JP4488430B2 - Invalid data area display suppression method for lossy compressed images - Google Patents

Description

  The present invention relates to a method for suppressing display of an invalid data area when displaying an irreversible compressed image including an invalid data area.

  Conventionally, various formats are used as the data format of image data. These can be roughly divided into a reversible compression format and an irreversible compression format. The lossless compression method is a method in which data before compression is completely equal to data that has undergone compression / decompression processing. The lossy compression method is a method in which data before compression and data that has undergone compression / decompression processing do not completely match. For example, PNG format and GIF format are lossless compression, and ECW format and JPEG 2000 format are lossy compression.

  In a lossless compression data format such as PNG format or GIF format, a method of specifying a specific pixel value as an invalid data pixel value (transparent pixel value) is known as a technique for not displaying an invalid data area in an image. ing. For example, in PNG and GIF formats, a specific color in the color palette can be specified as a transparent color, and most of the software that displays these images does not display pixels with the specified transparent color (i.e. Its position is the background pixel).

When saving images such as photographs, ECW format and JPEG 2000 format, which are not reversible but have a high compression ratio, are often more suitable than PNG and GIF formats, which are lossless compression methods. In these compression formats, a high compression rate is realized by slightly changing the original pixel value (= non-reversible). However, when the compressed data is expanded and displayed, the above-described change amount of the pixel value appears in the image as compression noise. Such an irreversible compression algorithm is designed to suppress this compression noise as much as possible while increasing the compression rate. For example, there is an algorithm that guarantees that the amount of change in pixel value is within a certain value. The compression algorithm is described in detail in, for example, Non-Patent Document 1 below.
"Image data compression" in Part I of Chapter 7 of the "New Image Analysis Handbook" (supervised by the University of Tokyo Press, Mikio Takagi and Yoko Shimoda)

  As described above, some reversible compression image data formats can specify a transparent color. However, in the lossy compression image data format, normally transparent pixels are not supported, and even if a specific pixel value is treated as a transparent pixel, the pixel that should originally have the transparent pixel value is compressed. Many pixels become non-transparent pixels due to noise, and there is a problem that expected display is not performed.

  FIG. 6 is a diagram showing the influence of compression noise on the invalid data area in the conventional method. The uncompressed image 601 is composed of an invalid data area 611 and a valid data area 612. The area painted black is the invalid data area 611, and the pixels in this area are previously set as transparent pixel values. Such an uncompressed image 601 is irreversibly compressed into a compressed image and displayed by image display software. This image display software has a function of not displaying pixels having transparent pixel values as invalid data areas. Reference numeral 602 denotes a display example of such a compressed image. Even a pixel having a transparent pixel value in the state of an uncompressed image may change to a value other than the transparent pixel value due to irreversible compression, and therefore, compression noise 623 occurs in that portion.

  FIG. 7 shows how transparent pixels become non-transparent pixels due to compression noise. Reference numeral 701 denotes an uncompressed state, the horizontal axis indicates the pixel position, and the vertical axis indicates whether the pixel is a transparent pixel or a non-transparent pixel. For example, if the transmissive pixel value is R = 0, G = 0, and B = 0, the pixel having this value is transmissive 711. Pixels having other values are non-transparent 712. A state in which such an uncompressed image is irreversibly compressed is indicated by 702. In contrast to the RGB value of the above-described transmission pixel value, the pixel value becomes R = 1, G = 0, and B = 0 due to the compression noise 723, and the transmission value is not transmitted. In other words, if the pixel value of the transmissive pixel is changed even by 1 because of the binary determination that it is not displayed if it is equal to the transmissive pixel value, and if it is not equal, it is determined as a non-transparent pixel. The display is directly affected by noise.

  An object of the present invention is to provide an invalid data area display suppression method capable of making compression noise inconspicuous even when image data having an invalid data area in an uncompressed state is irreversibly compressed. It is in.

In order to solve the above problems, the present invention is irreversible compressed image when displaying using a computer, the lossy compression of images having an effective data area and the invalid data region image by non-reversible compression on the background image In the invalid data area display suppression method in the above, the α band addition processing means provided in the computer sets the α band pixel value of the valid data area to 255 ( opaque ) for an image having the valid data area and the invalid data area , A step of giving 256- band α- band information in which the α-band pixel value of the invalid data area is 0 ( transparent ), and a step of irreversibly compressing the image to which the band information is added by an image compression processing unit included in the computer When the background image computer image display processing means comprising is to be superimposed on an image after the image and non-reversible compression after lossy compression The Get the alpha band pixel value alpha of the image after lossy compression, the in the pixel values of the corresponding positions 255-alpha background image, the corresponding position of the image after the lossy compression of the corresponding position And a step of weighting the pixel value by α and displaying the averaged value as the pixel value at the corresponding position .

  According to the present invention, band information for identifying an invalid data area is inserted into an image as an α band, and when displaying an image after irreversible compression, the band information is used as an opacity to display invalidity. The display of the data area can be suppressed, and thereby compression noise can be made inconspicuous even when irreversible compression is performed. The present invention is effective when displaying satellite images and aerial photographs on a map.

  Hereinafter, one example of the case of carrying out the present invention will be specifically described.

  FIG. 1 shows an example of a system for realizing the method according to the present invention. The system includes a storage device 101, a central processing unit 102, and a display device 103. The storage device 101 stores image files. The central processing unit 102 includes an α band addition processing unit 102, an image compression processing unit 122, and an image display processing unit 123. The display device 103 is a display that displays an image in response to an instruction from the image display processing unit 123. Such a system of the present embodiment is realized by executing predetermined software on a computer.

The central processing unit 102 performs image preparation processing and image display processing. First, in the image preparation process, an α band is added to the uncompressed image file on the storage device 101 by the α band adding processing unit 121 of the central processing unit 102, and the image compression processing unit 122 further compresses the image (uncompressed). Reversible compression). The compressed image data is stored in the storage device 101. Thereafter, the image display processing unit 123 performs image display processing to display the compressed image data. In this image display process, the compressed image data on the storage device 101 is read, and the image display processing unit 123 of the central processing unit 102 performs transparency processing using the α band and outputs the result to the display device 103. . The α band is an attribute of the Color class (http://java.sun.com/j2se/1.4/ja/docs/ja/api/java/awt/Color.html) of the Java (registered trademark) SDK, for example. It is a known and general technique as defined, and when drawing, according to the value of this α band when drawing the corresponding pixel,
-Transparent when α band value is 0 (= not displayed)
・ Continuously translucent as the value of α band increases from 0 (opacity increases gradually)
-When the value of the α band is 255, rendering is opaque.

  FIG. 2 shows an outline of the flow of processing in the system of FIG. Reference numeral 201 denotes an image of n bands before compression (for example, three bands if expressed in RGB. If a band of a frequency band such as infrared rays is assigned, the number of bands may be further increased). . This uncompressed image 201 has an invalid data area and a valid data area. The α band assigning processing unit 121 in FIG. 1 first assigns a band 221 (α band) having a pixel value of 255 in the valid data area 222 and 0 in the invalid data area 223 to the uncompressed image 201. The created image 202 is created. The image compression processing unit 122 in FIG. 1 performs irreversible compression on the image 202. Due to lossy compression, compression noise 233 is mixed in the α band 231 of the compressed image 203.

  The image display processing unit 123 in FIG. 1 displays the compressed image 203. At the time of this display, the pixel value of the α band 231 assigned to the compressed image 203 is used as the opacity. As a result, as shown in the display example 204, an image can be displayed so that compression noise is hardly noticeable. In the display example 204, 241 indicates the entire area of the image, 242 indicates the valid data area of the image, 243 indicates the invalid data area that is not displayed, and 244 indicates the background.

  The principle that compression noise can be displayed inconspicuously by using the above-described α band as opacity will be described. FIG. 3 is a diagram for explaining the influence of compression noise. The horizontal axis indicates the pixel position, and the vertical axis indicates the pixel value of the α band of the pixel. 301 indicates the pixel value of the α band at the stage of the uncompressed image 202. At this stage, since the pixel value of the α band takes either 0 or 255, the invalid data area and the valid data area can be clearly separated. Reference numeral 302 denotes a pixel value of the α band at the stage where the non-compressed image 202 is irreversibly compressed into a compressed image 203. At this stage, compression noise 321 is mixed, but this does not stand out. This is because a continuous value of opacity is used instead of a binary determination of whether or not the pixel is a transparent pixel. Of course, the alpha band that stores the opacity value is also affected by the compression noise and becomes a value other than 0 or 255 as shown in 321. It will be inconspicuous.

  FIG. 4 shows a flow of pixel drawing processing by the image display processing unit 123 of FIG. First, the pixel value P0 of the currently drawn background image is obtained for the pixel position to be drawn (step 401), the pixel value P1 of the image to be drawn thereon is obtained (step 402), and the α band is further obtained. The value α is acquired (step 403). Next, a value Q = (((255−α) P0 + αP1) / 255 is calculated by weighting P0 with 255-α and P1 with α (step 404). The calculated value is set as the pixel value at the target position (step 405).

  As the weighted average calculation in step 404, in addition to the method of calculating for each band, a method of calculating by converting to the HSB space may be applied. Here, even if noise δ is mixed into the α band value 0 of the transmissive pixel and the α band value becomes δ, Q = ((255−δ) P0 + δP1) / 255≈P0 and almost transparent Displayed as status. Conversely, even if noise δ is mixed in the α band value 255 of the non-transparent pixel and the α band value becomes 255−δ, Q = (δP0 + (255−δ) P1) / 255≈P1, It is displayed as a substantially non-transparent state. Therefore, compression noise does not stand out.

  The present invention is suitable for application when displaying satellite images and aerial photographs on a map. In ordinary snapshots, it is rare that an invalid data area is included in a part of the photo. However, in the case of satellite images and aerial photos, if the geometric correction to the map coordinates is performed, the difference between the sensor coordinates and the map coordinates at the time of shooting is lost. As a result, an invalid data area is generated. FIG. 5 shows such an example. The captured image 501 is expressed in sensor coordinates, and this is geometrically corrected to map coordinates. The geometrically corrected image 502 has a valid data area 521 and an invalid data area 522. In this case, the image data format of the irreversible compression method can be obtained by the method of the present invention so that the compression noise is not noticeable.

The figure which shows one example of the system which implement | achieves the method which concerns on this invention The figure which shows the outline | summary of the flow of a process in the system of this embodiment. Diagram for explaining the effect of compression noise The figure which shows the flow of the drawing process of the pixel by an image display process part Diagram showing invalid data area after geometric correction in satellite image and aerial photograph The figure which shows the influence of the compression noise with respect to the invalid data area in the conventional method The figure which shows the influence of compression noise in the conventional method

Explanation of symbols

  DESCRIPTION OF SYMBOLS 101 ... Memory | storage device, 102 ... Central processing unit, 103 ... Display apparatus, 121 ... (alpha) band provision process part, 122 ... Image compression process part, 123 ... Image display process part, 201 ... Original image, 202 ... Alpha band provision image, 203 ... Image after irreversible compression, 221 ... α band, 231 ... α band after irreversible compression, 204 ... Image display image.

Claims (1)

An invalid data area display suppression method in an irreversible compressed image when a computer is used to irreversibly compress an image having an effective data area and an invalid data area and display the irreversibly compressed image on a background image ,
The α band addition processing means provided in the computer sets the α band pixel value of the effective data area to 255 ( opaque ) and the α band pixel value of the invalid data area to 0 ( for an image having an effective data area and an invalid data area. Providing 256- band alpha band information to be transparent ) ;
Image compression processing means provided in the computer, irreversibly compresses the image to which the band information is added;
Image display processing means provided in the computer obtains an α band pixel value α of the image after irreversible compression at each corresponding position between the image after irreversible compression and a background image on which the image after irreversible compression is superimposed and displayed. The pixel value of the corresponding position of the background image is weighted by 255-α, the pixel value of the corresponding position of the image after the lossy compression is weighted by α, and an average value is obtained as the pixel of the corresponding position. A method for suppressing invalid data area display in a lossy compressed image , comprising: displaying as a value .

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