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, the method comprising:
parsing an image file comprising a base image and an HDR reconstruction image, wherein the HDR reconstruction image comprises a luma ratio component image and one or more residual chroma component images; and
generating an output HDR image using the base image and the HDR reconstruction image,
wherein generating the output HDR image further comprises:
generating an intermediate HDR image by multiplying the base image with the luma ratio component image; and
generating the output HDR image by adding the one or more residual chroma component images to the intermediate HDR image.
2. The method of claim 1 , wherein the base image is a tone-mapped image.
3. The method of claim 1 , wherein the base image in the image file comprises a JPEG image and the HDR reconstruction image is separate by the base image using at least one JPEG application segment marker.
4. The method of claim 1 , wherein the luma ratio component image comprises quantized log ratio pixel values.
5. The method of claim 1 , wherein the one or more residual chroma component images comprise normalized and quantized residual chroma pixel values.
6. The method of claim 3 , wherein the base image is decoded using JPEG decoding.
7. The method of claim 4 , wherein generating the output HDR image further comprises:
receiving quantization parameters related to the quantized log ratio pixel values; and
converting based at least on the quantized parameters the quantized log ratio pixel values into unquantized ratio luma values.
8. The method of claim 5 , wherein generating the output HDR image further comprises:
receiving quantization parameters related to the quantized residual chroma pixel values;
converting, based at least on the quantized parameters, the quantized residual chroma pixel values into intermediate residual chroma pixel values; and
generating unormalized chroma pixel values by multiplying the intermediate residual chroma pixel values by corresponding luminance values of the base image.
9. The method of claim 7 , 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
.
10. The method of claim 8 , 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
.
11. A decoder for decoding a high dynamic range image (HDR), the decoder comprising:
a processor configured to execute instructions to:
parse an image file comprising a base image and an HDR reconstruction image, wherein the HDR reconstruction image comprises a luma ratio component image and one or more residual chroma component images; and
to generate an output HDR image using the base image and the HDR reconstruction image,
wherein the output HDR image is generated by generating an intermediate HDR image by multiplying the base image with the luma ratio component image; and
generating the output HDR image by adding the one or more residual chroma component images to the intermediate HDR image.