view release on metacpan or  search on metacpan

webp-src/doc/webp-container-spec.txt  view on Meta::CPAN

**Notes**:

  * The frame disposal only applies to the _frame rectangle_, that is, the
    rectangle defined by _Frame X_, _Frame Y_, _frame width_ and _frame height_.
    It may or may not cover the whole canvas.

{:#alpha-blending}
  * **Alpha-blending**:

    Given that each of the R, G, B and A channels is 8-bit, and the RGB
    channels are _not premultiplied_ by alpha, the formula for blending
    'dst' onto 'src' is:

~~~~~
    blend.A = src.A + dst.A * (1 - src.A / 255)
    if blend.A = 0 then
      blend.RGB = 0
    else
      blend.RGB = (src.RGB * src.A +
                   dst.RGB * dst.A * (1 - src.A / 255)) / blend.A
~~~~~

webp-src/src/dsp/yuv.h  view on Meta::CPAN

// V = 0.4394 * R - 0.3679 * G - 0.0715 * B + 128
// We use 16bit fixed point operations for RGB->YUV conversion.
//
// 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)
// where Y is in the [16,235] range, and U/V in the [16,240] range.
// In the table-lookup version (WEBP_YUV_USE_TABLE), the common factor
// "1.164 * (Y-16)" can be handled as an offset in the VP8kClip[] table.
// So in this case the formulae should be read as:
//   R = 1.164 * [Y + 1.371 * (V-128)                  ] - 18.624
//   G = 1.164 * [Y - 0.698 * (V-128) - 0.336 * (U-128)] - 18.624
//   B = 1.164 * [Y                   + 1.733 * (U-128)] - 18.624
// once factorized. Here too, 16bit fixed precision is used.
//
// Author: Skal (pascal.massimino@gmail.com)

#ifndef WEBP_DSP_YUV_H_
#define WEBP_DSP_YUV_H_