解决gif导出后显示异常的现象

C++

    if (avpkt->size >= 6) {
        s->keyframe = memcmp(avpkt->data, gif87a_sig, 6) == 0 ||
                      memcmp(avpkt->data, gif89a_sig, 6) == 0;
    } else {
        s->keyframe = 0;
    }
   
   
        if (s->keyframe) {
       .......//这里代码先删除，方便理解
        } else {
            if ((ret = ff_reget_buffer(avctx, s->frame)) < 0)
                return ret;
        ........
        }
 

C++
  for (y = 0; y < height; y++) {
        int count = ff_lzw_decode(s->lzw, s->idx_line, width);
        if (count != width) {
            if (count)
                av_log(s->avctx, AV_LOG_ERROR, "LZW decode failed\n");
            goto decode_tail;
        }

        pr = ptr + pw;

        for (px = ptr, idx = s->idx_line; px < pr; px++, idx++) {
            if (*idx != s->transparent_color_index)
                *px = pal[*idx];
        }
}

C++
static int gif_read_image(GifState *s, AVFrame *frame)
{
    .....................
    /* process disposal method */
    if (s->gce_prev_disposal == GCE_DISPOSAL_BACKGROUND) {
        gif_fill_rect(frame, s->stored_bg_color, s->gce_l, s->gce_t, s->gce_w, s->gce_h);
    } else if (s->gce_prev_disposal == GCE_DISPOSAL_RESTORE) {
        gif_copy_img_rect(s->stored_img, (uint32_t *)frame->data[0],
            frame->linesize[0] / sizeof(uint32_t), s->gce_l, s->gce_t, s->gce_w, s->gce_h);
    }
    .....................
}

C++
static int gif_read_extension(GifState *s)
{
   .......
    switch(ext_code) {
    case GIF_GCE_EXT_LABEL:
  .......
        gce_flags    = bytestream2_get_byteu(&s->gb);
        bytestream2_skipu(&s->gb, 2);    // delay during which the frame is shown
   .......
 
        s->gce_disposal = (gce_flags >> 2) & 0x7;
      
        break;
    }
   .......

    return 0;
}

C++
static int flush_packet(AVFormatContext *s, AVPacket *new)
{
    ........
    /* graphic control extension block */
    avio_w8(pb, 0x21);
    avio_w8(pb, 0xf9);
    avio_w8(pb, 0x04); /* block size */
    avio_w8(pb, 1<<2 | (bcid >= 0));
    avio_wl16(pb, gif->duration);
    avio_w8(pb, bcid < 0 ? DEFAULT_TRANSPARENCY_INDEX : bcid);
    avio_w8(pb, 0x00);
    .......

    return 0;
}
 

C++
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;
    int honor_transparency = (s->flags & GF_TRANSDIFF) && s->last_frame;


    /* Crop image */
    if ((s->flags & GF_OFFSETTING) && s->last_frame && !palette) {
        const uint8_t *ref = s->last_frame->data[0];
        const int ref_linesize = s->last_frame->linesize[0];
        int x_end = avctx->width  - 1,
            y_end = avctx->height - 1;

        /* skip common lines */
        while (y_start < y_end) {
            if (memcmp(ref + y_start*ref_linesize, buf + y_start*linesize, width))
                break;
            y_start++;
        }
        while (y_end > y_start) {
            if (memcmp(ref + y_end*ref_linesize, buf + y_end*linesize, width))
                break;
            y_end--;
        }
        height = y_end + 1 - y_start;

        /* skip common columns */
        while (x_start < x_end) {
            int same_column = 1;
            for (y = y_start; y <= y_end; y++) {
                if (ref[y*ref_linesize + x_start] != buf[y*linesize + x_start]) {
                    same_column = 0;
                    break;
                }
            }
            if (!same_column)
                break;
            x_start++;
        }
        while (x_end > x_start) {
            int same_column = 1;
            for (y = y_start; y <= y_end; y++) {
                if (ref[y*ref_linesize + x_end] != buf[y*linesize + x_end]) {
                    same_column = 0;
                    break;
                }
            }
            if (!same_column)
                break;
            x_end--;
        }
        width = x_end + 1 - x_start;

    }
 

C++
int avcodec_open2(AVCodecContext *avctx, const AVCodec *codec, AVDictionary **options)
{

    if (codec->priv_data_size > 0) {
        if (!avctx->priv_data) {
            avctx->priv_data = av_mallocz(codec->priv_data_size);
            if (!avctx->priv_data) {
                ret = AVERROR(ENOMEM);
                goto end;
            }
            if (codec->priv_class) {
                *(const AVClass **)avctx->priv_data = codec->priv_class;
                av_opt_set_defaults(avctx->priv_data);
            }
        }
        if (codec->priv_class && (ret = av_opt_set_dict(avctx->priv_data, &tmp)) < 0)
            goto free_and_end;
    } else {
        avctx->priv_data = NULL;
    }
    if ((ret = av_opt_set_dict(avctx, &tmp)) < 0)
        goto free_and_end;
}

C++
#define OFFSET(x) offsetof(GIFContext, x)
#define FLAGS AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_ENCODING_PARAM
static const AVOption gif_options[] = {
    { "gifflags", "set GIF flags", OFFSET(flags), AV_OPT_TYPE_FLAGS, {.i64 = GF_OFFSETTING|GF_TRANSDIFF}, 0, INT_MAX, FLAGS, "flags" },
        { "offsetting", "enable picture offsetting", 0, AV_OPT_TYPE_CONST, {.i64=GF_OFFSETTING}, INT_MIN, INT_MAX, FLAGS, "flags" },
        { "transdiff", "enable transparency detection between frames", 0, AV_OPT_TYPE_CONST, {.i64=GF_TRANSDIFF}, INT_MIN, INT_MAX, FLAGS, "flags" },
    { NULL }
};