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optimize palette ordering
We compact the palette by weighted distance, favoring the green channel. Average gain on paletted file is ~0.5%, with gain up to 6-7% on some favorable cases. Encoding speed is unaffected. Disabled for alpha (or any single-channel input) Also: always use quality=20 for EncodePalette() since it doesn't make any real difference. Change-Id: I19fb14316a366f139a941b45aef5663a33c905e1
This commit is contained in:
Pascal Massimino
parent
f545feee64
commit
f82cb06afb
142
src/enc/vp8l.c
142
src/enc/vp8l.c
@ -29,8 +29,10 @@
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#define MAX_HUFF_IMAGE_SIZE (16 * 1024 * 1024)
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#define MAX_COLORS_FOR_GRAPH 64
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#define OPTIMIZE_MIN_NUM_COLORS 8
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// -----------------------------------------------------------------------------
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// Palette
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// Palette optimization
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static int CompareColors(const void* p1, const void* p2) {
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const uint32_t a = *(const uint32_t*)p1;
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@ -38,6 +40,86 @@ static int CompareColors(const void* p1, const void* p2) {
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assert(a != b);
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return (a < b) ? -1 : 1;
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}
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static WEBP_INLINE int Distance(int a, int b) {
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return abs(a - b);
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}
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static int ColorDistance(uint32_t col1, uint32_t col2) {
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int score = 0;
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// we favor grouping green channel in the palette
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score += Distance((col1 >> 0) & 0xff, (col2 >> 0) & 0xff) * 5;
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score += Distance((col1 >> 8) & 0xff, (col2 >> 8) & 0xff) * 8;
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score += Distance((col1 >> 16) & 0xff, (col2 >> 16) & 0xff) * 5;
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score += Distance((col1 >> 24) & 0xff, (col2 >> 24) & 0xff) * 1;
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return score;
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}
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static void SwapColor(uint32_t* const col1, uint32_t* const col2) {
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if (col1 != col2) {
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const uint32_t tmp = *col1;
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*col1 = *col2;
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*col2 = tmp;
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}
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}
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static int ShouldRestoreSortedPalette(int score_new, int score_orig) {
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if ((score_orig > 200) && (score_new + 100 > score_orig)) {
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return 1; // improvement not big enough
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}
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// if drop is less 20%, it's not enough
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if ((score_new + 100) > (score_orig + 100) * 80 / 100) {
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return 1;
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}
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if (score_orig > 500) { // if original palette was dispersed and...
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// improvement is not clear?
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if (score_new > 300) return 1;
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}
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return 0; // keep the new one
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}
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static void OptimizePalette(uint32_t palette[], int num_colors) {
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uint32_t palette_orig[MAX_PALETTE_SIZE];
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int score_orig = 0, score_new = 0;
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int i;
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// Compute original dispersion.
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assert(num_colors > 1 && num_colors <= MAX_PALETTE_SIZE);
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for (i = 1; i < num_colors; ++i) {
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score_orig += ColorDistance(palette[i], palette[i - 1]);
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}
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score_orig /= (num_colors - 1);
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// if score is already quite good, bail out at once.
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if (score_orig < 100) return;
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memcpy(palette_orig, palette, num_colors * sizeof(palette_orig[0]));
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// palette[0] contains the lowest ordered color already. Keep it.
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// Reorder subsequent palette colors by shortest distance to previous.
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for (i = 1; i < num_colors; ++i) {
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int j;
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int best_col = -1;
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int best_score = 0;
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const uint32_t prev_color = palette[i - 1];
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for (j = i; j < num_colors; ++j) {
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const int score = ColorDistance(palette[j], prev_color);
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if (best_col < 0 || score < best_score) {
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best_col = j;
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best_score = score;
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}
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}
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score_new += best_score;
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SwapColor(&palette[best_col], &palette[i]);
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}
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// dispersion is typically in range ~[100-1000]
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score_new /= (num_colors - 1);
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if (ShouldRestoreSortedPalette(score_new, score_orig)) {
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memcpy(palette, palette_orig, num_colors * sizeof(palette[0]));
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}
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}
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// -----------------------------------------------------------------------------
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// Palette
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// If number of colors in the image is less than or equal to MAX_PALETTE_SIZE,
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// creates a palette and returns true, else returns false.
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@ -52,6 +134,7 @@ static int AnalyzeAndCreatePalette(const WebPPicture* const pic,
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const uint32_t* argb = pic->argb;
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const int width = pic->width;
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const int height = pic->height;
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uint32_t all_color_bits;
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uint32_t last_pix = ~argb[0]; // so we're sure that last_pix != argb[0]
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for (y = 0; y < height; ++y) {
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@ -86,15 +169,24 @@ static int AnalyzeAndCreatePalette(const WebPPicture* const pic,
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// TODO(skal): could we reuse in_use[] to speed up EncodePalette()?
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num_colors = 0;
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all_color_bits = 0x00000000;
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for (i = 0; i < (int)(sizeof(in_use) / sizeof(in_use[0])); ++i) {
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if (in_use[i]) {
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palette[num_colors] = colors[i];
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all_color_bits |= colors[i];
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++num_colors;
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}
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}
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qsort(palette, num_colors, sizeof(*palette), CompareColors);
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*palette_size = num_colors;
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qsort(palette, num_colors, sizeof(*palette), CompareColors);
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// OptimizePalette() is not useful for single-channel (like alpha, e.g.).
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if (num_colors > OPTIMIZE_MIN_NUM_COLORS &&
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(all_color_bits & ~0x000000ffu) != 0 && // all red?
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(all_color_bits & ~0x0000ff00u) != 0 && // all green/alpha?
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(all_color_bits & ~0x00ff0000u) != 0) { // all blue?
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OptimizePalette(palette, num_colors);
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}
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return 1;
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}
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@ -881,6 +973,30 @@ static WebPEncodingError AllocateTransformBuffer(VP8LEncoder* const enc,
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return err;
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}
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static void MapToPalette(const uint32_t palette[], int num_colors,
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uint32_t* const last_pix, int* const last_idx,
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const uint32_t* src, uint8_t* dst, int width) {
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int x;
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int prev_idx = *last_idx;
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uint32_t prev_pix = *last_pix;
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for (x = 0; x < width; ++x) {
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const uint32_t pix = src[x];
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if (pix != prev_pix) {
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int i;
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for (i = 0; i < num_colors; ++i) {
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if (pix == palette[i]) {
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prev_idx = i;
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prev_pix = pix;
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break;
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}
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}
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}
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dst[x] = prev_idx;
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}
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*last_idx = prev_idx;
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*last_pix = prev_pix;
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}
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static void ApplyPalette(uint32_t* src, uint32_t* dst,
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uint32_t src_stride, uint32_t dst_stride,
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const uint32_t* palette, int palette_size,
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@ -914,19 +1030,8 @@ static void ApplyPalette(uint32_t* src, uint32_t* dst,
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uint32_t last_pix = palette[0];
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int last_idx = 0;
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for (y = 0; y < height; ++y) {
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for (x = 0; x < width; ++x) {
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const uint32_t pix = src[x];
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if (pix != last_pix) {
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for (i = 0; i < palette_size; ++i) {
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if (pix == palette[i]) {
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last_idx = i;
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last_pix = pix;
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break;
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}
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}
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}
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row[x] = last_idx;
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}
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MapToPalette(palette, palette_size, &last_pix, &last_idx,
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src, row, width);
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VP8LBundleColorMap(row, width, xbits, dst);
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src += src_stride;
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dst += dst_stride;
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@ -938,7 +1043,7 @@ static void ApplyPalette(uint32_t* src, uint32_t* dst,
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// Also, "enc->palette_" will be modified after this call and should not be used
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// later.
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static WebPEncodingError EncodePalette(VP8LBitWriter* const bw,
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VP8LEncoder* const enc, int quality) {
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VP8LEncoder* const enc) {
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WebPEncodingError err = VP8_ENC_OK;
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int i;
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const WebPPicture* const pic = enc->pic_;
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@ -978,8 +1083,7 @@ static WebPEncodingError EncodePalette(VP8LBitWriter* const bw,
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palette[i] = VP8LSubPixels(palette[i], palette[i - 1]);
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}
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err = EncodeImageNoHuffman(bw, palette, &enc->hash_chain_, enc->refs_,
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palette_size, 1, quality);
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palette_size, 1, 20 /* quality */);
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Error:
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WebPSafeFree(row);
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return err;
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@ -1080,7 +1184,7 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
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FinishEncParams(enc);
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if (enc->use_palette_) {
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err = EncodePalette(bw, enc, quality);
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err = EncodePalette(bw, enc);
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if (err != VP8_ENC_OK) goto Error;
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// Color cache is disabled for palette.
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enc->cache_bits_ = 0;
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