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 method for decoding a first image, the method comprising:
receiving, at a decoder, a compressed data structure representing a first image, the compressed data structure including:
a first representation of a second image, and
image data that is generated by an application of a first nonlinear function to ratios between (i) values of a parameter determined from pixel values of the first image and (ii) values of the parameter determined from corresponding pixel values in a second representation of the second image,
a dynamic range of the second image being lower than a dynamic range of the first image, and color values of the first image being scaled from corresponding original color values;
decompressing the compressed data structure to retrieve the first representation of the second image and the image data;
applying, by the decoder, a second nonlinear function to the image data to generate reconstructed image data, the second nonlinear function being an inverse function of the first nonlinear function, and the reconstructed image data including ratio data representing the ratios between (i) the values of the parameter determined from pixel values of the first image and (ii) values of the parameter determined from the corresponding pixel values in the second representation of the second image, the ratio data being adjusted to represent the original color values;
generating a representation of a reconstructed high dynamic range image by computing a pixel-wise product of the representation of the second image and the corresponding adjusted ratio data of the reconstructed image data; and
rendering the representation of the reconstructed high dynamic range image on a display device.
2. The method of claim 1 , wherein decompressing the compressed data structure comprises reading data from an MPEG file.
3. The method of claim 1 , further comprising decoding the image data from the compressed data structure.
4. The method of claim 1 , comprising applying a postcorrection function to the image data to generate corrected image data.
5. The method of claim 4 , comprising:
determining that a flag included in the compressed data structure indicates that precorrection was performed on the image data, and
responsive to determining that the flag included in the compressed data structure indicates that precorrection was performed on the image data, applying the postcorrection function to the image data to generate the corrected image data.
6. The method of claim 1 , wherein the values of the parameter determined from the second representation of the second image include luminance values of the second image.
7. The method of claim 1 , wherein the values of the parameter as determined from the second representation of the second image include chrominance values of the second image.
8. The method of claim 1 , wherein a spatial resolution of the second image is higher than a spatial resolution of the image data.
9. The method of claim 1 , wherein the ratios include ratios between the values of the parameter as determined from the second representation of the second image for individual pixels of the second image and luminance values for corresponding pixels of the first image.
10. The method of claim 1 , wherein the decompressing comprises dequantizing the compressed data structure in accordance with the MPEG video standard.
11. The method of claim 1 , wherein in the compressed data structure, the second image is included in an image portion of an image-file format, and the image data is included in an application extension portion of the image-file format.
12. The method of claim 11 , wherein the image-file format is one of: JPEG File Interchange Format (JIFF) or MPEG format.
13. The method of claim 1 , wherein the second representation of the second image is generated by first encoding a lower dynamic range version of the first image to produce an encoded image, and then decoding the encoded image.
14. The method of claim 1 , wherein the image data is encoded as a grey-scale image.
15. The method of claim 1 , wherein the image data is generated by first encoding the ratios to produce an encoded ratio image, and then decoding the encoded ratio image.
16. The method of claim 1 , wherein the first representation of the second image is identical to the second representation of the second image.