Exposure determining device and exposure determining method
There is provided an exposure determining device that determines whether exposure of an image represented by image data is appropriate. The exposure determining device includes a first evaluation value deriving unit that derives a maximum luminance reference value for each of blocks obtained by dividing the image into a plurality of blocks of the image data on the basis of luminance values of pixels constituting each block, and derives a first evaluation value for the image on the basis of the derived maximum luminance reference values for the blocks, and an exposure determining unit that determines whether exposure of the image is appropriate, on the basis of the derived first evaluation value.
1. An exposure determining device that determines whether exposure of an image represented by image data is appropriate, the exposure determining device comprising:
a first evaluation value deriving unit that divides the image into a plurality of blocks of the image data, derives a maximum luminance reference value for each block on the basis of luminance values of pixels constituting each block, and derives a first evaluation value for the image on the basis of the derived maximum luminance reference values for the blocks;
a second evaluation value deriving unit that derives a characteristic value of the image as a second evaluation value for the image on the basis of the image data; and
an exposure determining unit that determines whether exposure of the image is appropriate, on the basis of the derived first evaluation value and the derived second evaluation value.
2. The exposure determining device according to claim 1 ,
wherein the first evaluation value for the image is an average value of the maximum luminance reference values for the individual blocks, and
the exposure determining unit determines that exposure of the image is appropriate, when the first evaluation value exceeds a first threshold value.
3. The exposure determining device according to claim 1 ,
wherein the exposure determining unit determines whether exposure of the image is appropriate, on the basis of the derived second evaluation values as a first determining step, and determines whether exposure of the image is appropriate, on the basis of the derived first evaluation value, as a second determining step, when it is determined in the first determining step that exposure of the image is not appropriate, and
the first evaluation value deriving unit performs derivation of the first evaluation value on the basis of the image data, when it is determined by the exposure determining unit in the first determining step that exposure of the image is not appropriate.
4. The exposure determining device according to claim 1 ,
wherein the first evaluation value for the image is an average value of the maximum luminance reference values for the individual blocks,
the characteristic value of the image is an average value of luminance values of pixels that constitute the image, and
the exposure determining unit determines that exposure of the image is appropriate when it is determined in the first determining step that the second evaluation value exceeds a second threshold value, determines that exposure of the image is not appropriate when it is determined in the first determining step that the second evaluation does not exceed the second threshold value, and determines that exposure of the image is appropriate when it is determined in the second determining step that the first evaluation value exceeds a first threshold value.
5. The exposure determining device according to claim 1 ,
wherein the exposure determining unit determines whether exposure of the image is appropriate, on the basis of a threshold value function formula using first and second evaluation values as variables.
6. The exposure determining device according to claim 5 ,
wherein the first evaluation value for the image is an average value of the maximum luminance reference values for the individual blocks,
the characteristic value of the image is an average value of luminance values of pixels that constitute the image,
when the average value of the maximum luminance reference values for the individual blocks is defined as AvRmax, the average value of the luminance values of pixels is defined as Av, and constant numbers are defined as a and b (a and b being positive numbers), the threshold value function formula is represented by Equation AvRmax>−a·Av+b, and
when the threshold value function formula is satisfied, it is determined that exposure of the image is appropriate.
7. The exposure determining device according to claim 2 ,
wherein the average value of the maximum luminance reference values for the individual blocks is an average value of values that are obtained by weighting the maximum luminance reference values for the individual blocks.
8. The exposure determining device according to claim 7 ,
wherein when the image is an image that has been captured using predetermined photometry, weighting values for the individual blocks are set according to the predetermined photometry.
9. The exposure determining device according to claim 4 ,
wherein when the image is a JPEG image that is an image compressed in a JPEG format, the average value of the luminance values of pixels is calculated as an average value of discrete cosine components of a discrete cosine transform that is obtained for each block of 8×8 pixels constituting the JPEG image.
10. The exposure determining device according to claim 1 ,
wherein the maximum luminance reference value for each block is a maximum luminance value from among the luminance values of pixels that constitute the block.
11. The exposure determining device according to claim 1 ,
wherein when a thumbnail image for the image exists, it is determined whether exposure of the thumbnail image is appropriate, by using the thumbnail image instead of the image, and
when it is determined that exposure of the thumbnail image is appropriate, it is also determined that exposure of the image is appropriate.
12. An exposure determining method that determines whether exposure of an image represented by image data is appropriate, the exposure determining method comprising:
dividing the image into a plurality of blocks of the image data;
deriving a maximum luminance reference value for each block on the basis of luminance values of pixels constituting each block, and deriving a first evaluation value for the image on the basis of the derived maximum luminance reference values for the blocks;
deriving a characteristic value of the image as a second evaluation value for the image on the basis of the image data; and
determining whether exposure of the image is appropriate, on the basis of the derived first evaluation value and the derived second evaluation value.
13. The exposure determining method according to claim 12 ,
wherein the maximum luminance reference value for each block is a maximum luminance value from among luminance values of pixels that constitute each block.
14. A non-transitory computer-readable storage medium with a program stored thereon, wherein the program instructs a computer to determine whether exposure of an image represented by image data is appropriate by:
dividing the image into a plurality of blocks of the image data;
deriving a maximum luminance reference value for each block on the basis of luminance values of pixels constituting each block, and deriving a first evaluation value for the image on the basis of the derived maximum luminance reference values for the blocks;
deriving a characteristic value of the image as a second evaluation value for the image on the basis of the image data; and
determining whether exposure of the image is appropriate, on the basis of the derived first evaluation value and the derived second evaluation value.
15. The storage medium according to claim 14 ,
wherein the maximum luminance reference value for each block is a maximum luminance value from among luminance values of pixels that constitute each block.