static int gif_read_image(GifState *s, AVFrame *frame)
{
    int left, top, width, height, bits_per_pixel, code_size, flags, pw;
    int is_interleaved, has_local_palette, y, pass, y1, linesize, pal_size;
    uint32_t *ptr, *pal, *px, *pr, *ptr1;
    int ret;
    uint8_t *idx;

    /* At least 9 bytes of Image Descriptor. */
    if (bytestream2_get_bytes_left(&s->gb) < 9)
        return AVERROR_INVALIDDATA;

    left   = bytestream2_get_le16u(&s->gb);
    top    = bytestream2_get_le16u(&s->gb);
    width  = bytestream2_get_le16u(&s->gb);
    height = bytestream2_get_le16u(&s->gb);
    flags  = bytestream2_get_byteu(&s->gb);
    is_interleaved = flags & 0x40;
    has_local_palette = flags & 0x80;
    bits_per_pixel = (flags & 0x07) + 1;

    av_dlog(s->avctx, "image x=%d y=%d w=%d h=%d\n", left, top, width, height);

    if (has_local_palette) {
        pal_size = 1 << bits_per_pixel;

        if (bytestream2_get_bytes_left(&s->gb) < pal_size * 3)
            return AVERROR_INVALIDDATA;

        gif_read_palette(s, s->local_palette, pal_size);
        pal = s->local_palette;
    } else {
        if (!s->has_global_palette) {
            av_log(s->avctx, AV_LOG_ERROR, "picture doesn't have either global or local palette.\n");
            return AVERROR_INVALIDDATA;
        }

        pal = s->global_palette;
    }

    if (s->keyframe) {
        if (s->transparent_color_index == -1 && s->has_global_palette) {
            /* transparency wasn't set before the first frame, fill with background color */
            gif_fill(frame, s->bg_color);
        } else {
            /* otherwise fill with transparent color.
             * this is necessary since by default picture filled with 0x80808080. */
            gif_fill(frame, s->trans_color);
        }
    }

    /* verify that all the image is inside the screen dimensions */
    if (!width || width > s->screen_width || left >= s->screen_width) {
        av_log(s->avctx, AV_LOG_ERROR, "Invalid image width.\n");
        return AVERROR_INVALIDDATA;
    }
    if (!height || height > s->screen_height || top >= s->screen_height) {
        av_log(s->avctx, AV_LOG_ERROR, "Invalid image height.\n");
        return AVERROR_INVALIDDATA;
    }
    if (left + width > s->screen_width) {
        /* width must be kept around to avoid lzw vs line desync */
        pw = s->screen_width - left;
        av_log(s->avctx, AV_LOG_WARNING, "Image too wide by %d, truncating.\n",
               left + width - s->screen_width);
    } else {
        pw = width;
    }
    if (top + height > s->screen_height) {
        /* we don't care about the extra invisible lines */
        av_log(s->avctx, AV_LOG_WARNING, "Image too high by %d, truncating.\n",
               top + height - s->screen_height);
        height = s->screen_height - top;
    }

    /* 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);
    }

    s->gce_prev_disposal = s->gce_disposal;

    if (s->gce_disposal != GCE_DISPOSAL_NONE) {
        s->gce_l = left;  s->gce_t = top;
        s->gce_w = pw;    s->gce_h = height;

        if (s->gce_disposal == GCE_DISPOSAL_BACKGROUND) {
            if (s->transparent_color_index >= 0)
                s->stored_bg_color = s->trans_color;
            else
                s->stored_bg_color = s->bg_color;
        } else if (s->gce_disposal == GCE_DISPOSAL_RESTORE) {
            av_fast_malloc(&s->stored_img, &s->stored_img_size, frame->linesize[0] * frame->height);
            if (!s->stored_img)
                return AVERROR(ENOMEM);

            gif_copy_img_rect((uint32_t *)frame->data[0], s->stored_img,
                frame->linesize[0] / sizeof(uint32_t), left, top, pw, height);
        }
    }

    /* Expect at least 2 bytes: 1 for lzw code size and 1 for block size. */
    if (bytestream2_get_bytes_left(&s->gb) < 2)
        return AVERROR_INVALIDDATA;

    /* now get the image data */
    code_size = bytestream2_get_byteu(&s->gb);
    if ((ret = ff_lzw_decode_init(s->lzw, code_size, s->gb.buffer,
                                  bytestream2_get_bytes_left(&s->gb), FF_LZW_GIF)) < 0) {
        av_log(s->avctx, AV_LOG_ERROR, "LZW init failed\n");
        return ret;
    }

    /* read all the image */
    linesize = frame->linesize[0] / sizeof(uint32_t);
    ptr1 = (uint32_t *)frame->data[0] + top * linesize + left;
    ptr = ptr1;
    pass = 0;
    y1 = 0;
    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];
        }

        if (is_interleaved) {
            switch(pass) {
            default:
            case 0:
            case 1:
                y1 += 8;
                ptr += linesize * 8;
                if (y1 >= height) {
                    y1 = pass ? 2 : 4;
                    ptr = ptr1 + linesize * y1;
                    pass++;
                }
                break;
            case 2:
                y1 += 4;
                ptr += linesize * 4;
                if (y1 >= height) {
                    y1 = 1;
                    ptr = ptr1 + linesize;
                    pass++;
                }
                break;
            case 3:
                y1 += 2;
                ptr += linesize * 2;
                break;
            }
        } else {
            ptr += linesize;
        }
    }

 decode_tail:
    /* read the garbage data until end marker is found */
    ff_lzw_decode_tail(s->lzw);

    /* Graphic Control Extension's scope is single frame.
     * Remove its influence. */
    s->transparent_color_index = -1;
    s->gce_disposal = GCE_DISPOSAL_NONE;

    return 0;
}