Apparatuses and methods for HDR image encoding and decoding

A method of encoding a high dynamic range image, comprising:

• selecting a low dynamic range of luminances, or corresponding range of luma code values, within a total range of luminances covered by the high dynamic range image, the low dynamic range of luminances or corresponding range of luma code values being so large that image information encoded within it is usable for LDR rendering;
• selecting at least one complementary range within the total range of luminances, comprising mostly luminances not covered by the low dynamic range of luminances;
• mapping to luma codes of a first image having at least a luma component comprising N bit code words, pixel luminances of the high dynamic range image falling within the low dynamic range of luminances to luma code values according to a first mapping, and pixel luminances of the high dynamic range image falling within the at least one complementary range to luma code values according to a second mapping, wherein the first and second mappings map to disjunct subranges of a range of luma code values of the first image.

An image decoding unit arranged to obtain a high dynamic range image from a high dynamic range image encoding, comprising:

• a Low Dynamic Range (LDR) recovery unit arranged to determine a low dynamic range of luminances, or corresponding range of luma code values, within a total range of luminances covered by the high dynamic range image, the low dynamic range of luminances or corresponding range of luma code values being so large that image information encoded within it is usable for LDR rendering, and arranged to obtain a low dynamic range image from the high dynamic range image encoding;
• a High Dynamic Range (HDR) recovery unit arranged to determine at least one complementary range within the total range of luminances, comprising mostly luminances not covered by the low dynamic range of luminances, and arranged to determine at least one high dynamic range subimage from the high dynamic range image encoding corresponding to that at least one complementary range;
• an image composition unit arranged to compose the high dynamic range image from the low dynamic range image and the at least one high dynamic range subimage, whereby the low dynamic range image and the at least one high dynamic range subimage cover largely non-overlapping luminance subranges of the high dynamic range image.

A method of image decoding to obtain a high dynamic range image from a high dynamic range image encoding, comprising:

• determining a low dynamic range of luminances, or corresponding range of luma code values, within a total range of luminances covered by the high dynamic range image, the low dynamic range of luminances or corresponding range of luma code values being so large that image information encoded within it is usable for LDR rendering, and obtaining a low dynamic range image from the high dynamic range image encoding;
• determining at least one complementary range within the total range of luminances, comprising mostly luminances not covered by the low dynamic range of luminances, and determining at least one high dynamic range subimage from the high dynamic range image encoding corresponding to that at least one complementary range;
• composing the high dynamic range image from the low dynamic range image and the at least one high dynamic range subimage, whereby the low dynamic range image and the at least one high dynamic range subimage cover largely non-overlapping luminance subranges of the high dynamic range image.