METHODS AND APPARATUSES FOR ENCODING AN HDR IMAGES, AND METHODS AND APPARATUSES FOR USE OF SUCH ENCODED IMAGES
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Abstract
To enable a good HDR image or video coding technology, being able to yield high dynamic range images as well as low dynamic range images, we invented a method of encoding a high dynamic range image (M_HDR), comprising the steps of: converting the high dynamic range image to an image of lower luminance dynamic range (LDR_o) by applying a) scaling the high dynamic range image to a predetermined scale of the luma axis such as [0,1], b) applying a sensitivity tone mapping which changes the brightnesses of pixel colors falling within at least a subrange comprising the darker colors in the high dynamic range image, c) applying a gamma function, and d) applying an arbitrary monotonically increasing function mapping the lumas resulting from performing the steps b and c to output lumas of the lower dynamic range image (LDR_o); and outputting in an image signal (S_im) a codification of the pixel colors of the lower luminance dynamic range image (LDR_o), and outputting in the image signal (S_im) values encoding the functional behavior of the above color conversions as metadata, or values for the inverse functions, which metadata allows to reconstruct a high dynamic range image (Rec_HDR) from the lower luminance dynamic range image (LDR_o).
57 Citations
18 Claims
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2. (canceled)
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3. A method of encoding a high dynamic range image (M_HDR) as claimed in claim 1 in which the gamma function calculation uses a gamma value equal to 1/(2.4).
- 4. A method of encoding a high dynamic range image (M_HDR) as claimed in claim 1 comprising determining a gain value (gai) for mapping the maximum luma of the lower dynamic range image (LDR_o) to a specific value of the possible values in the reconstructed high dynamic range image (Rec_HDR), and encoding that gain value in the image signal (S_im).
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5. A method of encoding a high dynamic range image (M_HDR) as claimed in claim 1 comprising:
- after applying any of the above color mappings to determine the lower dynamic range image (LDR_o), applying a further technical tone mapping (301) to determine a second lower dynamic range image (LDR_i) which can be used to drive LDR displays as an alternative driving image alternative to the lower luminance dynamic range image (LDR_o), which technical tone mapping is determined by;
a) determining a first geometrical region of the lower luminance dynamic range image (LDR_o) for which the visibility of banding in the corresponding reconstructed high dynamic range image (Rec_HDR) is above an acceptable level, b) determine a range of lumas (L_u) for that region, c) determine a second range of pixel lumas (L_uu) adjacent on the luma axis to the range of lumas (L_u), wherein the second range is identified to fulfill the conditions that it has a number of lumas above a minimum number (MIN), and corresponds to a second geometrical image region which contains a texture which can be represented using less than the minimum number of codes in an LDR image (LDR_i) upon which to apply the functions yielding a reconstructed high dynamic range image (Rec_HDR) of sufficient visual quality for that second region, and d) determining a redistribution mapping function which redistributes the lumas of the first and second range of lumas, so that additional codes are available for the first range, and outputting in the image signal (S_im) values encoding the function shape of the redistribution mapping function or preferably its inverse.
- after applying any of the above color mappings to determine the lower dynamic range image (LDR_o), applying a further technical tone mapping (301) to determine a second lower dynamic range image (LDR_i) which can be used to drive LDR displays as an alternative driving image alternative to the lower luminance dynamic range image (LDR_o), which technical tone mapping is determined by;
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6. (canceled)
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8. (canceled)
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9. An image encoder arranged to encode a high dynamic range image (M_HDR), comprising:
a dynamic range conversion unit arranged to convert the high dynamic range image to an image of lower luminance dynamic range (LDR_o), the dynamic range conversion unit comprising;
a) a normalizer arranged to normalize the high dynamic range image to a luma axis ranging over [0,1] and to output normalized luminances (Yn_HDR), b) a gamma conversion unit arranged to apply a gamma function to the normalized luminances and to output gamma-converted luminances (xg), c) a first tone mapping unit arranged to apply a first tone mapping which yields lumas (v) which is defined as
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10-13. -13. (canceled)
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14. An image decoder arranged to receive a high dynamic range image signal (S_im) and comprising:
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a deformatter arranged to obtain a compressed pixellized lower dynamic range image (LDR_c) and parameter data (P) out of the image signal (S_im); and a decompressor arranged to apply at least an inverse DCT transform to the compressed pixellized lower dynamic range image (LDR_c) to obtain a pixellized lower dynamic range image (LDR_t); and
a dynamic range conversion unit (153) arranged to transform the lower dynamic range image (LDR_t) into a reconstructed high dynamic range image (Rec_HDR), wherein the dynamic range conversion unit comprises;
a) an arbitrary tone mapping unit arranged to apply an arbitrary monotonically increasing tone mapping, the parameters which define it (P_CC) being received in the parameter data (P), b) a first tone mapping unit arranged to apply a mapping as defined by a function of the form;
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15. (canceled)
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17. (canceled)
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18. A method of decoding a high dynamic range image signal (S_im) comprising:
obtaining a compressed pixellized lower dynamic range image (LDR_c) and parameter data (P) out of the image signal (S_im);
decompressing the compressed pixellized lower dynamic range image (LDR_c) by applying at least an inverse DCT transform to the compressed pixellized lower dynamic range image (LDR_c) to obtain a pixellized lower dynamic range image (LDR_t); and
transforming the lower dynamic range image (LDR_t) into a reconstructed high dynamic range image (Rec_HDR), by;
a) applying an arbitrary monotonically increasing tone mapping, the parameters which define it (P_CC) being received in the parameter data (P), b) applying a mapping as defined by at least one received parameter (RHO) the mapping being defined by a function of the form;
Specification