fix rescaling-with-alpha inaccuracy

(still missing YUVA decoding case for now)

https://code.google.com/p/webp/issues/detail?id=160

Change-Id: If723b4a5c0a303d0853ec9d839f995adce056095

Android.mk

@@ -39,6 +39,7 @@ LOCAL_SRC_FILES := \
     src/enc/tree.c \
     src/enc/vp8l.c \
     src/enc/webpenc.c \
+    src/utils/alpha_processing.c \
     src/utils/bit_reader.c \
     src/utils/bit_writer.c \
     src/utils/color_cache.c \

Makefile.vc

@@ -217,6 +217,7 @@ MUX_OBJS = \
     $(DIROBJ)\mux\muxread.obj \
 
 UTILS_DEC_OBJS = \
+    $(DIROBJ)\utils\alpha_processing.obj \
     $(DIROBJ)\utils\bit_reader.obj \
     $(DIROBJ)\utils\color_cache.obj \
     $(DIROBJ)\utils\filters.obj \

makefile.unix

@@ -147,6 +147,7 @@ MUX_OBJS = \
     src/mux/muxread.o \
 
 UTILS_DEC_OBJS = \
+    src/utils/alpha_processing.o \
     src/utils/bit_reader.o \
     src/utils/color_cache.o \
     src/utils/filters.o \
@@ -186,6 +187,7 @@ HDRS = \
     src/dsp/yuv.h \
     src/enc/cost.h \
     src/enc/vp8enci.h \
+    src/utils/alpha_processing.h \
     src/utils/bit_reader.h \
     src/utils/bit_writer.h \
     src/utils/color_cache.h \

src/dec/vp8l.c

@@ -18,6 +18,7 @@
 #include "./vp8li.h"
 #include "../dsp/lossless.h"
 #include "../dsp/yuv.h"
+#include "../utils/alpha_processing.h"
 #include "../utils/huffman.h"
 #include "../utils/utils.h"
 
@@ -417,12 +418,13 @@ static int AllocateAndInitRescaler(VP8LDecoder* const dec, VP8Io* const io) {
 // We have special "export" function since we need to convert from BGRA
 static int Export(WebPRescaler* const rescaler, WEBP_CSP_MODE colorspace,
                   int rgba_stride, uint8_t* const rgba) {
-  const uint32_t* const src = (const uint32_t*)rescaler->dst;
+  uint32_t* const src = (uint32_t*)rescaler->dst;
   const int dst_width = rescaler->dst_width;
   int num_lines_out = 0;
   while (WebPRescalerHasPendingOutput(rescaler)) {
     uint8_t* const dst = rgba + num_lines_out * rgba_stride;
     WebPRescalerExportRow(rescaler);
+    WebPMultARGBRow(src, dst_width, 1);
     VP8LConvertFromBGRA(src, dst_width, colorspace, dst);
     ++num_lines_out;
   }
@@ -430,18 +432,22 @@ static int Export(WebPRescaler* const rescaler, WEBP_CSP_MODE colorspace,
 }
 
 // Emit scaled rows.
-static int EmitRescaledRows(const VP8LDecoder* const dec,
+static int EmitRescaledRowsRGBA(const VP8LDecoder* const dec,
-                            const uint32_t* const data, int in_stride, int mb_h,
+                                uint8_t* in, int in_stride, int mb_h,
                             uint8_t* const out, int out_stride) {
   const WEBP_CSP_MODE colorspace = dec->output_->colorspace;
-  const uint8_t* const in = (const uint8_t*)data;
   int num_lines_in = 0;
   int num_lines_out = 0;
   while (num_lines_in < mb_h) {
-    const uint8_t* const row_in = in + num_lines_in * in_stride;
+    uint8_t* const row_in = in + num_lines_in * in_stride;
     uint8_t* const row_out = out + num_lines_out * out_stride;
-    num_lines_in += WebPRescalerImport(dec->rescaler, mb_h - num_lines_in,
+    const int lines_left = mb_h - num_lines_in;
-                                       row_in, in_stride);
+    const int needed_lines = WebPRescaleNeededLines(dec->rescaler, lines_left);
+    assert(needed_lines > 0 && needed_lines <= lines_left);
+    WebPMultARGBRows(row_in, in_stride,
+                     dec->rescaler->src_width, needed_lines, 0);
+    WebPRescalerImport(dec->rescaler, lines_left, row_in, in_stride);
+    num_lines_in += needed_lines;
     num_lines_out += Export(dec->rescaler, colorspace, out_stride, row_out);
   }
   return num_lines_out;
@@ -449,11 +455,10 @@ static int EmitRescaledRows(const VP8LDecoder* const dec,
 
 // Emit rows without any scaling.
 static int EmitRows(WEBP_CSP_MODE colorspace,
-                    const uint32_t* const data, int in_stride,
+                    const uint8_t* row_in, int in_stride,
                     int mb_w, int mb_h,
                     uint8_t* const out, int out_stride) {
   int lines = mb_h;
-  const uint8_t* row_in = (const uint8_t*)data;
   uint8_t* row_out = out;
   while (lines-- > 0) {
     VP8LConvertFromBGRA((const uint32_t*)row_in, mb_w, colorspace, row_out);
@@ -530,11 +535,12 @@ static void ConvertToYUVA(const uint32_t* const src, int width, int y_pos,
 
 static int ExportYUVA(const VP8LDecoder* const dec, int y_pos) {
   WebPRescaler* const rescaler = dec->rescaler;
-  const uint32_t* const src = (const uint32_t*)rescaler->dst;
+  uint32_t* const src = (uint32_t*)rescaler->dst;
   const int dst_width = rescaler->dst_width;
   int num_lines_out = 0;
   while (WebPRescalerHasPendingOutput(rescaler)) {
     WebPRescalerExportRow(rescaler);
+    WebPMultARGBRow(src, dst_width, 1);
     ConvertToYUVA(src, dst_width, y_pos, dec->output_);
     ++y_pos;
     ++num_lines_out;
@@ -543,28 +549,28 @@ static int ExportYUVA(const VP8LDecoder* const dec, int y_pos) {
 }
 
 static int EmitRescaledRowsYUVA(const VP8LDecoder* const dec,
-                                const uint32_t* const data,
+                                uint8_t* in, int in_stride, int mb_h) {
-                                int in_stride, int mb_h) {
-  const uint8_t* const in = (const uint8_t*)data;
   int num_lines_in = 0;
   int y_pos = dec->last_out_row_;
   while (num_lines_in < mb_h) {
-    const uint8_t* const row_in = in + num_lines_in * in_stride;
+    const int lines_left = mb_h - num_lines_in;
-    num_lines_in += WebPRescalerImport(dec->rescaler, mb_h - num_lines_in,
+    const int needed_lines = WebPRescaleNeededLines(dec->rescaler, lines_left);
-                                       row_in, in_stride);
+    WebPMultARGBRows(in, in_stride, dec->rescaler->src_width, needed_lines, 0);
+    WebPRescalerImport(dec->rescaler, lines_left, in, in_stride);
+    num_lines_in += needed_lines;
+    in += needed_lines * in_stride;
     y_pos += ExportYUVA(dec, y_pos);
   }
   return y_pos;
 }
 
 static int EmitRowsYUVA(const VP8LDecoder* const dec,
-                        const uint32_t* const data, int in_stride,
+                        const uint8_t* in, int in_stride,
                         int mb_w, int num_rows) {
   int y_pos = dec->last_out_row_;
-  const uint8_t* row_in = (const uint8_t*)data;
   while (num_rows-- > 0) {
-    ConvertToYUVA((const uint32_t*)row_in, mb_w, y_pos, dec->output_);
+    ConvertToYUVA((const uint32_t*)in, mb_w, y_pos, dec->output_);
-    row_in += in_stride;
+    in += in_stride;
     ++y_pos;
   }
   return y_pos;
@@ -575,11 +581,11 @@ static int EmitRowsYUVA(const VP8LDecoder* const dec,
 
 // Sets io->mb_y, io->mb_h & io->mb_w according to start row, end row and
 // crop options. Also updates the input data pointer, so that it points to the
-// start of the cropped window.
+// start of the cropped window. Note that pixels are in ARGB format even if
-// Note that 'pixel_stride' is in units of 'uint32_t' (and not 'bytes).
+// 'in_data' is uint8_t*.
 // Returns true if the crop window is not empty.
 static int SetCropWindow(VP8Io* const io, int y_start, int y_end,
-                         const uint32_t** const in_data, int pixel_stride) {
+                         uint8_t** const in_data, int pixel_stride) {
   assert(y_start < y_end);
   assert(io->crop_left < io->crop_right);
   if (y_end > io->crop_bottom) {
@@ -588,11 +594,11 @@ static int SetCropWindow(VP8Io* const io, int y_start, int y_end,
   if (y_start < io->crop_top) {
     const int delta = io->crop_top - y_start;
     y_start = io->crop_top;
-    *in_data += pixel_stride * delta;
+    *in_data += delta * pixel_stride;
   }
   if (y_start >= y_end) return 0;  // Crop window is empty.
 
-  *in_data += io->crop_left;
+  *in_data += io->crop_left * sizeof(uint32_t);
 
   io->mb_y = y_start - io->crop_top;
   io->mb_w = io->crop_right - io->crop_left;
@@ -666,17 +672,17 @@ static void ProcessRows(VP8LDecoder* const dec, int row) {
   // Emit output.
   {
     VP8Io* const io = dec->io_;
-    const uint32_t* rows_data = dec->argb_cache_;
+    uint8_t* rows_data = (uint8_t*)dec->argb_cache_;
-    if (!SetCropWindow(io, dec->last_row_, row, &rows_data, io->width)) {
+    const int in_stride = io->width * sizeof(uint32_t);  // in unit of RGBA
+    if (!SetCropWindow(io, dec->last_row_, row, &rows_data, in_stride)) {
       // Nothing to output (this time).
     } else {
       const WebPDecBuffer* const output = dec->output_;
-      const int in_stride = io->width * sizeof(*rows_data);
       if (output->colorspace < MODE_YUV) {  // convert to RGBA
         const WebPRGBABuffer* const buf = &output->u.RGBA;
         uint8_t* const rgba = buf->rgba + dec->last_out_row_ * buf->stride;
         const int num_rows_out = io->use_scaling ?
-            EmitRescaledRows(dec, rows_data, in_stride, io->mb_h,
+            EmitRescaledRowsRGBA(dec, rows_data, in_stride, io->mb_h,
                              rgba, buf->stride) :
             EmitRows(output->colorspace, rows_data, in_stride,
                      io->mb_w, io->mb_h, rgba, buf->stride);

src/enc/picture.c

@@ -16,6 +16,7 @@
 #include <math.h>
 
 #include "./vp8enci.h"
+#include "../utils/alpha_processing.h"
 #include "../utils/rescaler.h"
 #include "../utils/utils.h"
 #include "../dsp/dsp.h"
@@ -399,6 +400,28 @@ static void RescalePlane(const uint8_t* src,
   }
 }
 
+static void AlphaMultiplyARGB(WebPPicture* const pic, int inverse) {
+  uint32_t* ptr = pic->argb;
+  int y;
+  for (y = 0; y < pic->height; ++y) {
+    WebPMultARGBRow(ptr, pic->width, inverse);
+    ptr += pic->argb_stride;
+  }
+}
+
+static void AlphaMultiplyY(WebPPicture* const pic, int inverse) {
+  const uint8_t* ptr_a = pic->a;
+  if (ptr_a != NULL) {
+    uint8_t* ptr_y = pic->y;
+    int y;
+    for (y = 0; y < pic->height; ++y) {
+      WebPMultRow(ptr_y, ptr_a, pic->width, inverse);
+      ptr_y += pic->y_stride;
+      ptr_a += pic->a_stride;
+    }
+  }
+}
+
 int WebPPictureRescale(WebPPicture* pic, int width, int height) {
   WebPPicture tmp;
   int prev_width, prev_height;
@@ -429,9 +452,19 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
       WebPPictureFree(&tmp);
       return 0;
     }
+    // If present, we need to rescale alpha first (for AlphaMultiplyY).
+    if (pic->a != NULL) {
+      RescalePlane(pic->a, prev_width, prev_height, pic->a_stride,
+                   tmp.a, width, height, tmp.a_stride, work, 1);
+    }
 
+    // We take transparency into account on the luma plane only. That's not
+    // totally exact blending, but still is a good approximation.
+    AlphaMultiplyY(pic, 0);
     RescalePlane(pic->y, prev_width, prev_height, pic->y_stride,
                  tmp.y, width, height, tmp.y_stride, work, 1);
+    AlphaMultiplyY(&tmp, 1);
+
     RescalePlane(pic->u,
                  HALVE(prev_width), HALVE(prev_height), pic->uv_stride,
                  tmp.u,
@@ -441,10 +474,6 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
                  tmp.v,
                  HALVE(width), HALVE(height), tmp.uv_stride, work, 1);
 
-    if (tmp.a != NULL) {
-      RescalePlane(pic->a, prev_width, prev_height, pic->a_stride,
-                   tmp.a, width, height, tmp.a_stride, work, 1);
-    }
 #ifdef WEBP_EXPERIMENTAL_FEATURES
     if (tmp.u0 != NULL) {
       const int s = IS_YUV_CSP(tmp.colorspace, WEBP_YUV422) ? 2 : 1;
@@ -462,12 +491,16 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
       WebPPictureFree(&tmp);
       return 0;
     }
-
+    // In order to correctly interpolate colors, we need to apply the alpha
+    // weighting first (black-matting), scale the RGB values, and remove
+    // the premultiplication afterward (while preserving the alpha channel).
+    AlphaMultiplyARGB(pic, 0);
     RescalePlane((const uint8_t*)pic->argb, prev_width, prev_height,
                  pic->argb_stride * 4,
                  (uint8_t*)tmp.argb, width, height,
                  tmp.argb_stride * 4,
                  work, 4);
+    AlphaMultiplyARGB(&tmp, 1);
   }
   WebPPictureFree(pic);
   free(work);

src/utils/Makefile.am

@@ -9,6 +9,8 @@ common_HEADERS = ../webp/types.h
 commondir = $(includedir)/webp
 
 COMMON_SOURCES =
+COMMON_SOURCES += alpha_processing.c
+COMMON_SOURCES += alpha_processing.h
 COMMON_SOURCES += bit_reader.c
 COMMON_SOURCES += bit_reader.h
 COMMON_SOURCES += color_cache.c

src/utils/alpha_processing.c

@@ -0,0 +1,203 @@
+// 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 "./alpha_processing.h"
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+// 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
+
+void WebPMultARGBRow(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;
+      }
+    }
+  }
+}
+
+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 WebPMultRow(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);
+      }
+    }
+  }
+}
+
+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;
+  }
+}
+
+#undef KINV_255
+#undef HALF
+#undef MFIX
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}    // extern "C"
+#endif

src/utils/alpha_processing.h

@@ -0,0 +1,46 @@
+// 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)
+
+#ifndef WEBP_UTILS_ALPHA_PROCESSING_H_
+#define WEBP_UTILS_ALPHA_PROCESSING_H_
+
+#include "../webp/types.h"
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+// Pre-Multiply operation transforms x into x * A / 255  (where x=Y,R,G or B).
+// Un-Multiply operation transforms x into x * 255 / A.
+
+// Pre-Multiply or Un-Multiply (if 'inverse' is true) argb values in a row.
+void WebPMultARGBRow(uint32_t* const ptr, int width, int inverse);
+
+// Same a WebPMultARGBRow(), but for several rows.
+void WebPMultARGBRows(uint8_t* ptr, int stride, int width, int num_rows,
+                      int inverse);
+
+// Same for a row of single values, with side alpha values.
+void WebPMultRow(uint8_t* const ptr, const uint8_t* const alpha,
+                 int width, int inverse);
+
+// Same a WebPMultRow(), but for several 'num_rows' rows.
+void WebPMultRows(uint8_t* ptr, int stride,
+                  const uint8_t* alpha, int alpha_stride,
+                  int width, int num_rows, int inverse);
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}    // extern "C"
+#endif
+
+#endif    // WEBP_UTILS_ALPHA_PROCESSING_H_

src/utils/rescaler.c

@@ -123,6 +123,11 @@ uint8_t* WebPRescalerExportRow(WebPRescaler* const wrk) {
 //------------------------------------------------------------------------------
 // all-in-one calls
 
+int WebPRescaleNeededLines(const WebPRescaler* const wrk, int max_num_lines) {
+  const int num_lines = (wrk->y_accum + wrk->y_sub - 1) / wrk->y_sub;
+  return (num_lines > max_num_lines) ? max_num_lines : num_lines;
+}
+
 int WebPRescalerImport(WebPRescaler* const wrk, int num_lines,
                        const uint8_t* src, int src_stride) {
   int total_imported = 0;

src/utils/rescaler.h

@@ -38,7 +38,8 @@ typedef struct {
 } WebPRescaler;
 
 // Initialize a rescaler given scratch area 'work' and dimensions of src & dst.
-void WebPRescalerInit(WebPRescaler* const wrk, int src_width, int src_height,
+void WebPRescalerInit(WebPRescaler* const rescaler,
+                      int src_width, int src_height,
                       uint8_t* const dst,
                       int dst_width, int dst_height, int dst_stride,
                       int num_channels,
@@ -46,6 +47,11 @@ void WebPRescalerInit(WebPRescaler* const wrk, int src_width, int src_height,
                       int y_add, int y_sub,
                       int32_t* const work);
 
+// Returns the number of input lines needed next to produce one output line,
+// considering that the maximum available input lines are 'max_num_lines'.
+int WebPRescaleNeededLines(const WebPRescaler* const rescaler,
+                           int max_num_lines);
+
 // Import a row of data and save its contribution in the rescaler.
 // 'channel' denotes the channel number to be imported.
 void WebPRescalerImportRow(WebPRescaler* const rescaler,
@@ -64,10 +70,10 @@ int WebPRescalerHasPendingOutput(const WebPRescaler* const rescaler) {
 
 // Export one row from rescaler. Returns the pointer where output was written,
 // or NULL if no row was pending.
-uint8_t* WebPRescalerExportRow(WebPRescaler* const wrk);
+uint8_t* WebPRescalerExportRow(WebPRescaler* const rescaler);
 
 // Export as many rows as possible. Return the numbers of rows written.
-int WebPRescalerExport(WebPRescaler* const wrk);
+int WebPRescalerExport(WebPRescaler* const rescaler);
 
 //------------------------------------------------------------------------------