static int PreprocessARGB()

in Extended/libwebp/src/enc/picture_csp_enc.c [445:568]
103 lines of code
16 McCabe index (conditional complexity)

static int PreprocessARGB(const uint8_t* r_ptr,
                          const uint8_t* g_ptr,
                          const uint8_t* b_ptr,
                          int step, int rgb_stride,
                          WebPPicture* const picture) {
  // we expand the right/bottom border if needed
  const int w = (picture->width + 1) & ~1;
  const int h = (picture->height + 1) & ~1;
  const int uv_w = w >> 1;
  const int uv_h = h >> 1;
  uint64_t prev_diff_y_sum = ~0;
  int j, iter;

  // TODO(skal): allocate one big memory chunk. But for now, it's easier
  // for valgrind debugging to have several chunks.
  fixed_y_t* const tmp_buffer = SAFE_ALLOC(w * 3, 2, fixed_y_t);   // scratch
  fixed_y_t* const best_y_base = SAFE_ALLOC(w, h, fixed_y_t);
  fixed_y_t* const target_y_base = SAFE_ALLOC(w, h, fixed_y_t);
  fixed_y_t* const best_rgb_y = SAFE_ALLOC(w, 2, fixed_y_t);
  fixed_t* const best_uv_base = SAFE_ALLOC(uv_w * 3, uv_h, fixed_t);
  fixed_t* const target_uv_base = SAFE_ALLOC(uv_w * 3, uv_h, fixed_t);
  fixed_t* const best_rgb_uv = SAFE_ALLOC(uv_w * 3, 1, fixed_t);
  fixed_y_t* best_y = best_y_base;
  fixed_y_t* target_y = target_y_base;
  fixed_t* best_uv = best_uv_base;
  fixed_t* target_uv = target_uv_base;
  const uint64_t diff_y_threshold = (uint64_t)(3.0 * w * h);
  int ok;

  if (best_y_base == NULL || best_uv_base == NULL ||
      target_y_base == NULL || target_uv_base == NULL ||
      best_rgb_y == NULL || best_rgb_uv == NULL ||
      tmp_buffer == NULL) {
    ok = WebPEncodingSetError(picture, VP8_ENC_ERROR_OUT_OF_MEMORY);
    goto End;
  }
  assert(picture->width >= kMinDimensionIterativeConversion);
  assert(picture->height >= kMinDimensionIterativeConversion);

  WebPInitConvertARGBToYUV();

  // Import RGB samples to W/RGB representation.
  for (j = 0; j < picture->height; j += 2) {
    const int is_last_row = (j == picture->height - 1);
    fixed_y_t* const src1 = tmp_buffer + 0 * w;
    fixed_y_t* const src2 = tmp_buffer + 3 * w;

    // prepare two rows of input
    ImportOneRow(r_ptr, g_ptr, b_ptr, step, picture->width, src1);
    if (!is_last_row) {
      ImportOneRow(r_ptr + rgb_stride, g_ptr + rgb_stride, b_ptr + rgb_stride,
                   step, picture->width, src2);
    } else {
      memcpy(src2, src1, 3 * w * sizeof(*src2));
    }
    StoreGray(src1, best_y + 0, w);
    StoreGray(src2, best_y + w, w);

    UpdateW(src1, target_y, w);
    UpdateW(src2, target_y + w, w);
    UpdateChroma(src1, src2, target_uv, uv_w);
    memcpy(best_uv, target_uv, 3 * uv_w * sizeof(*best_uv));
    best_y += 2 * w;
    best_uv += 3 * uv_w;
    target_y += 2 * w;
    target_uv += 3 * uv_w;
    r_ptr += 2 * rgb_stride;
    g_ptr += 2 * rgb_stride;
    b_ptr += 2 * rgb_stride;
  }

  // Iterate and resolve clipping conflicts.
  for (iter = 0; iter < kNumIterations; ++iter) {
    const fixed_t* cur_uv = best_uv_base;
    const fixed_t* prev_uv = best_uv_base;
    uint64_t diff_y_sum = 0;

    best_y = best_y_base;
    best_uv = best_uv_base;
    target_y = target_y_base;
    target_uv = target_uv_base;
    for (j = 0; j < h; j += 2) {
      fixed_y_t* const src1 = tmp_buffer + 0 * w;
      fixed_y_t* const src2 = tmp_buffer + 3 * w;
      {
        const fixed_t* const next_uv = cur_uv + ((j < h - 2) ? 3 * uv_w : 0);
        InterpolateTwoRows(best_y, prev_uv, cur_uv, next_uv, w, src1, src2);
        prev_uv = cur_uv;
        cur_uv = next_uv;
      }

      UpdateW(src1, best_rgb_y + 0 * w, w);
      UpdateW(src2, best_rgb_y + 1 * w, w);
      UpdateChroma(src1, src2, best_rgb_uv, uv_w);

      // update two rows of Y and one row of RGB
      diff_y_sum += WebPSharpYUVUpdateY(target_y, best_rgb_y, best_y, 2 * w);
      WebPSharpYUVUpdateRGB(target_uv, best_rgb_uv, best_uv, 3 * uv_w);

      best_y += 2 * w;
      best_uv += 3 * uv_w;
      target_y += 2 * w;
      target_uv += 3 * uv_w;
    }
    // test exit condition
    if (iter > 0) {
      if (diff_y_sum < diff_y_threshold) break;
      if (diff_y_sum > prev_diff_y_sum) break;
    }
    prev_diff_y_sum = diff_y_sum;
  }
  // final reconstruction
  ok = ConvertWRGBToYUV(best_y_base, best_uv_base, picture);

 End:
  WebPSafeFree(best_y_base);
  WebPSafeFree(best_uv_base);
  WebPSafeFree(target_y_base);
  WebPSafeFree(target_uv_base);
  WebPSafeFree(best_rgb_y);
  WebPSafeFree(best_rgb_uv);
  WebPSafeFree(tmp_buffer);
  return ok;
}