Palette: use color cache to compress base colors
Get a list of palette base colors that are used in the above and
left blocks, referred to as "color cache". For each cache color,
signal if it is present in current block's palette, so that we
don't need to transmit their raw values.
When palette-delta-encoding is enabled, compression is improved
by 2% on keyframe and 1% overall for the screen_content testset.
Change-Id: I4cb027f1904aa9d0ab1c8f00ea9ee34bf5f16234
diff --git a/av1/encoder/rdopt.c b/av1/encoder/rdopt.c
index 6422842..61c3404 100644
--- a/av1/encoder/rdopt.c
+++ b/av1/encoder/rdopt.c
@@ -2429,6 +2429,28 @@
}
}
+#if CONFIG_PALETTE_DELTA_ENCODING
+// Bias toward using colors in the cache.
+// TODO(huisu): Try other schemes to improve compression.
+static void optimize_palette_colors(uint16_t *color_cache, int n_cache,
+ int n_colors, int stride,
+ float *centroids) {
+ if (n_cache <= 0) return;
+ for (int i = 0; i < n_colors * stride; i += stride) {
+ float min_diff = fabsf(centroids[i] - color_cache[0]);
+ int idx = 0;
+ for (int j = 1; j < n_cache; ++j) {
+ float this_diff = fabsf(centroids[i] - color_cache[j]);
+ if (this_diff < min_diff) {
+ min_diff = this_diff;
+ idx = j;
+ }
+ }
+ if (min_diff < 1.5) centroids[i] = color_cache[idx];
+ }
+}
+#endif // CONFIG_PALETTE_DELTA_ENCODING
+
static int rd_pick_palette_intra_sby(const AV1_COMP *const cpi, MACROBLOCK *x,
BLOCK_SIZE bsize, int palette_ctx,
int dc_mode_cost, MB_MODE_INFO *best_mbmi,
@@ -2515,6 +2537,14 @@
if (rows * cols > PALETTE_MAX_BLOCK_SIZE) return 0;
+#if CONFIG_PALETTE_DELTA_ENCODING
+ const MODE_INFO *above_mi = xd->above_mi;
+ const MODE_INFO *left_mi = xd->left_mi;
+ uint16_t color_cache[2 * PALETTE_MAX_SIZE];
+ const int n_cache =
+ av1_get_palette_cache(above_mi, left_mi, 0, color_cache);
+#endif // CONFIG_PALETTE_DELTA_ENCODING
+
for (n = colors > PALETTE_MAX_SIZE ? PALETTE_MAX_SIZE : colors; n >= 2;
--n) {
if (colors == PALETTE_MIN_SIZE) {
@@ -2529,6 +2559,9 @@
centroids[i] = lb + (2 * i + 1) * (ub - lb) / n / 2;
}
av1_k_means(data, centroids, color_map, rows * cols, n, 1, max_itr);
+#if CONFIG_PALETTE_DELTA_ENCODING
+ optimize_palette_colors(color_cache, n_cache, n, 1, centroids);
+#endif // CONFIG_PALETTE_DELTA_ENCODING
k = av1_remove_duplicates(centroids, n);
if (k < PALETTE_MIN_SIZE) {
// Too few unique colors to create a palette. And DC_PRED will work
@@ -2558,7 +2591,11 @@
av1_cost_bit(
av1_default_palette_y_mode_prob[bsize - BLOCK_8X8][palette_ctx],
1);
- palette_mode_cost += av1_palette_color_cost_y(pmi, cpi->common.bit_depth);
+ palette_mode_cost += av1_palette_color_cost_y(pmi,
+#if CONFIG_PALETTE_DELTA_ENCODING
+ color_cache, n_cache,
+#endif // CONFIG_PALETTE_DELTA_ENCODING
+ cpi->common.bit_depth);
for (i = 0; i < rows; ++i) {
for (j = (i == 0 ? 1 : 0); j < cols; ++j) {
int color_idx;
@@ -4517,6 +4554,13 @@
}
#endif // CONFIG_HIGHBITDEPTH
+#if CONFIG_PALETTE_DELTA_ENCODING
+ const MODE_INFO *above_mi = xd->above_mi;
+ const MODE_INFO *left_mi = xd->left_mi;
+ uint16_t color_cache[2 * PALETTE_MAX_SIZE];
+ const int n_cache = av1_get_palette_cache(above_mi, left_mi, 1, color_cache);
+#endif // CONFIG_PALETTE_DELTA_ENCODING
+
colors = colors_u > colors_v ? colors_u : colors_v;
if (colors > 1 && colors <= 64) {
int r, c, n, i, j;
@@ -4581,6 +4625,7 @@
}
av1_k_means(data, centroids, color_map, rows * cols, n, 2, max_itr);
#if CONFIG_PALETTE_DELTA_ENCODING
+ optimize_palette_colors(color_cache, n_cache, n, 2, centroids);
// Sort the U channel colors in ascending order.
for (i = 0; i < 2 * (n - 1); i += 2) {
int min_idx = i;
@@ -4620,7 +4665,11 @@
write_uniform_cost(n, color_map[0]) +
av1_cost_bit(
av1_default_palette_uv_mode_prob[pmi->palette_size[0] > 0], 1);
- this_rate += av1_palette_color_cost_uv(pmi, cpi->common.bit_depth);
+ this_rate += av1_palette_color_cost_uv(pmi,
+#if CONFIG_PALETTE_DELTA_ENCODING
+ color_cache, n_cache,
+#endif // CONFIG_PALETTE_DELTA_ENCODING
+ cpi->common.bit_depth);
for (i = 0; i < rows; ++i) {
for (j = (i == 0 ? 1 : 0); j < cols; ++j) {
int color_idx;
