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authorLaurent Pinchart <laurent.pinchart@ideasonboard.com>2022-08-17 23:38:32 +0300
committerLaurent Pinchart <laurent.pinchart@ideasonboard.com>2022-08-25 20:12:54 +0300
commita17f0eddc61486989a375e34dcdcbb15c3152a4a (patch)
treeb8061e9a3517ca55409163df256958d1bf292252 /Documentation/contributing.rst
parentba72e4202f1958028c9b40cd858fdfb46508335b (diff)
libcamera: color_space: Rename Jpeg to Sycc
The JPEG color space is badly name, as the JPEG specification (ITU-T T.81) doesn't define any particular color space: The interchange format does not specify a complete coded image representation. Application-dependent information, e.g. colour space, is outside the scope of this Specification. The JFIF specification (ITU-T T.871) is clearer as it requires ITU-R BT.601 YCbCr encoding and a full quantization range: The interpretations of Y, CB, and CR are derived from the E'Y, E'Cb, and E'Cr signals defined in the 625-line specification of Rec. ITU-R BT.601, but these signals are normalized so as to permit the usage of the full range of 256 levels of the 8-bit binary encoding of the Y component. It however doesn't specify color primaries or a transfer function explicitly. It only mentions the latter when describing the conversion from YCbCr to RGB: The inverse relationship for computing full scale 8-bit per colour channel gamma pre-corrected RGB values (following Rec. ITU-R BT.601 gamma pre-correction and colour primary specifications) from YCbCr colours (with 256 levels per component) can be computed as follows: [...] Given that ITU-R BT.601-5 (1995) didn't specify color primaries or a transfer function, and that the later ITU-R BT.601-7 (2011) version specifies color primaries for the 625-line variant that do not match sRGB, the JPEG color space in libcamera is badly named. This is confirmed by ITU-T T.871: As this Recommendation | International Standard is based on the prior informally-circulated JFIF version 1.02 specification that was produced in 1992, which referenced Rec. ITU-R BT.601 (formerly CCIR 601), it references that specification for definition of the E'Y, E'Cb, and E'Cr signals that correspond to the YCBCR values specified herein. However, since the development of the prior JFIF version 1.02 specification, additional industry specifications have been developed, Rec. ITU-R BT.601 has been updated, and common industry practice has emerged which often follows the sYCC specification in IEC 61966-2-1/Amd.1. The difference between the use of the colour interpretation specification in this Recommendation | International Standard and that of the sYCC specification may be considered negligible in practice. Rename the color space to sYCC, as its definition matches the sYCC standard, and indicate that it is typically used to encode JPEG images. Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com> Reviewed-by: Umang Jain <umang.jain@ideasonboard.com> Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
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