[FFmpeg-devel] [PATCH] libavcodec/gifenc: Only write palette entries that actually used
Derek Buitenhuis
derek.buitenhuis at gmail.com
Thu Feb 4 18:50:16 EET 2021
GIF palette entries are not compressed, and writing 256 entries,
which can be up to every frame, uses a significant amount of
space, especially in extreme cases, where palettes can be very
small.
Example, first six seconds of Tears of Steel, palette generated
with libimagequant, 320x240 resolution, and with transparency
optimization + per frame palette:
* Before patch: 186765 bytes
* After patch: 77901 bytes
Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
---
libavcodec/gif.c | 81 +++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 69 insertions(+), 12 deletions(-)
diff --git a/libavcodec/gif.c b/libavcodec/gif.c
index de41992851..c52db57edd 100644
--- a/libavcodec/gif.c
+++ b/libavcodec/gif.c
@@ -52,6 +52,7 @@ typedef struct GIFContext {
int flags;
int image;
uint32_t palette[AVPALETTE_COUNT]; ///< local reference palette for !pal8
+ size_t palette_count;
int palette_loaded;
int transparent_index;
uint8_t *tmpl; ///< temporary line buffer
@@ -62,6 +63,27 @@ enum {
GF_TRANSDIFF = 1<<1,
};
+static void shrink_palette(const uint32_t *src, uint32_t *dst, size_t *palette_count)
+{
+ size_t colors_seen = 0;
+
+ for (size_t i = 0; i < AVPALETTE_COUNT; i++) {
+ int seen = 0;
+ for (size_t c = 0; c < colors_seen; c++) {
+ if (src[i] == dst[c]) {
+ seen = 1;
+ break;
+ }
+ }
+ if (!seen) {
+ dst[colors_seen] = src[i];
+ colors_seen++;
+ }
+ }
+
+ *palette_count = colors_seen;
+}
+
static int is_image_translucent(AVCodecContext *avctx,
const uint8_t *buf, const int linesize)
{
@@ -83,7 +105,7 @@ static int is_image_translucent(AVCodecContext *avctx,
return 0;
}
-static int get_palette_transparency_index(const uint32_t *palette)
+static int get_palette_transparency_index(const uint32_t *palette, int palette_count)
{
int transparent_color_index = -1;
unsigned i, smallest_alpha = 0xff;
@@ -91,7 +113,7 @@ static int get_palette_transparency_index(const uint32_t *palette)
if (!palette)
return -1;
- for (i = 0; i < AVPALETTE_COUNT; i++) {
+ for (i = 0; i < palette_count; i++) {
const uint32_t v = palette[i];
if (v >> 24 < smallest_alpha) {
smallest_alpha = v >> 24;
@@ -266,6 +288,10 @@ static int gif_image_write_image(AVCodecContext *avctx,
int x_start = 0, y_start = 0, trans = s->transparent_index;
int bcid = -1, honor_transparency = (s->flags & GF_TRANSDIFF) && s->last_frame && !palette;
const uint8_t *ptr;
+ uint32_t shrunk_palette[AVPALETTE_COUNT] = { 0 };
+ size_t shrunk_palette_count;
+
+ memset(shrunk_palette, 0xff, AVPALETTE_SIZE);
if (!s->image && is_image_translucent(avctx, buf, linesize)) {
gif_crop_translucent(avctx, buf, linesize, &width, &height, &x_start, &y_start);
@@ -277,10 +303,21 @@ static int gif_image_write_image(AVCodecContext *avctx,
}
if (s->image || !avctx->frame_number) { /* GIF header */
- const uint32_t *global_palette = palette ? palette : s->palette;
+ uint32_t *global_palette;
+ uint32_t shrunk_global_palette[AVPALETTE_COUNT];
+ size_t global_palette_count;
+ unsigned pow2_global_palette_count;
const AVRational sar = avctx->sample_aspect_ratio;
int64_t aspect = 0;
+ if (palette) {
+ shrink_palette(palette, shrunk_global_palette, &global_palette_count);
+ global_palette = shrunk_global_palette;
+ } else {
+ global_palette = s->palette;
+ global_palette_count = s->palette_count;
+ }
+
if (sar.num > 0 && sar.den > 0) {
aspect = sar.num * 64LL / sar.den - 15;
if (aspect < 0 || aspect > 255)
@@ -291,17 +328,22 @@ static int gif_image_write_image(AVCodecContext *avctx,
bytestream_put_le16(bytestream, avctx->width);
bytestream_put_le16(bytestream, avctx->height);
- bcid = get_palette_transparency_index(global_palette);
+ bcid = get_palette_transparency_index(global_palette, global_palette_count);
- bytestream_put_byte(bytestream, 0xf7); /* flags: global clut, 256 entries */
+ pow2_global_palette_count = av_log2(global_palette_count - 1);
+
+ bytestream_put_byte(bytestream, 0xf0 | pow2_global_palette_count); /* flags: global clut, 256 entries */
bytestream_put_byte(bytestream, bcid < 0 ? DEFAULT_TRANSPARENCY_INDEX : bcid); /* background color index */
bytestream_put_byte(bytestream, aspect);
- for (int i = 0; i < 256; i++) {
+ for (int i = 0; i < 1 << (pow2_global_palette_count + 1); i++) {
const uint32_t v = global_palette[i] & 0xffffff;
bytestream_put_be24(bytestream, v);
}
}
+ if (palette)
+ shrink_palette(palette, shrunk_palette, &shrunk_palette_count);
+
if (honor_transparency && trans < 0) {
trans = pick_palette_entry(buf + y_start*linesize + x_start,
linesize, width, height);
@@ -312,7 +354,7 @@ static int gif_image_write_image(AVCodecContext *avctx,
if (trans < 0)
honor_transparency = 0;
- bcid = honor_transparency || disposal == GCE_DISPOSAL_BACKGROUND ? trans : get_palette_transparency_index(palette);
+ bcid = honor_transparency || disposal == GCE_DISPOSAL_BACKGROUND ? trans : get_palette_transparency_index(palette ? shrunk_palette : NULL, shrunk_palette_count);
/* graphic control extension */
bytestream_put_byte(bytestream, GIF_EXTENSION_INTRODUCER);
@@ -334,8 +376,9 @@ static int gif_image_write_image(AVCodecContext *avctx,
bytestream_put_byte(bytestream, 0x00); /* flags */
} else {
unsigned i;
- bytestream_put_byte(bytestream, 1<<7 | 0x7); /* flags */
- for (i = 0; i < AVPALETTE_COUNT; i++) {
+ unsigned pow2_count = av_log2(shrunk_palette_count - 1);
+ bytestream_put_byte(bytestream, 1<<7 | pow2_count); /* flags */
+ for (i = 0; i < 1 << (pow2_count + 1); i++) {
const uint32_t v = palette[i];
bytestream_put_be24(bytestream, v);
}
@@ -402,6 +445,7 @@ static av_cold int gif_encode_init(AVCodecContext *avctx)
if (avpriv_set_systematic_pal2(s->palette, avctx->pix_fmt) < 0)
av_assert0(avctx->pix_fmt == AV_PIX_FMT_PAL8);
+ s->palette_count = AVPALETTE_COUNT;
return 0;
}
@@ -420,13 +464,26 @@ static int gif_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
end = pkt->data + pkt->size;
if (avctx->pix_fmt == AV_PIX_FMT_PAL8) {
+ uint32_t shrunk_palette[AVPALETTE_COUNT];
+ size_t shrunk_palette_count;
+
palette = (uint32_t*)pict->data[1];
+ /*
+ * We memset to 0xff instead of 0x00 so that the transparency detection
+ * doesn't pick anything after the palette entries as the transparency
+ * index, and because GIF89a requires us to always write a power-of-2
+ * number of palette entries.
+ */
+ memset(shrunk_palette, 0xff, AVPALETTE_SIZE);
+ shrink_palette(palette, shrunk_palette, &shrunk_palette_count);
+
if (!s->palette_loaded) {
- memcpy(s->palette, palette, AVPALETTE_SIZE);
- s->transparent_index = get_palette_transparency_index(palette);
+ memcpy(s->palette, shrunk_palette, AVPALETTE_SIZE);
+ s->palette_count = shrunk_palette_count;
+ s->transparent_index = get_palette_transparency_index(s->palette, s->palette_count);
s->palette_loaded = 1;
- } else if (!memcmp(s->palette, palette, AVPALETTE_SIZE)) {
+ } else if (!memcmp(s->palette, shrunk_palette, AVPALETTE_SIZE)) {
palette = NULL;
}
}
--
2.30.0
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