gifdec.c

ffmpeg的完整源代码和作者自己写的文档。不但有在Linux的工程哦

        get_buffer(f, s->local_palette, 3 * (1 << bits_per_pixel));
        palette = s->local_palette;
    } else {
        palette = s->global_palette;
        bits_per_pixel = s->bits_per_pixel;
    }

    /* verify that all the image is inside the screen dimensions */
    if (left + width > s->screen_width ||
        top + height > s->screen_height)
        return AVERROR(EINVAL);

    /* build the palette */
    if (s->pix_fmt == PIX_FMT_RGB24) {
        line = av_malloc(width);
        if (!line)
            return AVERROR(ENOMEM);
    } else {
        n = (1 << bits_per_pixel);
        spal = palette;
        for(i = 0; i < n; i++) {
            s->image_palette[i] = (0xff << 24) |
                (spal[0] << 16) | (spal[1] << 8) | (spal[2]);
            spal += 3;
        }
        for(; i < 256; i++)
            s->image_palette[i] = (0xff << 24);
        /* handle transparency */
        if (s->transparent_color_index >= 0)
            s->image_palette[s->transparent_color_index] = 0;
        line = NULL;
    }

    /* now get the image data */
    s->f = f;
    code_size = get_byte(f);
    GLZWDecodeInit(s, code_size);

    /* read all the image */
    linesize = s->image_linesize;
    ptr1 = s->image_buf + top * linesize + (left * 3);
    ptr = ptr1;
    pass = 0;
    y1 = 0;
    for (y = 0; y < height; y++) {
        if (s->pix_fmt == PIX_FMT_RGB24) {
            /* transcode to RGB24 */
            GLZWDecode(s, line, width);
            d = ptr;
            sptr = line;
            for(x = 0; x < width; x++) {
                spal = palette + sptr[0] * 3;
                d[0] = spal[0];
                d[1] = spal[1];
                d[2] = spal[2];
                d += 3;
                sptr++;
            }
        } else {
            GLZWDecode(s, ptr, width);
        }
        if (is_interleaved) {
            switch(pass) {
            default:
            case 0:
            case 1:
                y1 += 8;
                ptr += linesize * 8;
                if (y1 >= height) {
                    y1 = 4;
                    if (pass == 0)
                        ptr = ptr1 + linesize * 4;
                    else
                        ptr = ptr1 + linesize * 2;
                    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;
        }
    }
    av_free(line);

    /* read the garbage data until end marker is found */
    while (!s->eob_reached)
        GetCode(s);
    return 0;
}

static int gif_read_extension(GifState *s)
{
    ByteIOContext *f = s->f;
    int ext_code, ext_len, i, gce_flags, gce_transparent_index;

    /* extension */
    ext_code = get_byte(f);
    ext_len = get_byte(f);
#ifdef DEBUG
    printf("gif: ext_code=0x%x len=%d\n", ext_code, ext_len);
#endif
    switch(ext_code) {
    case 0xf9:
        if (ext_len != 4)
            goto discard_ext;
        s->transparent_color_index = -1;
        gce_flags = get_byte(f);
        s->gce_delay = get_le16(f);
        gce_transparent_index = get_byte(f);
        if (gce_flags & 0x01)
            s->transparent_color_index = gce_transparent_index;
        else
            s->transparent_color_index = -1;
        s->gce_disposal = (gce_flags >> 2) & 0x7;
#ifdef DEBUG
        printf("gif: gce_flags=%x delay=%d tcolor=%d disposal=%d\n",
               gce_flags, s->gce_delay,
               s->transparent_color_index, s->gce_disposal);
#endif
        ext_len = get_byte(f);
        break;
    }

    /* NOTE: many extension blocks can come after */
 discard_ext:
    while (ext_len != 0) {
        for (i = 0; i < ext_len; i++)
            get_byte(f);
        ext_len = get_byte(f);
#ifdef DEBUG
        printf("gif: ext_len1=%d\n", ext_len);
#endif
    }
    return 0;
}

static int gif_read_header1(GifState *s)
{
    ByteIOContext *f = s->f;
    uint8_t sig[6];
    int ret, v, n;
    int has_global_palette;

    /* read gif signature */
    ret = get_buffer(f, sig, 6);
    if (ret != 6)
        return -1;
    if (memcmp(sig, gif87a_sig, 6) != 0 &&
        memcmp(sig, gif89a_sig, 6) != 0)
        return -1;

    /* read screen header */
    s->transparent_color_index = -1;
    s->screen_width = get_le16(f);
    s->screen_height = get_le16(f);
    if(   (unsigned)s->screen_width  > 32767
       || (unsigned)s->screen_height > 32767){
        av_log(NULL, AV_LOG_ERROR, "picture size too large\n");
        return -1;
    }

    v = get_byte(f);
    s->color_resolution = ((v & 0x70) >> 4) + 1;
    has_global_palette = (v & 0x80);
    s->bits_per_pixel = (v & 0x07) + 1;
    s->background_color_index = get_byte(f);
    get_byte(f);                /* ignored */
#ifdef DEBUG
    printf("gif: screen_w=%d screen_h=%d bpp=%d global_palette=%d\n",
           s->screen_width, s->screen_height, s->bits_per_pixel,
           has_global_palette);
#endif
    if (has_global_palette) {
        n = 1 << s->bits_per_pixel;
        get_buffer(f, s->global_palette, n * 3);
    }
    return 0;
}

static int gif_parse_next_image(GifState *s)
{
    ByteIOContext *f = s->f;
    int ret, code;

    for (;;) {
        code = url_fgetc(f);
#ifdef DEBUG
        printf("gif: code=%02x '%c'\n", code, code);
#endif
        switch (code) {
        case ',':
            if (gif_read_image(s) < 0)
                return AVERROR(EIO);
            ret = 0;
            goto the_end;
        case ';':
            /* end of image */
            ret = AVERROR(EIO);
            goto the_end;
        case '!':
            if (gif_read_extension(s) < 0)
                return AVERROR(EIO);
            break;
        case EOF:
        default:
            /* error or errneous EOF */
            ret = AVERROR(EIO);
            goto the_end;
        }
    }
  the_end:
    return ret;
}

static int gif_read_header(AVFormatContext * s1,
                           AVFormatParameters * ap)
{
    GifState *s = s1->priv_data;
    ByteIOContext *f = &s1->pb;
    AVStream *st;

    s->f = f;
    if (gif_read_header1(s) < 0)
        return -1;

    /* allocate image buffer */
    s->image_linesize = s->screen_width * 3;
    s->image_buf = av_malloc(s->screen_height * s->image_linesize);
    if (!s->image_buf)
        return AVERROR(ENOMEM);
    s->pix_fmt = PIX_FMT_RGB24;
    /* now we are ready: build format streams */
    st = av_new_stream(s1, 0);
    if (!st)
        return -1;

    st->codec->codec_type = CODEC_TYPE_VIDEO;
    st->codec->codec_id = CODEC_ID_RAWVIDEO;
    st->codec->time_base.den = 5;
    st->codec->time_base.num = 1;
    /* XXX: check if screen size is always valid */
    st->codec->width = s->screen_width;
    st->codec->height = s->screen_height;
    st->codec->pix_fmt = PIX_FMT_RGB24;
    return 0;
}

static int gif_read_packet(AVFormatContext * s1,
                           AVPacket * pkt)
{
    GifState *s = s1->priv_data;
    int ret;

    ret = gif_parse_next_image(s);
    if (ret < 0)
        return ret;

    /* XXX: avoid copying */
    if (av_new_packet(pkt, s->screen_width * s->screen_height * 3)) {
        return AVERROR(EIO);
    }
    pkt->stream_index = 0;
    memcpy(pkt->data, s->image_buf, s->screen_width * s->screen_height * 3);
    return 0;
}

static int gif_read_close(AVFormatContext *s1)
{
    GifState *s = s1->priv_data;
    av_free(s->image_buf);
    return 0;
}

AVInputFormat gif_demuxer =
{
    "gif",
    "gif format",
    sizeof(GifState),
    gif_video_probe,
    gif_read_header,
    gif_read_packet,
    gif_read_close,
};