src/dsp/alpha_processing.c

// Copyright 2013 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Utilities for processing transparent channel.
//
// Author: Skal (pascal.massimino@gmail.com)

#include <assert.h>
#include "./dsp.h"

// Tables can be faster on some platform but incur some extra binary size (~2k).
// #define USE_TABLES_FOR_ALPHA_MULT

// -----------------------------------------------------------------------------

#define MFIX 24    // 24bit fixed-point arithmetic
#define HALF ((1u << MFIX) >> 1)
#define KINV_255 ((1u << MFIX) / 255u)

static uint32_t Mult(uint8_t x, uint32_t mult) {
  const uint32_t v = (x * mult + HALF) >> MFIX;
  assert(v <= 255);  // <- 24bit precision is enough to ensure that.
  return v;
}

#ifdef USE_TABLES_FOR_ALPHA_MULT

static const uint32_t kMultTables[2][256] = {
  {    // (255u << MFIX) / alpha
    0x00000000, 0xff000000, 0x7f800000, 0x55000000, 0x3fc00000, 0x33000000,
    0x2a800000, 0x246db6db, 0x1fe00000, 0x1c555555, 0x19800000, 0x172e8ba2,
    0x15400000, 0x139d89d8, 0x1236db6d, 0x11000000, 0x0ff00000, 0x0f000000,
    0x0e2aaaaa, 0x0d6bca1a, 0x0cc00000, 0x0c249249, 0x0b9745d1, 0x0b1642c8,
    0x0aa00000, 0x0a333333, 0x09cec4ec, 0x0971c71c, 0x091b6db6, 0x08cb08d3,
    0x08800000, 0x0839ce73, 0x07f80000, 0x07ba2e8b, 0x07800000, 0x07492492,
    0x07155555, 0x06e45306, 0x06b5e50d, 0x0689d89d, 0x06600000, 0x063831f3,
    0x06124924, 0x05ee23b8, 0x05cba2e8, 0x05aaaaaa, 0x058b2164, 0x056cefa8,
    0x05500000, 0x05343eb1, 0x05199999, 0x05000000, 0x04e76276, 0x04cfb2b7,
    0x04b8e38e, 0x04a2e8ba, 0x048db6db, 0x0479435e, 0x04658469, 0x045270d0,
    0x04400000, 0x042e29f7, 0x041ce739, 0x040c30c3, 0x03fc0000, 0x03ec4ec4,
    0x03dd1745, 0x03ce540f, 0x03c00000, 0x03b21642, 0x03a49249, 0x03976fc6,
    0x038aaaaa, 0x037e3f1f, 0x03722983, 0x03666666, 0x035af286, 0x034fcace,
    0x0344ec4e, 0x033a5440, 0x03300000, 0x0325ed09, 0x031c18f9, 0x0312818a,
    0x03092492, 0x03000000, 0x02f711dc, 0x02ee5846, 0x02e5d174, 0x02dd7baf,
    0x02d55555, 0x02cd5cd5, 0x02c590b2, 0x02bdef7b, 0x02b677d4, 0x02af286b,
    0x02a80000, 0x02a0fd5c, 0x029a1f58, 0x029364d9, 0x028ccccc, 0x0286562d,
    0x02800000, 0x0279c952, 0x0273b13b, 0x026db6db, 0x0267d95b, 0x026217ec,
    0x025c71c7, 0x0256e62a, 0x0251745d, 0x024c1bac, 0x0246db6d, 0x0241b2f9,
    0x023ca1af, 0x0237a6f4, 0x0232c234, 0x022df2df, 0x02293868, 0x02249249,
    0x02200000, 0x021b810e, 0x021714fb, 0x0212bb51, 0x020e739c, 0x020a3d70,
    0x02061861, 0x02020408, 0x01fe0000, 0x01fa0be8, 0x01f62762, 0x01f25213,
    0x01ee8ba2, 0x01ead3ba, 0x01e72a07, 0x01e38e38, 0x01e00000, 0x01dc7f10,
    0x01d90b21, 0x01d5a3e9, 0x01d24924, 0x01cefa8d, 0x01cbb7e3, 0x01c880e5,
    0x01c55555, 0x01c234f7, 0x01bf1f8f, 0x01bc14e5, 0x01b914c1, 0x01b61eed,
    0x01b33333, 0x01b05160, 0x01ad7943, 0x01aaaaaa, 0x01a7e567, 0x01a5294a,
    0x01a27627, 0x019fcbd2, 0x019d2a20, 0x019a90e7, 0x01980000, 0x01957741,
    0x0192f684, 0x01907da4, 0x018e0c7c, 0x018ba2e8, 0x018940c5, 0x0186e5f0,
    0x01849249, 0x018245ae, 0x01800000, 0x017dc11f, 0x017b88ee, 0x0179574e,
    0x01772c23, 0x01750750, 0x0172e8ba, 0x0170d045, 0x016ebdd7, 0x016cb157,
    0x016aaaaa, 0x0168a9b9, 0x0166ae6a, 0x0164b8a7, 0x0162c859, 0x0160dd67,
    0x015ef7bd, 0x015d1745, 0x015b3bea, 0x01596596, 0x01579435, 0x0155c7b4,
    0x01540000, 0x01523d03, 0x01507eae, 0x014ec4ec, 0x014d0fac, 0x014b5edc,
    0x0149b26c, 0x01480a4a, 0x01466666, 0x0144c6af, 0x01432b16, 0x0141938b,
    0x01400000, 0x013e7063, 0x013ce4a9, 0x013b5cc0, 0x0139d89d, 0x01385830,
    0x0136db6d, 0x01356246, 0x0133ecad, 0x01327a97, 0x01310bf6, 0x012fa0be,
    0x012e38e3, 0x012cd459, 0x012b7315, 0x012a150a, 0x0128ba2e, 0x01276276,
    0x01260dd6, 0x0124bc44, 0x01236db6, 0x01222222, 0x0120d97c, 0x011f93bc,
    0x011e50d7, 0x011d10c4, 0x011bd37a, 0x011a98ef, 0x0119611a, 0x01182bf2,
    0x0116f96f, 0x0115c988, 0x01149c34, 0x0113716a, 0x01124924, 0x01112358,
    0x01100000, 0x010edf12, 0x010dc087, 0x010ca458, 0x010b8a7d, 0x010a72f0,
    0x01095da8, 0x01084a9f, 0x010739ce, 0x01062b2e, 0x01051eb8, 0x01041465,
    0x01030c30, 0x01020612, 0x01010204, 0x01000000 },
  {   // alpha * KINV_255
    0x00000000, 0x00010101, 0x00020202, 0x00030303, 0x00040404, 0x00050505,
    0x00060606, 0x00070707, 0x00080808, 0x00090909, 0x000a0a0a, 0x000b0b0b,
    0x000c0c0c, 0x000d0d0d, 0x000e0e0e, 0x000f0f0f, 0x00101010, 0x00111111,
    0x00121212, 0x00131313, 0x00141414, 0x00151515, 0x00161616, 0x00171717,
    0x00181818, 0x00191919, 0x001a1a1a, 0x001b1b1b, 0x001c1c1c, 0x001d1d1d,
    0x001e1e1e, 0x001f1f1f, 0x00202020, 0x00212121, 0x00222222, 0x00232323,
    0x00242424, 0x00252525, 0x00262626, 0x00272727, 0x00282828, 0x00292929,
    0x002a2a2a, 0x002b2b2b, 0x002c2c2c, 0x002d2d2d, 0x002e2e2e, 0x002f2f2f,
    0x00303030, 0x00313131, 0x00323232, 0x00333333, 0x00343434, 0x00353535,
    0x00363636, 0x00373737, 0x00383838, 0x00393939, 0x003a3a3a, 0x003b3b3b,
    0x003c3c3c, 0x003d3d3d, 0x003e3e3e, 0x003f3f3f, 0x00404040, 0x00414141,
    0x00424242, 0x00434343, 0x00444444, 0x00454545, 0x00464646, 0x00474747,
    0x00484848, 0x00494949, 0x004a4a4a, 0x004b4b4b, 0x004c4c4c, 0x004d4d4d,
    0x004e4e4e, 0x004f4f4f, 0x00505050, 0x00515151, 0x00525252, 0x00535353,
    0x00545454, 0x00555555, 0x00565656, 0x00575757, 0x00585858, 0x00595959,
    0x005a5a5a, 0x005b5b5b, 0x005c5c5c, 0x005d5d5d, 0x005e5e5e, 0x005f5f5f,
    0x00606060, 0x00616161, 0x00626262, 0x00636363, 0x00646464, 0x00656565,
    0x00666666, 0x00676767, 0x00686868, 0x00696969, 0x006a6a6a, 0x006b6b6b,
    0x006c6c6c, 0x006d6d6d, 0x006e6e6e, 0x006f6f6f, 0x00707070, 0x00717171,
    0x00727272, 0x00737373, 0x00747474, 0x00757575, 0x00767676, 0x00777777,
    0x00787878, 0x00797979, 0x007a7a7a, 0x007b7b7b, 0x007c7c7c, 0x007d7d7d,
    0x007e7e7e, 0x007f7f7f, 0x00808080, 0x00818181, 0x00828282, 0x00838383,
    0x00848484, 0x00858585, 0x00868686, 0x00878787, 0x00888888, 0x00898989,
    0x008a8a8a, 0x008b8b8b, 0x008c8c8c, 0x008d8d8d, 0x008e8e8e, 0x008f8f8f,
    0x00909090, 0x00919191, 0x00929292, 0x00939393, 0x00949494, 0x00959595,
    0x00969696, 0x00979797, 0x00989898, 0x00999999, 0x009a9a9a, 0x009b9b9b,
    0x009c9c9c, 0x009d9d9d, 0x009e9e9e, 0x009f9f9f, 0x00a0a0a0, 0x00a1a1a1,
    0x00a2a2a2, 0x00a3a3a3, 0x00a4a4a4, 0x00a5a5a5, 0x00a6a6a6, 0x00a7a7a7,
    0x00a8a8a8, 0x00a9a9a9, 0x00aaaaaa, 0x00ababab, 0x00acacac, 0x00adadad,
    0x00aeaeae, 0x00afafaf, 0x00b0b0b0, 0x00b1b1b1, 0x00b2b2b2, 0x00b3b3b3,
    0x00b4b4b4, 0x00b5b5b5, 0x00b6b6b6, 0x00b7b7b7, 0x00b8b8b8, 0x00b9b9b9,
    0x00bababa, 0x00bbbbbb, 0x00bcbcbc, 0x00bdbdbd, 0x00bebebe, 0x00bfbfbf,
    0x00c0c0c0, 0x00c1c1c1, 0x00c2c2c2, 0x00c3c3c3, 0x00c4c4c4, 0x00c5c5c5,
    0x00c6c6c6, 0x00c7c7c7, 0x00c8c8c8, 0x00c9c9c9, 0x00cacaca, 0x00cbcbcb,
    0x00cccccc, 0x00cdcdcd, 0x00cecece, 0x00cfcfcf, 0x00d0d0d0, 0x00d1d1d1,
    0x00d2d2d2, 0x00d3d3d3, 0x00d4d4d4, 0x00d5d5d5, 0x00d6d6d6, 0x00d7d7d7,
    0x00d8d8d8, 0x00d9d9d9, 0x00dadada, 0x00dbdbdb, 0x00dcdcdc, 0x00dddddd,
    0x00dedede, 0x00dfdfdf, 0x00e0e0e0, 0x00e1e1e1, 0x00e2e2e2, 0x00e3e3e3,
    0x00e4e4e4, 0x00e5e5e5, 0x00e6e6e6, 0x00e7e7e7, 0x00e8e8e8, 0x00e9e9e9,
    0x00eaeaea, 0x00ebebeb, 0x00ececec, 0x00ededed, 0x00eeeeee, 0x00efefef,
    0x00f0f0f0, 0x00f1f1f1, 0x00f2f2f2, 0x00f3f3f3, 0x00f4f4f4, 0x00f5f5f5,
    0x00f6f6f6, 0x00f7f7f7, 0x00f8f8f8, 0x00f9f9f9, 0x00fafafa, 0x00fbfbfb,
    0x00fcfcfc, 0x00fdfdfd, 0x00fefefe, 0x00ffffff }
};

static WEBP_INLINE uint32_t GetScale(uint32_t a, int inverse) {
  return kMultTables[!inverse][a];
}

#else

static WEBP_INLINE uint32_t GetScale(uint32_t a, int inverse) {
  return inverse ? (255u << MFIX) / a : a * KINV_255;
}

#endif    // USE_TABLES_FOR_ALPHA_MULT

static void MultARGBRow(uint32_t* const ptr, int width, int inverse) {
  int x;
  for (x = 0; x < width; ++x) {
    const uint32_t argb = ptr[x];
    if (argb < 0xff000000u) {      // alpha < 255
      if (argb <= 0x00ffffffu) {   // alpha == 0
        ptr[x] = 0;
      } else {
        const uint32_t alpha = (argb >> 24) & 0xff;
        const uint32_t scale = GetScale(alpha, inverse);
        uint32_t out = argb & 0xff000000u;
        out |= Mult(argb >>  0, scale) <<  0;
        out |= Mult(argb >>  8, scale) <<  8;
        out |= Mult(argb >> 16, scale) << 16;
        ptr[x] = out;
      }
    }
  }
}

static void MultRow(uint8_t* const ptr, const uint8_t* const alpha,
                    int width, int inverse) {
  int x;
  for (x = 0; x < width; ++x) {
    const uint32_t a = alpha[x];
    if (a != 255) {
      if (a == 0) {
        ptr[x] = 0;
      } else {
        const uint32_t scale = GetScale(a, inverse);
        ptr[x] = Mult(ptr[x], scale);
      }
    }
  }
}

#undef KINV_255
#undef HALF
#undef MFIX

void (*WebPMultARGBRow)(uint32_t* const ptr, int width, int inverse);
void (*WebPMultRow)(uint8_t* const ptr, const uint8_t* const alpha,
                    int width, int inverse);

//------------------------------------------------------------------------------
// Generic per-plane calls

void WebPMultARGBRows(uint8_t* ptr, int stride, int width, int num_rows,
                      int inverse) {
  int n;
  for (n = 0; n < num_rows; ++n) {
    WebPMultARGBRow((uint32_t*)ptr, width, inverse);
    ptr += stride;
  }
}

void WebPMultRows(uint8_t* ptr, int stride,
                  const uint8_t* alpha, int alpha_stride,
                  int width, int num_rows, int inverse) {
  int n;
  for (n = 0; n < num_rows; ++n) {
    WebPMultRow(ptr, alpha, width, inverse);
    ptr += stride;
    alpha += alpha_stride;
  }
}

//------------------------------------------------------------------------------
// Premultiplied modes

// non dithered-modes

// (x * a * 32897) >> 23 is bit-wise equivalent to (int)(x * a / 255.)
// for all 8bit x or a. For bit-wise equivalence to (int)(x * a / 255. + .5),
// one can use instead: (x * a * 65793 + (1 << 23)) >> 24
#if 1     // (int)(x * a / 255.)
#define MULTIPLIER(a)   ((a) * 32897U)
#define PREMULTIPLY(x, m) (((x) * (m)) >> 23)
#else     // (int)(x * a / 255. + .5)
#define MULTIPLIER(a) ((a) * 65793U)
#define PREMULTIPLY(x, m) (((x) * (m) + (1U << 23)) >> 24)
#endif

static void ApplyAlphaMultiply(uint8_t* rgba, int alpha_first,
                               int w, int h, int stride) {
  while (h-- > 0) {
    uint8_t* const rgb = rgba + (alpha_first ? 1 : 0);
    const uint8_t* const alpha = rgba + (alpha_first ? 0 : 3);
    int i;
    for (i = 0; i < w; ++i) {
      const uint32_t a = alpha[4 * i];
      if (a != 0xff) {
        const uint32_t mult = MULTIPLIER(a);
        rgb[4 * i + 0] = PREMULTIPLY(rgb[4 * i + 0], mult);
        rgb[4 * i + 1] = PREMULTIPLY(rgb[4 * i + 1], mult);
        rgb[4 * i + 2] = PREMULTIPLY(rgb[4 * i + 2], mult);
      }
    }
    rgba += stride;
  }
}
#undef MULTIPLIER
#undef PREMULTIPLY

// rgbA4444

#define MULTIPLIER(a)  ((a) * 0x1111)    // 0x1111 ~= (1 << 16) / 15

static WEBP_INLINE uint8_t dither_hi(uint8_t x) {
  return (x & 0xf0) | (x >> 4);
}

static WEBP_INLINE uint8_t dither_lo(uint8_t x) {
  return (x & 0x0f) | (x << 4);
}

static WEBP_INLINE uint8_t multiply(uint8_t x, uint32_t m) {
  return (x * m) >> 16;
}

static WEBP_INLINE void ApplyAlphaMultiply4444(uint8_t* rgba4444,
                                               int w, int h, int stride,
                                               int rg_byte_pos /* 0 or 1 */) {
  while (h-- > 0) {
    int i;
    for (i = 0; i < w; ++i) {
      const uint32_t rg = rgba4444[2 * i + rg_byte_pos];
      const uint32_t ba = rgba4444[2 * i + (rg_byte_pos ^ 1)];
      const uint8_t a = ba & 0x0f;
      const uint32_t mult = MULTIPLIER(a);
      const uint8_t r = multiply(dither_hi(rg), mult);
      const uint8_t g = multiply(dither_lo(rg), mult);
      const uint8_t b = multiply(dither_hi(ba), mult);
      rgba4444[2 * i + rg_byte_pos] = (r & 0xf0) | ((g >> 4) & 0x0f);
      rgba4444[2 * i + (rg_byte_pos ^ 1)] = (b & 0xf0) | a;
    }
    rgba4444 += stride;
  }
}
#undef MULTIPLIER

static void ApplyAlphaMultiply_16b(uint8_t* rgba4444,
                                   int w, int h, int stride) {
#ifdef WEBP_SWAP_16BIT_CSP
  ApplyAlphaMultiply4444(rgba4444, w, h, stride, 1);
#else
  ApplyAlphaMultiply4444(rgba4444, w, h, stride, 0);
#endif
}

void (*WebPApplyAlphaMultiply)(uint8_t*, int, int, int, int);
void (*WebPApplyAlphaMultiply4444)(uint8_t*, int, int, int);

//------------------------------------------------------------------------------
// Init function

void WebPInitAlphaProcessing(void) {
  WebPMultARGBRow = MultARGBRow;
  WebPMultRow = MultRow;
  WebPApplyAlphaMultiply = ApplyAlphaMultiply;
  WebPApplyAlphaMultiply4444 = ApplyAlphaMultiply_16b;
}