[FFmpeg-devel] [PATCH] fix for transparencies in animated gifs (requires feedback)
Bjorn Roche
bjorn at giphy.com
Tue Oct 24 19:40:22 EEST 2017
Support for transparencies in animated gifs requires modifying both
libavcodec/gif.c and libavformat/gif.c because both the graphics
control extension (handled by libavformat/gif.c) and the raw frame data
(handled by libavcodec/gif.c) must be changed. This is because
transparencies in GIF can be used both to create a transparent image,
and to provide optimization.
How transparencies are interpreted in a given frame is controlled by
the “disposal method”, which must be set appropriately in the graphics
control extension.
The “in place” disposal method is used when transparency indicates
optimization, and the “background” disposal method is used when
transparency is intended to be preserved. In order to support both
disposal methods, libavcodec/gif.c must signal to libavformat/gif.c
which disposal method is required for every frame. This is done with a
new side data type: AV_PKT_DATA_GIF_FRAME_DISPOSAL. This requires a
change to avcodec.h
Unfortunately, the addition of a new side data type causes some of the
FATE tests to fail. This is not addressed here.
This patch assumes paletteuse has already been patched to support
transparency. (e.g. lavfi/paletteuse: fix to support transparency)
Feedback I definitely need:
- I’ve done a fair bit of testing, but I’m sure I missed some important
cases.
- I don’t know if/how to update the FATE tests.
---
libavcodec/avcodec.h | 6 ++
libavcodec/gif.c | 196 +++++++++++++++++++++++++++++++++++++++++++++++++--
libavformat/gif.c | 16 ++++-
3 files changed, 212 insertions(+), 6 deletions(-)
diff --git a/libavcodec/avcodec.h b/libavcodec/avcodec.h
index 52cc5b0ca0..82a5328ce1 100644
--- a/libavcodec/avcodec.h
+++ b/libavcodec/avcodec.h
@@ -1599,6 +1599,12 @@ enum AVPacketSideDataType {
*/
AV_PKT_DATA_CONTENT_LIGHT_LEVEL,
+ /**
+ * The disposal method that should be used with the frame. If missing,
+ * the frame will not be disposed. This contains exactly one byte.
+ */
+ AV_PKT_DATA_GIF_FRAME_DISPOSAL,
+
/**
* ATSC A53 Part 4 Closed Captions. This metadata should be associated with
* a video stream. A53 CC bitstream is stored as uint8_t in AVPacketSideData.data.
diff --git a/libavcodec/gif.c b/libavcodec/gif.c
index d9c99d52cf..6c839d99d2 100644
--- a/libavcodec/gif.c
+++ b/libavcodec/gif.c
@@ -74,11 +74,35 @@ static int pick_palette_entry(const uint8_t *buf, int linesize, int w, int h)
return -1;
}
-static int gif_image_write_image(AVCodecContext *avctx,
- uint8_t **bytestream, uint8_t *end,
- const uint32_t *palette,
- const uint8_t *buf, const int linesize,
- AVPacket *pkt)
+// returns true if any of the pixels are transparent
+static int is_image_translucent(AVCodecContext *avctx,
+ const uint32_t *palette,
+ const uint8_t *buf, const int linesize)
+{
+ GIFContext *s = avctx->priv_data;
+ int trans = s->transparent_index;
+ int p;
+ const int m = avctx->width * avctx->height ;
+
+ if (trans < 0) {
+ return 0;
+ }
+
+ for (p=0; p<m; ++p) {
+ if (buf[p] == trans) {
+ return 1;
+ }
+ }
+ return 0;
+}
+
+// writes an opaque image. ie an image with no transparency.
+// it also works, and should be used, for a first image.
+static int gif_image_write_opaque(AVCodecContext *avctx,
+ uint8_t **bytestream, uint8_t *end,
+ const uint32_t *palette,
+ const uint8_t *buf, const int linesize,
+ AVPacket *pkt)
{
GIFContext *s = avctx->priv_data;
int len = 0, height = avctx->height, width = avctx->width, x, y;
@@ -137,6 +161,11 @@ static int gif_image_write_image(AVCodecContext *avctx,
width, height, x_start, y_start, avctx->width, avctx->height);
}
+ uint8_t *frame_disposal = av_packet_new_side_data(pkt, AV_PKT_DATA_GIF_FRAME_DISPOSAL, 1);
+ if (!frame_disposal)
+ return AVERROR(ENOMEM);
+ *frame_disposal = GCE_DISPOSAL_INPLACE;
+
/* image block */
bytestream_put_byte(bytestream, GIF_IMAGE_SEPARATOR);
bytestream_put_le16(bytestream, x_start);
@@ -214,6 +243,163 @@ static int gif_image_write_image(AVCodecContext *avctx,
return 0;
}
+// wrtites an image that may contain transparency
+// this should work for opaque images as well, but will be less optimized.
+static int gif_image_write_translucent(AVCodecContext *avctx,
+ uint8_t **bytestream, uint8_t *end,
+ const uint32_t *palette,
+ const uint8_t *buf, const int linesize,
+ AVPacket *pkt)
+{
+ GIFContext *s = avctx->priv_data;
+ int len = 0, height = avctx->height, width = avctx->width, y;
+ int x_start = 0, y_start = 0, trans = s->transparent_index;
+ const uint8_t *ptr;
+
+ /* Crop Image */
+ if ((s->flags & GF_OFFSETTING) && trans >=0) {
+ const int w = avctx->width;
+ const int h = avctx->height;
+ int x_end = w - 1,
+ y_end = h - 1;
+
+ // crop top
+ while (y_start < y_end) {
+ int i;
+ int is_trans = 1;
+ for (i=0; i<w; ++i) {
+ if( buf[w*y_start+i] != trans ) {
+ is_trans = 0;
+ break;
+ }
+ }
+ if (!is_trans)
+ break;
+ y_start++;
+ }
+
+ // crop bottom
+ while (y_end < h) {
+ int i;
+ int is_trans = 1;
+ for (i=0; i<w; ++i) {
+ if (buf[w*y_end+i] != trans) {
+ is_trans = 0;
+ break;
+ }
+ }
+ if (!is_trans)
+ break;
+ y_end--;
+ }
+
+ // crop left
+ while (x_start < x_end) {
+ int i;
+ int is_trans = 1;
+ for (i=y_start; i<y_end; ++i) {
+ if (buf[w*i+x_start] != trans) {
+ is_trans = 0;
+ break;
+ }
+ }
+ if (!is_trans)
+ break;
+ x_start++;
+ }
+
+ // crop right
+ while (x_end < w) {
+ int i;
+ int is_trans = 1;
+ for (i=y_start; i<y_end; ++i) {
+ if (buf[w*i+x_end] != trans) {
+ is_trans = 0;
+ break;
+ }
+ }
+ if (!is_trans)
+ break;
+ x_end--;
+ }
+
+ height = y_end + 1 - y_start;
+ width = x_end + 1 - x_start;
+ av_log(avctx, AV_LOG_DEBUG,"%dx%d image at pos (%d;%d) [area:%dx%d]\n",
+ width, height, x_start, y_start, avctx->width, avctx->height);
+ }
+
+
+ uint8_t *frame_disposal = av_packet_new_side_data(pkt, AV_PKT_DATA_GIF_FRAME_DISPOSAL, 1);
+ if (!frame_disposal)
+ return AVERROR(ENOMEM);
+ *frame_disposal = GCE_DISPOSAL_BACKGROUND;
+
+ /* image block */
+ bytestream_put_byte(bytestream, GIF_IMAGE_SEPARATOR);
+ bytestream_put_le16(bytestream, x_start);
+ bytestream_put_le16(bytestream, y_start);
+ bytestream_put_le16(bytestream, width);
+ bytestream_put_le16(bytestream, height);
+
+ if (!palette) {
+ bytestream_put_byte(bytestream, 0x00); /* flags */
+ } else {
+ unsigned i;
+ bytestream_put_byte(bytestream, 1<<7 | 0x7); /* flags */
+ for (i = 0; i < AVPALETTE_COUNT; i++) {
+ const uint32_t v = palette[i];
+ bytestream_put_be24(bytestream, v);
+ }
+ }
+
+ bytestream_put_byte(bytestream, 0x08);
+
+ ff_lzw_encode_init(s->lzw, s->buf, s->buf_size,
+ 12, FF_LZW_GIF, put_bits);
+
+ ptr = buf + y_start*linesize + x_start;
+
+ for (y = 0; y < height; y++) {
+ len += ff_lzw_encode(s->lzw, ptr, width);
+ ptr += linesize;
+ }
+
+ len += ff_lzw_encode_flush(s->lzw, flush_put_bits);
+
+ ptr = s->buf;
+ while (len > 0) {
+ int size = FFMIN(255, len);
+ bytestream_put_byte(bytestream, size);
+ if (end - *bytestream < size)
+ return -1;
+ bytestream_put_buffer(bytestream, ptr, size);
+ ptr += size;
+ len -= size;
+ }
+ bytestream_put_byte(bytestream, 0x00); /* end of image block */
+
+ return 0;
+}
+
+static int gif_image_write_image(AVCodecContext *avctx,
+ uint8_t **bytestream, uint8_t *end,
+ const uint32_t *palette,
+ const uint8_t *buf, const int linesize,
+ AVPacket *pkt)
+{
+ GIFContext *s = avctx->priv_data;
+ if (!s->last_frame) {
+ return gif_image_write_opaque(avctx, bytestream, end, palette, buf, linesize, pkt);
+ }
+
+ if (is_image_translucent(avctx, palette, buf, linesize)) {
+ return gif_image_write_translucent(avctx, bytestream, end, palette, buf, linesize, pkt);
+ } else {
+ return gif_image_write_opaque(avctx, bytestream, end, palette, buf, linesize, pkt);
+ }
+}
+
static av_cold int gif_encode_init(AVCodecContext *avctx)
{
GIFContext *s = avctx->priv_data;
diff --git a/libavformat/gif.c b/libavformat/gif.c
index 01d98a27b0..b4a8b6aa94 100644
--- a/libavformat/gif.c
+++ b/libavformat/gif.c
@@ -144,6 +144,8 @@ static int flush_packet(AVFormatContext *s, AVPacket *new)
AVIOContext *pb = s->pb;
const uint32_t *palette;
AVPacket *pkt = gif->prev_pkt;
+ uint8_t *disposal;
+ uint8_t packed;
if (!pkt)
return 0;
@@ -157,16 +159,28 @@ static int flush_packet(AVFormatContext *s, AVPacket *new)
}
bcid = get_palette_transparency_index(palette);
+ disposal = av_packet_get_side_data(pkt, AV_PKT_DATA_GIF_FRAME_DISPOSAL, &size);
+ if (disposal && size != 1) {
+ av_log(s, AV_LOG_ERROR, "Invalid gif frame disposal extradata\n");
+ return AVERROR_INVALIDDATA;
+ }
+
if (new && new->pts != AV_NOPTS_VALUE)
gif->duration = av_clip_uint16(new->pts - gif->prev_pkt->pts);
else if (!new && gif->last_delay >= 0)
gif->duration = gif->last_delay;
/* graphic control extension block */
+ if (disposal) {
+ packed = (0xff & (*disposal)<<2) | (bcid >= 0);
+ } else {
+ packed = 1<<2 | (bcid >= 0);
+ }
+
avio_w8(pb, 0x21);
avio_w8(pb, 0xf9);
avio_w8(pb, 0x04); /* block size */
- avio_w8(pb, 1<<2 | (bcid >= 0));
+ avio_w8(pb, packed);
avio_wl16(pb, gif->duration);
avio_w8(pb, bcid < 0 ? DEFAULT_TRANSPARENCY_INDEX : bcid);
avio_w8(pb, 0x00);
--
2.14.1
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