CN114862706B - Tone mapping method for keeping gradient direction of image - Google Patents
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Abstract
A color gradation mapping method for keeping the gradient direction of an image belongs to the technical field of image processing. The invention solves the problems that the details and the texture of the LDR image reconstructed by adopting the traditional color gradation mapping algorithm are not prominent, and the visual effect of the image is poor. The method comprises the steps of firstly selecting diagonal pixels of an HDR image; secondly, carrying out color level mapping on the brightness of diagonal pixels; thirdly, deducing a brightness calculation method capable of keeping the gradient direction by utilizing a Roberts operator; from time to time, the brightness of all pixels at the upper left of the diagonal is recurrently deduced according to the brightness of the diagonal pixels and a brightness calculation method for keeping the gradient direction; and finally, deducing the brightness of all pixels at the lower right of the diagonal line according to the brightness of the diagonal line pixels and a brightness calculation method for keeping the gradient direction. The LDR image reconstructed by the method has the advantages of more prominent local details and textures, higher image definition and better overall visual effect. The method can be applied to the technical field of image processing.
Description
Technical Field
The invention belongs to the technical field of image processing, and particularly relates to a tone mapping method for keeping the gradient direction of an image.
Background
Since the dynamic range of a conventional display is much smaller than a High Dynamic Range (HDR) image fused by a multi-exposure image synthesis method, the tone mapping algorithm with a dynamic range compression function is increasingly receiving attention from academia. The traditional tone mapping algorithm mainly has the defects that the details and textures of a reconstructed Low Dynamic Range (LDR) image are not prominent, and the visual effect of the image is poor.
Disclosure of Invention
The invention aims to provide a color gradation mapping method for keeping the gradient direction of an image, which aims to solve the problems that the details and the texture of an LDR image reconstructed by adopting a traditional color gradation mapping algorithm are not prominent, and the visual effect of the image is poor.
The technical scheme adopted by the invention for solving the technical problems is as follows:
a tone mapping method for maintaining the gradient direction of an image specifically comprises the following steps:
step one, selecting diagonal pixels of an HDR image;
step two, recording a set formed by the diagonal pixels selected in the step one as a set C, and respectively carrying out color level mapping on each pixel in the set C to obtain the brightness of each pixel after the color level mapping;
and thirdly, taking the brightness obtained after the tone mapping in the second step as the brightness of a corresponding pixel in the reconstructed LDR image, recording a set formed by the pixels which are not selected in the first step in the HDR image as a set E, and calculating the brightness of the pixels in the set E in the reconstructed LDR image according to the brightness value of the pixels in the set E in the HDR image and the brightness after the tone mapping to obtain the reconstructed LDR image.
Further, in the first step, a diagonal pixel of the HDR image is selected, and the selected pixel comprises two parts;
wherein, the selection mode of some pixels is as follows:
step one, selecting a pixel at the leftmost lower corner of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
selecting a pixel which is positioned at the upper right of the current pixel and is adjacent to the current pixel;
step three, taking the pixel selected in the step two as the current pixel;
step one four, the process from the step two to the step one three is repeated until the selected pixel reaches the right boundary of the HDR image;
step one five, in the process from the step one to the step four, all selected pixels are used as a part of selected pixels;
the other part of the pixels are selected in the following way:
step six, selecting an upper adjacent pixel of a leftmost lower corner pixel of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
seventhly, selecting a pixel which is positioned at the upper right of the current pixel and is immediately adjacent to the current pixel;
step one eight, the pixel selected in the step one seven is taken as the current pixel;
step one nine, the process from the step one seven to the step one eight is repeated until the selected pixel reaches the right side boundary of the HDR image;
and step one ten, in the process of executing the step one six to the step one nine, all the selected pixels are taken as the other part of the selected pixels.
Further, the specific process of the second step is as follows:
for any pixel in the set C, a coordinate point of the pixel in the HDR image is represented as (m, n), and a luminance value of the pixel in the HDR image is represented as L (m, n), so that luminance I (m, n) after tone mapping corresponding to the pixel is:
where max (C) and min (C) represent the maximum and minimum luminance of the pixel in set C in the HDR image, respectively, D min Indicating the minimum brightness, D, of the display max Representing the maximum brightness of the display.
Further, the specific process of the third step is as follows:
step three, firstly, the gradient of a pixel positioned at a coordinate point (x, y) in the HDR image is determined
And
expressed as:
wherein, L (x, y) represents the luminance of the pixel at the coordinate point (x, y) in the HDR image, L (x +1, y + 1) represents the luminance of the pixel at the coordinate point (x +1, y + 1) in the HDR image, L (x +1, y) represents the luminance of the pixel at the coordinate point (x +1, y) in the HDR image, and L (x, y + 1) represents the luminance of the pixel at the coordinate point (x, y + 1) in the HDR image;
gradient of pixel at coordinate point (x, y) in LDR image
And
expressed as:
wherein I (x, y) represents the luminance of a pixel at coordinate point (x, y) in the LDR image, I (x +1, y + 1) represents the luminance of a pixel at coordinate point (x +1, y + 1) in the LDR image, I (x +1, y) represents the luminance of a pixel at coordinate point (x +1, y) in the LDR image, and I (x, y + 1) represents the luminance of a pixel at coordinate point (x, y + 1) in the LDR image;
wherein phi is I (x, y) represents the direction of the gradient of the pixel at coordinate point (x, y) in the LDR image, φ L (x, y) represents the direction of the gradient of the pixel at coordinate point (x, y) in the HDR image；
In order to make the LDR image consistent with the HDR image gradient direction, it needs to guarantee:
obtaining the brightness I (x, y) of the pixel at the coordinate point (x, y) in the LDR image by sorting the formula (5):
step two, calculating the brightness of the pixel positioned on the upper left of the diagonal pixel selected in the step one according to the formula (6) and the brightness after the tone mapping obtained in the step two;
step three, sorting the formula (6) to obtain the brightness I (x +1, y + 1) of the pixel located at the coordinate point (x +1, y + 1) in the LDR image:
step three, calculating the brightness of the pixel positioned at the right lower part of the diagonal pixel selected in the step one according to the brightness after the tone mapping obtained in the formula (7) and the step two;
and step three, obtaining a reconstructed LDR image after the brightness calculated in the step two and the step three is taken as the brightness of the corresponding pixel in the LDR image.
Further, the specific process of the third step is as follows:
step three, step two, the pixel which is positioned right above the pixel selected in the step one ten and is most adjacent to the pixel selected in the step one ten is taken as a first group of pixels;
calculating the brightness of the first group of pixels according to the formula (6) and the brightness after the tone mapping obtained in the step two;
step two, taking the pixel which is positioned right above the first group of pixels and is most adjacent to the first group of pixels as a second group of pixels;
calculating the brightness of the second group of pixels according to the formula (6) and the brightness of the first group of pixels;
and the like until the brightness of all the pixels positioned at the upper left of the diagonal pixel selected in the step one is deducted.
Further, the specific process of the third step and the fourth step is as follows:
step three, taking the pixel which is positioned right below the pixel selected in the step one, and is most adjacent to the pixel selected in the step one as a group 1 pixel;
calculating the brightness of the 1 st group of pixels according to the formula (7) and the brightness obtained in the step two after the tone mapping;
step three, taking the pixel which is positioned right below the 1 st group of pixels and is most adjacent to the 1 st group of pixels as a 2 nd group of pixels;
calculating the brightness of the 2 nd group pixel according to the formula (7) and the brightness of the 1 st group pixel;
and the like until the brightness of all the pixels positioned at the lower right of the diagonal pixel selected in the step one is deducted.
The invention has the beneficial effects that:
the method comprises the steps of firstly, selecting diagonal pixels of an HDR image; secondly, carrying out color gradation mapping on diagonal line pixel brightness; thirdly, deducing a brightness calculation method capable of keeping the gradient direction by utilizing a Roberts operator; from time to time, the brightness of all pixels at the upper left of the diagonal is recurrently deduced according to the brightness of the diagonal pixels and a brightness calculation method for keeping the gradient direction; and finally, deducing the brightness of all pixels at the lower right of the diagonal line according to the brightness of the diagonal line pixels and a brightness calculation method for keeping the gradient direction. Experimental results show that the LDR image reconstructed by the method has more prominent local details and textures, higher image definition and better overall visual effect.
Drawings
FIG. 1 is a schematic diagram of a diagonal pixel selection for an HDR image of size 10 × 5;
the selected pixel is marked as 0 in the figure;
FIG. 2 is a schematic diagram of diagonal pixels corresponding to an HDR image of size 11 × 11;
FIG. 3 is a schematic diagram of the top left first group pixel brightness recursion process;
the first group of pixels is marked with 1;
FIG. 4 is a schematic diagram of the top left second group pixel brightness recursion process;
the second group of pixels is numbered 2;
FIG. 5 is a schematic diagram of the bottom right first set of pixel recursion processes;
the 1 st group of pixels in the figure is marked with 1;
FIG. 6 is a schematic diagram of the bottom right second set of pixel recursion processes;
the 2 nd group of pixels is numbered 2 in the figure;
FIG. 7a is an HDR image one;
FIG. 7b is a chart of the tone mapping results of FIG. 7 a;
FIG. 8a is an HDR image two;
FIG. 8b is a chart of the tone mapping results of FIG. 8 a;
FIG. 9a is an HDR image three;
fig. 9b is a diagram of the tone mapping result of fig. 9 a.
Detailed Description
In a first embodiment, a method for tone mapping with maintained gradient direction of an image in this embodiment includes the following steps:
step one, selecting diagonal pixels of an HDR image;
step two, recording a set formed by the diagonal pixels selected in the step one as a set C, and respectively carrying out color level mapping on each pixel in the set C to obtain the brightness of each pixel after the color level mapping;
and thirdly, taking the brightness obtained after the tone mapping in the second step as the brightness of a corresponding pixel in the reconstructed LDR image, recording a set formed by the pixels which are not selected in the first step in the HDR image as a set E, and calculating the brightness of the pixels in the set E in the reconstructed LDR image according to the brightness value of the pixels in the set E in the HDR image and the brightness after the tone mapping to obtain the reconstructed LDR image.
The second embodiment is as follows: this embodiment will be described with reference to fig. 1. The first embodiment is different from the first embodiment in that, in the first step, a diagonal pixel of the HDR image is selected, and the selected pixel includes two parts;
wherein, the selection mode of some pixels is as follows:
step one, selecting a pixel at the leftmost lower corner of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
selecting a pixel which is positioned at the upper right of the current pixel (when the step is executed for the first time, the current pixel is the initial pixel in the step one, and the current pixel is the current pixel determined in the step one in the previous iteration process from the step of executing the step for the second time) and is adjacent to the current pixel;
step three, taking the pixel selected in the step two as the current pixel;
step four, repeating the process from the step two to the step three until the selected pixel reaches the right side boundary of the HDR image;
step five, in the process from the step one to the step four, all selected pixels are used as a part of selected pixels;
the other part of the pixels are selected in the following way:
step six, selecting an upper adjacent pixel of a leftmost lower corner pixel of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
step seven, selecting a pixel which is positioned at the upper right of the current pixel (when the step is executed for the first time, the current pixel is the initial pixel in the step six, and from the step of executing the step for the second time, the current pixel is the current pixel determined in the step eight in the previous iteration process) and is closely adjacent to the current pixel;
step one eight, taking the pixel selected in the step one seven as the current pixel;
step one nine, the process from the step one seven to the step one eight is repeated until the selected pixel reaches the right side boundary of the HDR image;
and step one ten, in the process of executing the step one six to the step one nine, all the selected pixels are used as the other part of the selected pixels.
Other steps and parameters are the same as those in the first embodiment.
The third concrete implementation mode: the difference between this embodiment and the first or second embodiment is that the specific process of the second step is:
for any pixel in the set C, if a coordinate point of the pixel in the HDR image is represented as (m, n), (m, n) represents a plane position where the pixel is located, and a luminance value of the pixel in the HDR image is represented as L (m, n), then the luminance I (m, n) after tone mapping corresponding to the pixel is:
where max (C) and min (C) represent the maximum and minimum luminance of the pixel in set C in the HDR image, respectively, D min Representing the minimum brightness, D, of the display max Representing the maximum brightness of the display.
Other steps and parameters are the same as those in the first or second embodiment.
The fourth concrete implementation mode is as follows: the difference between this embodiment and one of the first to third embodiments is that the specific process of step three is:
step three, according to a Roberts operator, gradient of a pixel located at a coordinate point (x, y) in the HDR image is determined
And
expressed as:
where L (x, y) represents the luminance of a pixel at coordinate point (x, y) in the HDR image, L (x +1, y + 1) represents the luminance of a pixel at coordinate point (x +1, y + 1) in the HDR image, L (x +1, y) represents the luminance of a pixel at coordinate point (x +1, y) in the HDR image, and L (x, y + 1) represents the luminance of a pixel at coordinate point (x, y + 1) in the HDR image;
gradient of pixel at coordinate point (x, y) in LDR image
And
expressed as:
wherein I (x, y) represents the luminance of a pixel at coordinate point (x, y) in the LDR image, I (x +1, y + 1) represents the luminance of a pixel at coordinate point (x +1, y + 1) in the LDR image, I (x +1, y) represents the luminance of a pixel at coordinate point (x +1, y) in the LDR image, and I (x, y + 1) represents the luminance of a pixel at coordinate point (x, y + 1) in the LDR image;
wherein phi is I (x, y) represents the direction of the gradient of the pixel at coordinate point (x, y) in the LDR image, φ L (x, y) represents the direction of the gradient of the pixel located at coordinate point (x, y) in the HDR image;
in order to make the LDR image consistent with the HDR image gradient direction, it is necessary to ensure that:
obtaining the brightness I (x, y) of the pixel at the coordinate point (x, y) in the LDR image by sorting the formula (5):
step two, calculating the brightness of the pixel positioned on the upper left of the diagonal pixel selected in the step one according to the formula (6) and the brightness after the tone mapping obtained in the step two;
step three, sorting the formula (6) to obtain the brightness I (x +1, y + 1) of the pixel located at the coordinate point (x +1, y + 1) in the LDR image:
step three, calculating the brightness of the pixel positioned at the right lower part of the diagonal pixel selected in the step one according to the brightness after the tone mapping obtained in the formula (7) and the step two;
and step three, obtaining a reconstructed LDR image after the brightness calculated in the step three and the step three is taken as the brightness of the corresponding pixel in the LDR image.
Other steps and parameters are the same as those in one of the first to third embodiments.
The fifth concrete implementation mode: the present embodiment will be described with reference to fig. 2, 3, and 4. The difference between this embodiment and one of the first to fourth embodiments is that the specific process of the second step is:
step three, step two, the pixel which is positioned right above the pixel selected in the step one ten and is most adjacent to the pixel selected in the step one ten is taken as a first group of pixels;
calculating the brightness of the first group of pixels according to the formula (6) and the brightness obtained in the step two after the tone mapping;
step two, taking the pixel which is positioned right above the first group of pixels and is most adjacent to the first group of pixels as a second group of pixels;
calculating the brightness of the second group of pixels according to the formula (6) and the brightness of the first group of pixels;
and the like until the brightness of all the pixels positioned at the upper left of the diagonal pixel selected in the step one is deducted.
Other steps and parameters are the same as in one of the first to fourth embodiments.
The sixth specific implementation mode: this embodiment will be described with reference to fig. 2, 5, and 6. The difference between this embodiment and one of the first to fifth embodiments is that the specific process of step three and step four is:
step three, step four, the pixel which is positioned right below the pixel selected in the step one, step five and is most adjacent to the pixel selected in the step one is taken as the pixel of the group 1;
calculating the brightness of the 1 st group of pixels according to the formula (7) and the brightness after the tone mapping obtained in the step two;
step three, taking the pixel which is positioned right below the 1 st group of pixels and is most adjacent to the 1 st group of pixels as a 2 nd group of pixels;
calculating the brightness of the 2 nd group pixel according to the formula (7) and the brightness of the 1 st group pixel;
and so on until the brightness of all the pixels which are positioned at the right lower part of the diagonal pixel selected in the step one is deducted.
Other steps and parameters are the same as those in one of the first to fifth embodiments.
Results and analysis of the experiments
In the experimental part, a notebook computer with a CPU of i7-11800H and a memory of 16G is used as hardware simulation. The software simulation platform is matlab 2016a. The algorithm input is an HDR image with HDR suffix HDR, and the algorithm output is a jpeg format image. The experimental simulation results of the algorithm are shown in fig. 7a, 7b, 8a, 8b, 9a and 9 b.
The experimental results show that: the original HDR image has highlight effect and shadow effect, the highlight effect and the shadow effect cause that the human eye cannot clearly distinguish the image content of highlight areas and shadow areas, and the highlight areas bring strong irritation to the human eye. The image quality of directly displaying HDR images cannot be guaranteed. The color gradation mapping algorithm provided by the invention can effectively compress the dynamic range of the image, the visibility of the image details and the texture of a highlight area and a shadow area is effectively improved, the overall brightness of an LDR image obtained by color gradation mapping is more suitable for being watched by human eyes, and the image quality is obviously improved. Therefore, the method can realize the compression of the dynamic range on the premise of ensuring the image quality, thereby displaying on an LDR display with high quality.
The above-described calculation examples of the present invention are merely to describe the calculation model and the calculation flow of the present invention in detail, and are not intended to limit the embodiments of the present invention. It will be apparent to those skilled in the art that other variations and modifications can be made on the basis of the foregoing description, and it is not intended to exhaust all of the embodiments, and all obvious variations and modifications which fall within the scope of the invention are intended to be included within the scope of the invention.
Claims (4)
1. A tone mapping method for maintaining the gradient direction of an image is characterized by comprising the following steps:
step one, selecting diagonal pixels of an HDR image;
in the first step, diagonal pixels of the HDR image are selected, and the selected pixels comprise two parts;
wherein, the selection mode of a part of pixels is as follows:
step one, selecting a pixel at the leftmost lower corner of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
selecting a pixel which is positioned at the upper right of the current pixel and is adjacent to the current pixel;
step three, taking the pixel selected in the step two as the current pixel;
step four, repeating the process from the step two to the step three until the selected pixel reaches the right side boundary of the HDR image;
step one five, in the process from the step one to the step four, all selected pixels are used as a part of selected pixels;
the other part of the pixels are selected in the following way:
step six, selecting an upper adjacent pixel of a leftmost lower corner pixel of the HDR image as an initial pixel, and taking the initial pixel as a current pixel;
seventhly, selecting a pixel which is positioned at the upper right of the current pixel and is immediately adjacent to the current pixel;
step one eight, the pixel selected in the step one seven is taken as the current pixel;
step one nine, the process from the step one seven to the step one eight is repeated until the selected pixel reaches the right side boundary of the HDR image;
step one ten, in the process of executing the step one six to the step one nine, all selected pixels are taken as another part of selected pixels;
step two, recording a set formed by the diagonal pixels selected in the step one as a set C, and respectively carrying out color level mapping on each pixel in the set C to obtain the brightness of each pixel after the color level mapping;
step three, the brightness obtained in the step two after tone mapping is used as the brightness of a corresponding pixel in a reconstructed LDR image, a set formed by pixels which are not selected in the step one in the HDR image is recorded as a set E, the brightness of the pixels in the set E in the reconstructed LDR image is calculated according to the brightness value of the pixels in the HDR image in the set E and the brightness after tone mapping, and the reconstructed LDR image is obtained;
the specific process of the third step is as follows:
step three, firstly, gradient of a pixel positioned at a coordinate point (x, y) in the HDR image is determined
And
expressed as:
wherein, L (x, y) represents the luminance of the pixel at the coordinate point (x, y) in the HDR image, L (x +1, y + 1) represents the luminance of the pixel at the coordinate point (x +1, y + 1) in the HDR image, L (x +1, y) represents the luminance of the pixel at the coordinate point (x +1, y) in the HDR image, and L (x, y + 1) represents the luminance of the pixel at the coordinate point (x, y + 1) in the HDR image;
gradient of pixel at coordinate point (x, y) in LDR image
And
expressed as:
wherein I (x, y) represents the luma of a pixel at coordinate point (x, y) in the LDR image, I (x +1, y + 1) represents the luma of a pixel at coordinate point (x +1, y + 1) in the LDR image, I (x +1, y) represents the luma of a pixel at coordinate point (x +1, y) in the LDR image, and I (x, y + 1) represents the luma of a pixel at coordinate point (x, y + 1) in the LDR image;
wherein phi is I (x, y) represents the direction of the gradient of the pixel at coordinate point (x, y) in the LDR image, φ L (x, y) represents the direction of the gradient of the pixel located at coordinate point (x, y) in the HDR image;
in order to make the LDR image consistent with the HDR image gradient direction, it needs to guarantee:
obtaining the brightness I (x, y) of the pixel at the coordinate point (x, y) in the LDR image by sorting the formula (5):
step two, calculating the brightness of the pixel positioned on the upper left of the diagonal pixel selected in the step one according to the formula (6) and the brightness after the tone mapping obtained in the step two;
step three, sorting the formula (6) to obtain the brightness I (x +1, y + 1) of the pixel located at the coordinate point (x +1, y + 1) in the LDR image:
step three, calculating the brightness of the pixel positioned at the right lower part of the diagonal pixel selected in the step one according to the brightness after the tone mapping obtained in the formula (7) and the step two;
and step three, obtaining a reconstructed LDR image after the brightness calculated in the step two and the step three is taken as the brightness of the corresponding pixel in the LDR image.
2. The method for tone mapping with preserved gradient direction of image as claimed in claim 1, wherein the specific process of the second step is:
for any pixel in the set C, if the coordinate point of the pixel in the HDR image is represented as (m, n), and the luminance value of the pixel in the HDR image is represented as L (m, n), the luminance I (m, n) after tone mapping corresponding to the pixel is:
where max (C) and min (C) represent the maximum and minimum luminance of the pixel in set C in the HDR image, respectively, D min Representing the minimum brightness, D, of the display max Representing the maximum brightness of the display.
3. The method according to claim 2, wherein the specific process of the second step is as follows:
step three, step two, the pixel which is positioned right above the pixel selected in the step one ten and is most adjacent to the pixel selected in the step one ten is taken as a first group of pixels;
calculating the brightness of the first group of pixels according to the formula (6) and the brightness obtained in the step two after the tone mapping;
step two, taking the pixel which is positioned right above the first group of pixels and is most adjacent to the first group of pixels as a second group of pixels;
calculating the brightness of the second group of pixels according to the formula (6) and the brightness of the first group of pixels;
and so on until the brightness of all pixels above and to the left of the diagonal pixel selected in step one is deducted.
4. The method according to claim 3, wherein the specific process of step three or four is as follows:
step three, taking the pixel which is positioned right below the pixel selected in the step one, and is most adjacent to the pixel selected in the step one as a group 1 pixel;
calculating the brightness of the 1 st group of pixels according to the formula (7) and the brightness after the tone mapping obtained in the step two;
step three, taking the pixel which is positioned right below the 1 st group of pixels and is most adjacent to the 1 st group of pixels as a 2 nd group of pixels;
calculating the brightness of the 2 nd group pixel according to the formula (7) and the brightness of the 1 st group pixel;
and so on until the brightness of all the pixels which are positioned at the right lower part of the diagonal pixel selected in the step one is deducted.
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