Blame src/enc/near_lossless_enc.c

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// Copyright 2014 Google Inc. All Rights Reserved.
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//
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// Use of this source code is governed by a BSD-style license
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// that can be found in the COPYING file in the root of the source
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// tree. An additional intellectual property rights grant can be found
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// in the file PATENTS. All contributing project authors may
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// be found in the AUTHORS file in the root of the source tree.
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// -----------------------------------------------------------------------------
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//
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// Near-lossless image preprocessing adjusts pixel values to help
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// compressibility with a guarantee of maximum deviation between original and
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// resulting pixel values.
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//
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// Author: Jyrki Alakuijala (jyrki@google.com)
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// Converted to C by Aleksander Kramarz (akramarz@google.com)
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#include <assert.h>
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#include <stdlib.h>
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#include "src/dsp/lossless_common.h"
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#include "src/utils/utils.h"
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#include "src/enc/vp8li_enc.h"
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#if (WEBP_NEAR_LOSSLESS == 1)
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#define MIN_DIM_FOR_NEAR_LOSSLESS 64
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#define MAX_LIMIT_BITS             5
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// Quantizes the value up or down to a multiple of 1<
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// choosing the closer one, resolving ties using bankers' rounding.
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static uint32_t FindClosestDiscretized(uint32_t a, int bits) {
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  const uint32_t mask = (1u << bits) - 1;
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  const uint32_t biased = a + (mask >> 1) + ((a >> bits) & 1);
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  assert(bits > 0);
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  if (biased > 0xff) return 0xff;
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  return biased & ~mask;
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}
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// Applies FindClosestDiscretized to all channels of pixel.
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static uint32_t ClosestDiscretizedArgb(uint32_t a, int bits) {
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  return
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      (FindClosestDiscretized(a >> 24, bits) << 24) |
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      (FindClosestDiscretized((a >> 16) & 0xff, bits) << 16) |
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      (FindClosestDiscretized((a >> 8) & 0xff, bits) << 8) |
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      (FindClosestDiscretized(a & 0xff, bits));
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}
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// Checks if distance between corresponding channel values of pixels a and b
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// is within the given limit.
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static int IsNear(uint32_t a, uint32_t b, int limit) {
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  int k;
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  for (k = 0; k < 4; ++k) {
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    const int delta =
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        (int)((a >> (k * 8)) & 0xff) - (int)((b >> (k * 8)) & 0xff);
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    if (delta >= limit || delta <= -limit) {
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      return 0;
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    }
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  }
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  return 1;
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}
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static int IsSmooth(const uint32_t* const prev_row,
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                    const uint32_t* const curr_row,
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                    const uint32_t* const next_row,
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                    int ix, int limit) {
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  // Check that all pixels in 4-connected neighborhood are smooth.
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  return (IsNear(curr_row[ix], curr_row[ix - 1], limit) &&
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          IsNear(curr_row[ix], curr_row[ix + 1], limit) &&
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          IsNear(curr_row[ix], prev_row[ix], limit) &&
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          IsNear(curr_row[ix], next_row[ix], limit));
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}
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// Adjusts pixel values of image with given maximum error.
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static void NearLossless(int xsize, int ysize, const uint32_t* argb_src,
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                         int stride, int limit_bits, uint32_t* copy_buffer,
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                         uint32_t* argb_dst) {
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  int x, y;
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  const int limit = 1 << limit_bits;
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  uint32_t* prev_row = copy_buffer;
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  uint32_t* curr_row = prev_row + xsize;
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  uint32_t* next_row = curr_row + xsize;
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  memcpy(curr_row, argb_src, xsize * sizeof(argb_src[0]));
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  memcpy(next_row, argb_src + stride, xsize * sizeof(argb_src[0]));
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  for (y = 0; y < ysize; ++y, argb_src += stride, argb_dst += xsize) {
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    if (y == 0 || y == ysize - 1) {
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      memcpy(argb_dst, argb_src, xsize * sizeof(argb_src[0]));
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    } else {
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      memcpy(next_row, argb_src + stride, xsize * sizeof(argb_src[0]));
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      argb_dst[0] = argb_src[0];
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      argb_dst[xsize - 1] = argb_src[xsize - 1];
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      for (x = 1; x < xsize - 1; ++x) {
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        if (IsSmooth(prev_row, curr_row, next_row, x, limit)) {
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          argb_dst[x] = curr_row[x];
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        } else {
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          argb_dst[x] = ClosestDiscretizedArgb(curr_row[x], limit_bits);
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        }
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      }
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    }
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    {
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      // Three-way swap.
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      uint32_t* const temp = prev_row;
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      prev_row = curr_row;
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      curr_row = next_row;
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      next_row = temp;
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    }
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  }
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}
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int VP8ApplyNearLossless(const WebPPicture* const picture, int quality,
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                         uint32_t* const argb_dst) {
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  int i;
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  const int xsize = picture->width;
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  const int ysize = picture->height;
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  const int stride = picture->argb_stride;
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  uint32_t* const copy_buffer =
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      (uint32_t*)WebPSafeMalloc(xsize * 3, sizeof(*copy_buffer));
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  const int limit_bits = VP8LNearLosslessBits(quality);
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  assert(argb_dst != NULL);
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  assert(limit_bits > 0);
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  assert(limit_bits <= MAX_LIMIT_BITS);
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  if (copy_buffer == NULL) {
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    return 0;
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  }
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  // For small icon images, don't attempt to apply near-lossless compression.
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  if ((xsize < MIN_DIM_FOR_NEAR_LOSSLESS &&
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       ysize < MIN_DIM_FOR_NEAR_LOSSLESS) ||
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      ysize < 3) {
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    for (i = 0; i < ysize; ++i) {
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      memcpy(argb_dst + i * xsize, picture->argb + i * picture->argb_stride,
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             xsize * sizeof(*argb_dst));
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    }
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    WebPSafeFree(copy_buffer);
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    return 1;
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  }
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  NearLossless(xsize, ysize, picture->argb, stride, limit_bits, copy_buffer,
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               argb_dst);
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  for (i = limit_bits - 1; i != 0; --i) {
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    NearLossless(xsize, ysize, argb_dst, xsize, i, copy_buffer, argb_dst);
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  }
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  WebPSafeFree(copy_buffer);
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  return 1;
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}
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#else  // (WEBP_NEAR_LOSSLESS == 1)
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// Define a stub to suppress compiler warnings.
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extern void VP8LNearLosslessStub(void);
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void VP8LNearLosslessStub(void) {}
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#endif  // (WEBP_NEAR_LOSSLESS == 1)