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lavc/dvdsubenc: improve color distance function. 

Consider the color space as an hypercone with apex alpha=0
and base alpha=1 instead of an hypercube.
Make the encoder consider very transparent colors more similar
even if the hue is very different.
This corresponds roughly to using the alpha as a weight for the
color difference.
Only 4 bits of alpha are used, because this is what dvdsub uses,
and it avoids overflows.

Fix trac ticket #2005.
This commit is contained in:
Nicolas George 2013-01-01 17:06:04 +01:00
parent 8dbbaf568e
commit 5ed5e90f2a
1 changed files with 8 additions and 3 deletions
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11
libavcodec/dvdsubenc.c
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@ -94,10 +94,14 @@ static void dvd_encode_rle(uint8_t **pq,
static int color_distance(uint32_t a, uint32_t b)
{
int r = 0, d, i;
int alpha_a = 8, alpha_b = 8;
for (i = 0; i < 32; i += 8) {
d = ((a >> i) & 0xFF) - ((b >> i) & 0xFF);
for (i = 24; i >= 0; i -= 8) {
d = alpha_a * (int)((a >> i) & 0xFF) -
alpha_b * (int)((b >> i) & 0xFF);
r += d * d;
alpha_a = a >> 28;
alpha_b = b >> 28;
}
return r;
}
@ -130,7 +134,8 @@ static void count_colors(AVCodecContext *avctx, unsigned hits[33],
if (match) {
best_d = INT_MAX;
for (j = 0; j < 16; j++) {
d = color_distance(color & 0xFFFFFF, dvdc->global_palette[j]);
d = color_distance(0xFF000000 | color,
0xFF000000 | dvdc->global_palette[j]);
if (d < best_d) {
best_d = d;
best_j = j;