yuv.h: update RGB<->YUV coefficients in comment

The values for the R/G/B floating point formulas resembled
https://fourcc.org/fccyvrgb.php and Video Demystified, but the fixed
point values are more closely aligned to rounded values from
https://en.wikipedia.org/wiki/YCbCr and BT.601.

The R/G/B formulas with the values prior to this change are added to
sharpyuv_csp.c as they align with the fixed values. The origin of those
coefficients is unclear. For consistency between library versions we'll
leave them as is.

Bug: webp:375011696
Change-Id: Id3f2a57530eee700cc52a899b32b25b5c015e89b

sharpyuv/sharpyuv_csp.c

@@ -60,6 +60,10 @@ void SharpYuvComputeConversionMatrix(const SharpYuvColorSpace* yuv_color_space,
 
 // Matrices are in YUV_FIX fixed point precision.
 // WebP's matrix, similar but not identical to kRec601LimitedMatrix
+// Derived using the following formulas:
+// Y = 0.2569 * R + 0.5044 * G + 0.0979 * B + 16
+// U = -0.1483 * R - 0.2911 * G + 0.4394 * B + 128
+// V = 0.4394 * R - 0.3679 * G - 0.0715 * B + 128
 static const SharpYuvConversionMatrix kWebpMatrix = {
   {16839, 33059, 6420, 16 << 16},
   {-9719, -19081, 28800, 128 << 16},

src/dsp/yuv.h

@@ -11,15 +11,15 @@
 //
 // The exact naming is Y'CbCr, following the ITU-R BT.601 standard.
 // More information at: https://en.wikipedia.org/wiki/YCbCr
-// Y = 0.2569 * R + 0.5044 * G + 0.0979 * B + 16
-// U = -0.1483 * R - 0.2911 * G + 0.4394 * B + 128
-// V = 0.4394 * R - 0.3679 * G - 0.0715 * B + 128
+// Y = 0.2568 * R + 0.5041 * G + 0.0979 * B + 16
+// U = -0.1482 * R - 0.2910 * G + 0.4392 * B + 128
+// V = 0.4392 * R - 0.3678 * G - 0.0714 * B + 128
 // We use 16bit fixed point operations for RGB->YUV conversion (YUV_FIX).
 //
 // For the Y'CbCr to RGB conversion, the BT.601 specification reads:
 //   R = 1.164 * (Y-16) + 1.596 * (V-128)
-//   G = 1.164 * (Y-16) - 0.813 * (V-128) - 0.391 * (U-128)
-//   B = 1.164 * (Y-16)                   + 2.018 * (U-128)
+//   G = 1.164 * (Y-16) - 0.813 * (V-128) - 0.392 * (U-128)
+//   B = 1.164 * (Y-16)                   + 2.017 * (U-128)
 // where Y is in the [16,235] range, and U/V in the [16,240] range.
 //
 // The fixed-point implementation used here is: