[FFmpeg-devel] [PATCH 2/2 v3] avcodec/gifenc: Only write frame palette entries that actually used

Paul B Mahol onemda at gmail.com
Sun Feb 21 20:57:21 EET 2021


I would prefer with memory log removed....

Probably fine anyway.

On Sat, Feb 20, 2021 at 8:24 PM Derek Buitenhuis <derek.buitenhuis at gmail.com>
wrote:

> 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: 77895 bytes
>
> Signed-off-by: Derek Buitenhuis <derek.buitenhuis at gmail.com>
> ---
> Changes since v2:
>     * shrunk_buf allocated only once per encoder instance.
>     * Does not use temporary pointers during remapping.
> ---
>  libavcodec/gif.c | 68 +++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 65 insertions(+), 3 deletions(-)
>
> diff --git a/libavcodec/gif.c b/libavcodec/gif.c
> index 8c07ee2769..ff0d50cb44 100644
> --- a/libavcodec/gif.c
> +++ b/libavcodec/gif.c
> @@ -47,6 +47,7 @@ typedef struct GIFContext {
>      const AVClass *class;
>      LZWState *lzw;
>      uint8_t *buf;
> +    uint8_t *shrunk_buf;
>      int buf_size;
>      AVFrame *last_frame;
>      int flags;
> @@ -63,6 +64,38 @@ enum {
>      GF_TRANSDIFF  = 1<<1,
>  };
>
> +static void shrink_palette(const uint32_t *src, uint8_t *map,
> +                           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];
> +            map[i] = colors_seen;
> +            colors_seen++;
> +        }
> +    }
> +
> +    *palette_count = colors_seen;
> +}
> +
> +static void remap_frame_to_palette(const uint8_t *src, int src_linesize,
> +                                   uint8_t *dst, int dst_linesize,
> +                                   int w, int h, uint8_t *map)
> +{
> +    for (int i = 0; i < h; i++)
> +        for (int j = 0; j < w; j++)
> +            dst[i * dst_linesize + j] = map[src[i * src_linesize + j]];
> +}
> +
>  static int is_image_translucent(AVCodecContext *avctx,
>                                  const uint8_t *buf, const int linesize)
>  {
> @@ -267,6 +300,17 @@ 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];
> +    uint8_t map[AVPALETTE_COUNT] = { 0 };
> +    size_t shrunk_palette_count = 0;
> +
> +    /*
> +     * 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);
>
>      if (!s->image && is_image_translucent(avctx, buf, linesize)) {
>          gif_crop_translucent(avctx, buf, linesize, &width, &height,
> &x_start, &y_start);
> @@ -335,9 +379,14 @@ static int gif_image_write_image(AVCodecContext
> *avctx,
>
>      if (palette || !s->use_global_palette) {
>          const uint32_t *pal = palette ? palette : s->palette;
> +        unsigned pow2_count;
>          unsigned i;
> -        bytestream_put_byte(bytestream, 1<<7 | 0x7); /* flags */
> -        for (i = 0; i < AVPALETTE_COUNT; i++) {
> +
> +        shrink_palette(pal, map, shrunk_palette, &shrunk_palette_count);
> +        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 = pal[i];
>              bytestream_put_be24(bytestream, v);
>          }
> @@ -350,7 +399,19 @@ static int gif_image_write_image(AVCodecContext
> *avctx,
>      ff_lzw_encode_init(s->lzw, s->buf, s->buf_size,
>                         12, FF_LZW_GIF, 1);
>
> -    ptr = buf + y_start*linesize + x_start;
> +    if (shrunk_palette_count) {
> +        if (!s->shrunk_buf) {
> +            s->shrunk_buf = av_malloc(avctx->height * linesize);
> +            if (!s->shrunk_buf) {
> +                av_log(avctx, AV_LOG_ERROR, "Could not allocated remapped
> frame buffer.\n");
> +                return AVERROR(ENOMEM);
> +            }
> +        }
> +        remap_frame_to_palette(buf, linesize, s->shrunk_buf, linesize,
> avctx->width, avctx->height, map);
> +        ptr = s->shrunk_buf + y_start*linesize + x_start;
> +    } else {
> +        ptr = buf + y_start*linesize + x_start;
> +    }
>      if (honor_transparency) {
>          const int ref_linesize = s->last_frame->linesize[0];
>          const uint8_t *ref = s->last_frame->data[0] +
> y_start*ref_linesize + x_start;
> @@ -464,6 +525,7 @@ static int gif_encode_close(AVCodecContext *avctx)
>
>      av_freep(&s->lzw);
>      av_freep(&s->buf);
> +    av_freep(&s->shrunk_buf);
>      s->buf_size = 0;
>      av_frame_free(&s->last_frame);
>      av_freep(&s->tmpl);
> --
> 2.30.0
>
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