Apparatus and methods for encoding, decoding and representing high dynamic range images
A data structure defining a high dynamic range image comprises a tone map having a reduced dynamic range and HDR information. The high dynamic range image can be reconstructed from the tone map and the HDR information. The data structure can be backwards compatible with legacy hardware or software viewers. The data structure may comprise a JFIF file having the tone map encoded as a JPEG image with the HDR information in an application extension or comment field of the JFIF file, or a MPEG file having the tone map encoded as a MPEG image with the HDR information in a video or audio channel of the MPEG file. Apparatus and methods for encoding or decoding the data structure may apply pre- or post correction to compensate for lossy encoding of the high dynamic range information.
1. A decoding method comprising:
receiving a JPEG compressed file, the JPEG compressed file includes a low dynamic range (LDR) image and a transformed ratio image, the transformed ratio image separated as an application extension from the LDR image;
decompressing the compressed JPEG file to retrieve the LDR image and the transformed ratio image, wherein the decompressing comprises dequantizing a quantized transformed ratio image to generate a transformed ratio image;
applying an inverse non-linear function to the transformed ratio image to generate a decoded ratio image;
reconstructing a reconstructed HDR image, the reconstructed HDR image determined by the LDR image and the decoded ratio image; and
providing the reconstructed HDR image for display,
wherein the LDR image has a lower dynamic range than the reconstructed HDR image, and
wherein each pixel value of the decoded ratio image is from a ratio of a luminance of a pixel of an HDR image and a luminance of a corresponding pixel of the LDR image.
2. The method of claim 1 wherein the LDR image is a tone map corresponding to the HDR image.
3. The method of claim 1 wherein the ratio image is ASCII text in the JPEG compressed file.
4. The method of claim 1 further comprising rendering the reconstructed HDR image on a display.
5. The method of claim 1 wherein the JPEG compressed file further includes a flag indicative of HDR processing.
6. The method of claim 1 wherein the receiving of the JPEG compressed file is performed by decoder that includes a processor.
7. A method for decoding a high dynamic range (HDR) image, the method comprising:
receiving a compressed file, the compressed file includes a low dynamic range (LDR) image, wherein the LDR image has a lower dynamic range than the HDR image;
decompressing the compressed file to retrieve the LDR image, wherein the decompressing comprises dequantizing a quantized trasformed ratio image to generate a transformed ratio image;
receiving a transformed ratio image;
applying an inverse non-linear function to the transformed ratio image to generate a decoded ratio image, wherein each pixel value of the decoded ratio image is a ratio of a luminance of a pixel of the HDR image and a luminance of a corresponding pixel of the LDR image;
reconstructing a reconstructed HDR image, the reconstructed HDR image determined by the LDR image and the decoded ratio image, and
providing the reconstructed HDR image for display.
8. The method of claim 7 wherein the LDR image is a tone map corresponding to the HDR image.
9. The method of claim 7 wherein the ratio image is ASCII text in the compressed file.
10. The method of claim 7 further comprising rendering the reconstructed HDR image on a display.
11. The method of claim 7 wherein the compressed file further includes a flag indicative of HDR processing.
12. The method of claim 7 wherein the receiving of the compressed file is performed by decoder that includes a processor.
13. A non-transitory computer-readable storage medium encoded with instructions for causing a processor to perform the method of claim 7 .
14. The method of claim 1 , wherein the inverse function is an inverse logarithm function or an inverse square root function.
15. The method of claim 7 , wherein the inverse function is an inverse logarithm function or an inverse square root function.
16. The method of claim 7 , further comprising the steps of:
applying a post-correction to the decoded ratio image prior to reconstructing the reconstructed HDR image, wherein the post-correction comprises calculating a corrected ratio image based on a function of the decoded ratio image, a set of luminance values of tone map data generated from the HDR image, and a set of downsampled luminance values of tone map data generated from the HDR image.
17. The method of claim 16 , wherein calculating the corrected ratio image comprises calculating:
RI
CORRECTED
=
RI
×
(
L
(
TM
)
L
(
TM
R
)
)
σ
or a mathematical equivalent thereof, where RI CORRECTED is the corrected ratio image, RI is the decoded ratio image, L(TM) is the set of luminance values of tone map data generated from the HDR image, L(TM R ) is the set of downsampled luminance values of tone map data generated from the HDR image, and σ is a measure of variance in the ratio of L(TM) to L(TM R ).
18. The method of claim 1 , wherein reconstructing the reconstructed HDR image further comprises multiplying, for each pixel of the LDR image, a luminance of a pixel of the LDR image by a luminance of a corresponding pixel of the decoded ratio image to reconstruct the reconstructed HDR image.