diff options
| -rw-r--r-- | libavcodec/xan.c | 56 |
1 files changed, 56 insertions, 0 deletions
diff --git a/libavcodec/xan.c b/libavcodec/xan.c index 3e4ca2e030..1d697c6a84 100644 --- a/libavcodec/xan.c +++ b/libavcodec/xan.c @@ -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++; |