when the second process is performed Coordinate conversion is performed by adding 0.5 to each value of coordinate values (x, y, z), and opacity or / and color values in the voxel coordinate system are acquired. Then, the subject image in the volume rendering image obtained in the second processing is shifted in the range of a maximum of one pixel compared to the volume rendering image obtained in the first processing, and these are averaged by superimposing them. The level difference at the edge portion is eliminated. The method described in the first embodiment can also be used when performing sub-sampling. In the second embodiment of the present invention, a specific example will be described. The configuration of the ray casting apparatus 100 is the same as that of the first embodiment.

  When sub-sampling is performed, it is necessary to perform coordinate conversion after adding an offset by intermediate coordinates. With respect to normal integer coordinates, it is possible to determine in advance as a coordinate conversion parameter as to which direction and how wide the intermediate coordinates are added. For example, when three intermediate coordinates are added along each of the x-axis, y-axis, and z-axis, the coordinate conversion processing is (x, y, z), (x + 0.25, y + 0.25, z + 0.25), ( x + 0.5, y + 0.5, z + 0.5), (x + 0.75, y + 0.75, z + 0.75). In this case, the sub-sample offsets dx = dy = dz = 0.25 in the x-axis direction, the y-axis direction, and the z-axis direction may be determined as coordinate conversion parameters. For the yz direction, a subsample offset different from the x direction can be added. However, if there are many types of intermediate coordinates, the processing load is increased and the image quality is not necessarily improved. Therefore, the intermediate coordinates are only dx = dy = dz = 0.5, and (x, y, z) and (x + 0. 5, y + 0.5, z + 0.5), and coordinate conversion may be performed, and the starting point coordinate search and ray casting processing may be executed twice and averaged.

  When dx = dy = dz = 0.25, add three intermediate coordinates of dx = dy = dz = 0.25, 0.5, 0.75 to each of the x, y, and z directions. Since coordinate conversion parameter calculation S220, starting point coordinate search S230, and ray casting processing S240 are respectively performed, it is necessary to similarly determine the starting point coordinates for each of the three intermediate coordinates and recreate the search control table. There is. The starting point coordinate search unit 123 records the result of searching for the starting point coordinate corresponding to each of the three intermediate coordinates in another search control table. In this case, four search control tables are created. When subsampling is performed by adding intermediate coordinates different from the x direction for the yz coordinates, the search control table is increased correspondingly and created. Even when coordinate conversion is performed in consideration of intermediate coordinates, the starting point coordinates can be efficiently searched using the procedure shown in FIGS.

  The ray casting processing unit 124 refers to the search control table corresponding to the intermediate coordinates for each pixel of the volume rendering image, acquires the origin coordinates, and similarly performs the ray casting processing for the intermediate coordinates. Further, the ray casting method can be efficiently performed also at intermediate coordinates by the same method as in FIG.

  According to the ray casting apparatus 100 according to the second embodiment, even when the image quality of a volume rendering image is improved by sub-sampling, the starting point coordinate search and the ray casting process can be performed efficiently. Thereby, even a general computer such as a personal computer can improve the image quality of the volume rendering image by sub-sampling within a practical time.

<Embodiment 3>
When the user views the volume rendering image, the viewpoint (the rotation angles Rx, Ry, Rz around the X axis, the Y axis, and the Z axis in the coordinate conversion parameter) and the magnification (enlargement / reduction scale Scale in the coordinate conversion parameter) are displayed. It may change continuously on the screen. The ray casting apparatus 100 redraws the volume rendering image corresponding to the changed viewpoint and magnification. At this time, the user changes viewpoint parameters (rotation angles Rx, Ry, Rz about the X axis, the Y axis, and the Z axis) by dragging the mouse on the screen, for example. If the volume rendering image can be calculated and updated in real time while the user is dragging the mouse, the operational feeling for the user can be improved. However, generating a high-definition volume rendering image at a level that can be used for diagnosis or the like on a general computer such as a personal computer is heavy in computational load, and continues to update the volume rendering image following the user's mouse operation. It is difficult. Therefore, in the third embodiment of the present invention, a method of suppressing the calculation load while the user changes the viewpoint by mouse drag will be described. The configuration of the ray casting apparatus 100 is the same as that of the first embodiment.

  FIG. 8 is an example of a volume rendering image generated by the ray casting apparatus 100 when the viewpoint is changed by the user dragging the mouse. The left figure of FIG. 8 is an image while the user is dragging the mouse. The right figure of FIG. 8 is an image after the user finishes the mouse drag.

  The ray casting apparatus 100 receives an input of a coordinate conversion parameter that instructs the user to change the viewpoint by dragging the mouse. The ray casting processing unit 124 and the display unit 125 generate an image by thinning out the pixels of the volume rendering image while the user is dragging the mouse, and display the enlarged image on the output destination apparatus or device. Output to. For example, an image that is thinned out every four pixels in the xy direction is generated, and enlarged and displayed four times in length and width. The left figure of FIG. 8 is an example of an image with pixels thinned out. Thereby, while the user is dragging the mouse, the calculation load can be suppressed to such an extent that a volume rendering image can be generated in real time. However, the pixel thinning condition needs to be changed as appropriate depending on the CPU performance of the personal computer and the number of voxels in the source voxel image. In the example of FIG. 8, the source voxel image is x: 512 × y: 512 × z: 370 and the Z-direction magnification is 1.4, and the volume for diagnostic use of 512 × 512 pixels shown in the right diagram of FIG. In the case of generating a rendering image, if the image is 128 × 128 pixels thinned out every four pixels in the vertical and horizontal directions shown in the left diagram of FIG. 8, the calculation load is suppressed to 1/16 and real time (0. 1 second or less). In the left diagram of FIG. 8, thinning is not performed in the z direction, but thinning can also be performed in every four pixels in the z direction. In this case, the calculation load is suppressed to 1/64. Even a tablet terminal having a processing capability lower than that of a general personal computer can be generated in real time.

  When the user completes the mouse drag (that is, when the user releases the mouse button), the ray casting apparatus 100 receives a signal to that effect. The ray casting processing unit 124 and the display unit 125 generate a volume rendering image as shown in the right diagram of FIG. 8 without thinning out pixels after the completion of dragging, and process the image data according to the output destination apparatus or device. Output.

<Modification of the present invention>
The present invention is not limited to the embodiments described above, and includes various modifications. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described. Further, a part of the configuration of an embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of an embodiment. In addition, it is possible to add, delete, and replace other configurations for a part of the configuration of each embodiment.

  In the above embodiment, the voxel image creation unit 121 may perform smoothing on the voxel image. For example, a representative value (for example, an average value) of opacity of adjacent voxels (for example, 26 adjacent voxels) in each of the x, y, and z directions of a certain voxel is adopted as the opacity of the central voxel. In addition, a voxel image can be created. At this time, if smoothing is not applied to the color value, the calculation load of smoothing can be reduced to ¼ and blurring of the generated volume rendering image can be suppressed. When referring to a voxel, the method according to the present invention can be implemented together with coordinate transformation, but in this case, since the voxel referred to in the voxel coordinate system is overlapped, smoothing processing is performed. Since the computation load may increase on the contrary, it is desirable that the smoothing of the voxels is performed before the search for the origin coordinates and the ray casting process (for example, step S210).

  In the above embodiment, the voxel image creation unit 121 calculates a shadow value for each voxel of the voxel image, and multiplies the color value of each voxel to add a shadow to the volume rendering image. You may do it. The diffuse reflection component in the shade is calculated as a three-dimensional gradient vector of a three-dimensional change in opacity of 26 or six adjacent voxels adjacent to each voxel (two adjacent in the xyz coordinate axis directions). The calculation can be performed by taking the inner product of the unit vector of the vector and a predefined light source vector (unit vector indicating the three-dimensional direction of parallel light). Since it is necessary to refer to adjacent voxels when calculating the gradient vector of opacity, the smoothing process is performed before the search for the origin coordinates and the ray casting process (for example, step S210) in the same manner as the smoothing. In such a case, it is desirable that the smoothing process be performed.

  In the above embodiment, an example of complementing odd-numbered pixels in the search control table has been described, but even-numbered pixels may be complemented instead. The same applies to any of the xy directions. Alternatively, the origin coordinates may be searched for every vertical and horizontal M (> 1) pixels instead of every other vertical and horizontal pixels, and the M pixels sandwiched between them may be interpolated based on both adjacent pixels. In this case, the calculation load can be further suppressed.

  In the above embodiment, the voxel image creation unit 121, the coordinate conversion unit 122, the starting point coordinate search unit 123, and the ray casting processing unit 124 are described as individual modules, but two or more of these are configured as an integrated module. You can also Furthermore, the CPU 110 can execute these modules as one ray casting program as a whole.

  Each of the above-described configurations, functions, processing units, processing means, and the like may be realized by hardware by designing a part or all of them with, for example, an integrated circuit. Each of the above-described configurations, functions, and the like may be realized by software by interpreting and executing a program that realizes each function by the processor. Information such as programs, tables, and files for realizing each function can be stored in a memory, a hard disk, a recording device such as an SSD (Solid State Drive), or a portable recording medium such as an IC card or an SD card. When a program is stored on a portable recording medium, the program can be executed by reading the recording medium from the drive.

  FIG. 9 is a configuration example when each functional unit is configured as a program. For example, the above-described storage device or storage medium may be used as the storage unit 130, and each functional unit from the voxel image creation unit 121 to the ray casting processing unit 124 may be configured as a program and stored in the storage unit 130. When the CPU 110 reads out and executes these programs from the storage unit 130, the function of the ray casting apparatus 100 can be implemented. Then, the volume rendering image generated by the ray casting processing unit 124 is passed to the display unit 125 so that it can be visually browsed.

100: Ray casting device 110: CPU
121: Voxel image creation unit 122: Coordinate conversion unit 123: Origin coordinate search unit 124: Ray casting processing unit 125: Display unit 130: Storage unit

Claims (16)

A ray casting program for causing a computer to execute a process of generating a volume rendering image using a ray casting method on a tomographic image acquired by tomographic imaging of an object,
A voxel image creating step for creating a voxel image of the object having the color value and opacity set based on the voxel value of the tomographic image from the tomographic image;
This is a parameter for converting a three-dimensional (x, y, z) viewpoint coordinate system based on the viewpoint (x, y, 0) with respect to the object to the coordinate system of the voxel image. A coordinate conversion parameter calculation step for calculating a coordinate conversion parameter used in common,
By referring to the voxel image while performing coordinate conversion using the coordinate conversion parameter from the viewpoint along the z direction of the viewpoint coordinate system, opacity is not zero among the z coordinates of the viewpoint coordinate system. An origin coordinate search step for setting an object as an origin coordinate;
A pixel of the volume rendering image is obtained by obtaining a color value and opacity of the voxel of the voxel image while performing coordinate transformation using the coordinate transformation parameter along the z direction of the viewpoint coordinate system starting from the origin coordinate. A ray casting step for calculating a pixel value of (x, y);
A ray casting program characterized in that In the starting point coordinate search step, the computer,
Searching for the first opaque voxel whose opacity is not 0 by obtaining the opacity of the voxel image at intervals of 2 voxels or more from the viewpoint along the z direction of the viewpoint coordinate system;
The first transparent voxel having an opacity of 0 is searched by obtaining the opacity of the voxel image for each voxel in the direction of the viewpoint from the first voxel and along the z direction of the viewpoint coordinate system. Step,
Identifying the z-coordinate in the viewpoint coordinate system of a voxel with non-opacity of 0 adjacent to the first transparent voxel as the origin coordinate,
The ray casting program according to claim 1, wherein: The volume rendering image has pixels in each of the xy directions,
In the starting point coordinate search step, the computer,
A first step of searching for a starting point coordinate corresponding to the pixel by referring to the corresponding voxel value for a part of the pixels of the volume rendering image, and setting the starting point coordinate;
While referring to the origin coordinates of the pixels of the volume rendering image searched in the first step, the origin coordinates corresponding to other pixels of the volume rendering image not searched in the first step are set. Two steps,
The ray casting program according to claim 1, wherein the ray casting program is executed. The volume rendering image has pixels in each of the xy directions,
In the starting point coordinate search step, the computer,
A first step of searching the starting point coordinates for skipping M (M is an integer of 1 or more) pixels along the x and y directions of the volume rendering image, and setting the starting point coordinates;
When the origin coordinates corresponding to the first pixel searched for in the first step and the origin coordinates corresponding to the adjacent second pixel after skipping M pixels in the x direction are the same, the first pixel and the A second step of setting the same z coordinate value as the starting point coordinate corresponding to the first pixel as the starting point coordinate corresponding to each of the M pixels arranged in the x direction between the second pixel and the second pixel;
When the starting point coordinates corresponding to the third pixel set in the first step and the second step are the same as the starting point coordinates corresponding to the adjacent fourth pixel after skipping M pixels in the y direction, The same z-coordinate value as the starting point coordinate corresponding to the third pixel is set as the starting point coordinate corresponding to each of the M pixels arranged in the y direction between the third pixel and the fourth pixel. The third step,
The ray casting program according to claim 1, wherein the ray casting program is executed. In the starting point coordinate search step, the computer is made to create a search control table that records the result after setting the starting point coordinates for all the pixels of the volume rendering image,
5. The ray casting step according to claim 1, wherein, in the ray casting step, the computer is configured to calculate a pixel value of the volume rendering image using the starting point coordinates recorded in the search control table as a starting point. The ray casting program according to any one of the above. In the starting point coordinate search step, when referring to the voxel image while performing coordinate conversion using the coordinate conversion parameter, the computer refers to the opacity without referring to the color value of the voxel, The ray casting program according to claim 1, wherein the starting point coordinates are searched. In the starting point coordinate search step, the computer,
When the first opaque voxel is not found within the range of the voxel image, the step of setting a value representing that as the starting point coordinate is executed,
In the ray casting step, the computer
When a value indicating that the first opaque voxel has not been found is set as the starting point coordinates, the processing for calculating the pixel value of the volume rendering image with the starting point coordinates as a starting point is skipped, and a predetermined background color The ray casting program according to claim 2, wherein a pixel value indicating is set. In the ray casting step, the computer
An opacity determination step of determining whether or not the voxel image is a transparent voxel having an opacity of 0 by obtaining the opacity of each voxel of the voxel image along the z direction of the viewpoint coordinate system from the viewpoint.
An opaque voxel search that starts from the transparent voxel determined in the opacity determination step and obtains the opacity of each voxel of the voxel image along the z-direction until reaching an opaque voxel whose opacity is not zero. Step,
Skipping the process of calculating the pixel value of the volume rendering image for a series of voxels from the transparent voxel to the voxel located immediately before in the z-direction of the opaque voxel;
The ray casting program according to claim 1, wherein the ray casting program is executed. In the opaque voxel search step, the computer
Searching for the first opaque voxel whose opacity is not zero by obtaining the opacity of the voxel image for each voxel interval of two or more voxels along the z direction, starting from the transparent voxel;
Searching for the first transparent voxel with zero opacity by obtaining the opacity of the voxel image for each voxel along the z direction in the direction from the first opaque voxel to the transparent voxel. ,
Controlling the ray casting step to resume from opaque voxels with non-zero opacity adjacent to the first transparent voxel;
The ray casting program according to claim 8, wherein the ray casting program is executed. The ray casting program further causes the computer to execute a step of receiving an input of a coordinate conversion parameter for instructing a coordinate conversion parameter for changing the viewpoint, and each time the input of the coordinate conversion parameter is received, the coordinate conversion parameter calculation is performed. Step, the origin coordinate search step and the ray casting step are executed to repeatedly display the volume rendering image,
In the starting point coordinate search step and the ray casting step, a thinning process for thinning out at least a part of the pixels of the volume rendering image is performed during a period during which the computer receives the input of the coordinate conversion parameter. After that, set the starting point coordinates and calculate the pixel value,
In the starting point coordinate search step and the ray casting step, the computer sets the starting point coordinate and calculates a pixel value without performing the thinning process when the input of the coordinate conversion parameter is stopped. The ray casting program according to any one of claims 1 to 9. In the voxel image creation step, the computer is caused to execute a step of calculating the opacity of the target voxel,
In the voxel image creating step, the computer is caused to execute a step of updating the opacity of the target voxel by an average value of opacity of each of the 26 voxels adjacent to the target voxel. The ray casting program according to any one of claims 1 to 10. In the starting point coordinate search step, when setting the starting point coordinates by referring to the voxel image while performing coordinate conversion using the coordinate conversion parameter, the intermediate coordinates between the voxels of the voxel image are set in the computer. In consideration of the coordinate transformation, the origin coordinate search step is executed N (N> 1) times,
In the ray casting step, when calculating the pixel value of the volume rendering image by obtaining the color value and opacity of the voxel image while performing coordinate conversion using the coordinate conversion parameter, Performing coordinate transformation in consideration of coordinates, causing the ray casting step to be executed N times, and calculating pixel values of the volume rendering image by averaging the pixel values of the calculated N volume rendering images. The ray casting program according to any one of claims 1 to 11, wherein the ray casting program is characterized in that: When calculating a pixel value of a volume rendering image generated by using the ray casting method for a tomographic image acquired by tomographic imaging of an object, the color value set based on the voxel value of the tomographic image and an For the voxel image of the object having transparency, the starting point is the z-coordinate of the opaque voxel that is the first starting point in the z-direction of the viewpoint coordinate system from the viewpoint and whose opacity is not zero, which is the starting point for executing the ray casting method. Search control data having a structure set as coordinates,
In the search control data, the starting point coordinates are set for each pixel of the volume rendering image,
At least a part of the starting point coordinates corresponding to each of the pixels is set to the same value for M pixels (M> 2) continuous in the x direction and / or the y direction,
At least a part of the origin coordinates corresponding to each of the pixels is set to a value indicating that there is no opaque voxel with non-opacity from the viewpoint to the end in the z direction of the viewpoint coordinate system. Search control data to be performed. When calculating pixel values of a volume rendering image generated using the ray casting method for a tomographic image acquired by tomographic imaging of an object, viewpoint coordinates from the viewpoint, which is the starting point for executing the ray casting method A search control data generation method for generating search control data describing, as a starting point coordinate, an opaque voxel having an opacity that is initially zero in the z direction of the system,
By referring to the voxel image of the object from the viewpoint with respect to the object along the z direction of the viewpoint coordinate system, the z coordinate in the viewpoint coordinate system of the opaque voxel in which the opacity of the voxel image is not 0 is set as the origin coordinate. An origin coordinate search step to search;
Recording the starting point coordinates in the search control data;
Have
The volume rendering image has pixels in each of the xy directions,
The origin coordinate search step includes:
The origin coordinate is searched by referring to the voxel value of the corresponding voxel image for skipping M (M is an integer of 1 or more) pixels along the x and y directions of the volume rendering image. The first step of setting
When the starting point coordinates corresponding to the first pixel searched for in the first step and the starting point coordinates corresponding to the adjacent second pixel after skipping M pixels in the x direction are equal, the first pixel and the second pixel A second step of setting the same z coordinate value as the starting point coordinate corresponding to the first pixel as the starting point coordinate corresponding to each of the M pixels arranged in the x direction between
In the case where the starting point coordinates corresponding to the third pixel set in the first step and the second step are the same as the starting point coordinates corresponding to the adjacent fourth pixel after skipping M pixels in the y direction, the third A third z-coordinate value that is the same as the starting point coordinate corresponding to the third pixel is set as the starting point coordinate corresponding to each of the M pixels arranged in the y direction between the pixel and the fourth pixel. Steps,
A search control data generation method characterized by comprising: In the first step of the starting point coordinate search step, if there is no opaque voxel with non-opacity from the viewpoint to the end in the z direction of the viewpoint coordinate system, a value indicating that is set as the starting point coordinate. The search control data generation method according to claim 14. A ray casting apparatus that generates a volume rendering image using a ray casting method for a tomographic image acquired by tomographic imaging of an object,
A voxel image creating unit that creates a voxel image of the object having a color value and opacity as a voxel value from the tomographic image;
Parameters for converting the coordinate system of the voxel image into a three-dimensional (x, y, z) viewpoint coordinate system based on the viewpoint (x, y, 0) with respect to the object, A coordinate conversion parameter calculator that calculates coordinate conversion parameters used in common,
By referring to the voxel image while performing coordinate conversion using the coordinate conversion parameter from the viewpoint along the z direction of the viewpoint coordinate system, opacity is not zero among the z coordinates of the viewpoint coordinate system. An origin coordinate search unit for setting an object as an origin coordinate,
A pixel of the volume rendering image is obtained by obtaining a color value and opacity of the voxel of the voxel image while performing coordinate transformation using the coordinate transformation parameter along the z direction of the viewpoint coordinate system starting from the origin coordinate. A ray casting processing unit for calculating a pixel value of (x, y);
A ray casting apparatus characterized by comprising:

Images