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Add explanation and alternative implementation for strange xan

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gamma correction.

Originally committed as revision 25779 to svn://svn.ffmpeg.org/ffmpeg/trunk
This commit is contained in:
Reimar Döffinger 2010-11-21 10:37:26 +00:00
parent 24ae353dfb
commit 3354b05187
1 changed files with 56 additions and 0 deletions
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56
libavcodec/xan.c
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@ -40,6 +40,8 @@
// for av_memcpy_backptr
#include "libavutil/lzo.h"
#define RUNTIME_GAMMA 0
#define VGA__TAG MKTAG('V', 'G', 'A', ' ')
#define PALT_TAG MKTAG('P', 'A', 'L', 'T')
#define SHOT_TAG MKTAG('S', 'H', 'O', 'T')
@ -359,6 +361,53 @@ static void xan_wc3_decode_frame(XanContext *s) {
static void xan_wc4_decode_frame(XanContext *s) {
}
#if RUNTIME_GAMMA
static inline unsigned mul(unsigned a, unsigned b)
{
return (a * b) >> 16;
}
static inline unsigned pow4(unsigned a)
{
unsigned square = mul(a, a);
return mul(square, square);
}
static inline unsigned pow5(unsigned a)
{
return mul(pow4(a), a);
}
static uint8_t gamma_corr(uint8_t in) {
unsigned lo, hi = 0xff40, target;
int i = 15;
in = (in << 2) | (in >> 6);
/* equivalent float code:
if (in >= 252)
return 253;
return round(pow(in / 256.0, 0.8) * 256);
*/
lo = target = in << 8;
do {
unsigned mid = (lo + hi) >> 1;
unsigned pow = pow5(mid);
if (pow > target) hi = mid;
else lo = mid;
} while (--i);
return (pow4((lo + hi) >> 1) + 0x80) >> 8;
}
#else
/**
* This is a gamma correction that xan3 applies to all palette entries.
*
* There is a peculiarity, namely that the values are clamped to 253 -
* it seems likely that this table was calculated by a buggy fixed-point
* implementation, the one above under RUNTIME_GAMMA behaves like this for
* example.
* The exponent value of 0.8 can be explained by this as well, since 0.8 = 4/5
* and thus pow(x, 0.8) is still easy to calculate.
* Also, the input values are first rotated to the left by 2.
*/
static const uint8_t gamma_lookup[256] = {
0x00, 0x09, 0x10, 0x16, 0x1C, 0x21, 0x27, 0x2C,
0x31, 0x35, 0x3A, 0x3F, 0x43, 0x48, 0x4C, 0x50,
@ -393,6 +442,7 @@ static const uint8_t gamma_lookup[256] = {
0xCE, 0xD1, 0xD5, 0xD8, 0xDB, 0xDF, 0xE2, 0xE5,
0xE9, 0xEC, 0xEF, 0xF2, 0xF6, 0xF9, 0xFC, 0xFD
};
#endif
static int xan_decode_frame(AVCodecContext *avctx,
void *data, int *data_size,
@ -425,9 +475,15 @@ static int xan_decode_frame(AVCodecContext *avctx,
s->palettes = tmpptr;
tmpptr += s->palettes_count * AVPALETTE_COUNT;
for (i = 0; i < PALETTE_COUNT; i++) {
#if RUNTIME_GAMMA
int r = gamma_corr(*buf++);
int g = gamma_corr(*buf++);
int b = gamma_corr(*buf++);
#else
int r = gamma_lookup[*buf++];
int g = gamma_lookup[*buf++];
int b = gamma_lookup[*buf++];
#endif
*tmpptr++ = (r << 16) | (g << 8) | b;
}
s->palettes_count++;