• Nigel Tao's avatar
    image/color: tweak the YCbCr to RGBA conversion formula. · c2023a07
    Nigel Tao authored
    Before, calling the RGBA method of YCbCr color would return red values
    in the range [0x0080, 0xff80]. After, the range is [0x0000, 0xffff] and
    is consistent with what Gray colors' RGBA method returns. In particular,
    pure black, pure white and every Gray color in between are now exactly
    representable as a YCbCr color.
    
    This fixes a regression from Go 1.4 (where YCbCr{0x00, 0x80, 0x80} was
    no longer equivalent to pure black), introduced by golang.org/cl/8073 in
    the Go 1.5 development cycle. In Go 1.4, the +0x80 rounding was not
    noticable when Cb == 0x80 && Cr == 0x80, because the YCbCr to RGBA
    conversion truncated to 8 bits before multiplying by 0x101, so the
    output range was [0x0000, 0xffff].
    
    The TestYCbCrRoundtrip fuzzy-match tolerance grows from 1 to 2 because
    the YCbCr to RGB conversion now maps to an ever-so-slightly larger
    range, along with the usual imprecision of accumulating rounding errors.
    
    Also s/int/int32/ in ycbcr.go. The conversion shouldn't overflow either
    way, as int is always at least 32 bits, but it does make it clearer that
    the computation doesn't depend on sizeof(int).
    
    Fixes #11691
    
    Change-Id: I538ca0adf7e040fa96c5bc8b3aef4454535126b9
    Reviewed-on: https://go-review.googlesource.com/12220Reviewed-by: 's avatarRob Pike <r@golang.org>
    c2023a07
gen.go 3.78 KB