Encoding, decoding, and representing high dynamic range images
Techniques are provided to encode and decode image data comprising a tone mapped (TM) image with HDR reconstruction data in the form of luminance ratios and color residual values. In an example embodiment, luminance ratio values and residual values in color channels of a color space are generated on an individual pixel basis based on a high dynamic range (HDR) image and a derivative tone-mapped (TM) image that comprises one or more color alterations that would not be recoverable from the TM image with a luminance ratio image. The TM image with HDR reconstruction data derived from the luminance ratio values and the color-channel residual values may be outputted in an image file to a downstream device, for example, for decoding, rendering, and/or storing. The image file may be decoded to generate a restored HDR image free of the color alterations.
1. A method to decode a high dynamic range (HDR) image with a processor, the method comprising:
parsing an image file to generate a base image and HDR reconstruction data, wherein the HDR reconstruction data comprises a quantized luma ratio image and one or more sets of quantized residual chroma values;
extracting quantization parameters from the HDR reconstruction data;
generating a dequantized luma ratio image and one or more sets of dequantized residual chroma values based on the quantization parameters, the quantized luma ratio image, and the one or more sets of quantized residual chroma values;
linearizing the dequantized luma ratio image to generate a linearized luma ratio image; and
generating an output HDR image based on the one or more sets of dequantized residual chroma values, the linearized luma ratio image, and the base image.
2. The method of claim 1 , wherein generating the output HDR image further comprises:
generating an intermediate image by adding the one or more sets of dequantized residual chroma values to the base image; and
generating the output HDR image by multiplying the intermediate image with the linearized luma ratio image.
3. The method of claim 1 , wherein the base image is a tone-mapped image.
4. The method of claim 1 , wherein the base image in the image file comprises a JPEG image and the HDR reconstruction data is separated by the base image using at least one JPEG application segment marker.
5. The method of claim 1 , wherein the quantized luma ratio image comprises quantized log ratio pixel values.
6. The method of claim 1 , wherein the one or more sets of quantized residual chroma values comprise normalized and quantized residual chroma pixel values.
7. The method of claim 4 , wherein the base image is decoded using JPEG decoding.
8. The method of claim 5 , wherein the quantization parameters related to the quantized log ratio pixel values comprise a minimum log ratio pixel value (lr min ) and a maximum log ratio pixel value (lr max ), and given a quantized log ratio pixel value (H), a corresponding unquantized log ratio luma value lr is determined as
lr
=
H
(
lr
max
-
lr
min
255
)
+
lr
min
.
9. The method of claim 6 , wherein a first set of quantization parameters related to quantized residual chroma pixel values comprise a minimum residual chroma pixel value (Cb min ) and a maximum residual chroma pixel value (Cb max ), and given a quantized residual chroma pixel value (U), a corresponding intermediate residual chroma pixel value Cb is determined as
Cb
=
U
(
Cb
max
-
Cb
min
255
)
+
Cb
min
.
10. The method of claim 1 , wherein linearizing the dequantized luma ratio image comprises applying an inverse logarithm function to pixels of the dequantized luma ratio image.
11. A decoder for decoding a high dynamic range image (HDR), the decoder comprising:
an input to receive an image file;
a processor configured to execute instructions to:
parse the image file to generate a base image and HDR reconstruction data, wherein the HDR reconstruction data comprises a quantized luma ratio image and one or more sets of quantized residual chroma values;
extract quantization parameters from the HDR reconstruction data;
generate a dequantized luma ratio image and one or more sets of dequantized residual chroma values based on the quantization parameters, the quantized luma ratio image, and the one or more sets of quantized residual chroma values;
linearize the dequantized luma ratio image to generate a linearized luma ratio image; and
generate an output HDR image based on the one or more sets of dequantized residual chroma values, the linearized luma ratio image, and the base image; and
an output to output the generated output HDR image.
12. The decoder of claim 11 wherein to generate the output HDR image further comprises instructions to:
generate an intermediate image by adding the one or more sets of dequantized residual chroma values to the base image; and
generate the output HDR image by multiplying the intermediate image with the linearized luma ratio image.
13. A non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing a method in accordance with claim 1 .