FFmpeg/libavcodec/xsubdec.c

#include "avcodec.h"
#include "bitstream.h"
#include "bytestream.h"

static int decode_init(AVCodecContext *avctx) {
    avctx->pix_fmt = PIX_FMT_PAL8;
    return 0;
}

static const uint8_t runbits[8] = { 2, 2, 6, 6, 10, 10, 14, 14 };

static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
                        uint8_t *buf, int buf_size) {
    AVSubtitle *sub = data;
    uint8_t *buf_end = buf + buf_size;
    uint8_t *bitmap;
    int w, h, x, y, rlelen, i;
    GetBitContext gb;

    // check that at least header fits
    if (buf_size < 27 + 7 * 2 + 4 * 3) {
        av_log(avctx, AV_LOG_ERROR, "coded frame too small\n");
        return -1;
    }

    // read header
    w = bytestream_get_le16(&buf);
    h = bytestream_get_le16(&buf);
    if (avcodec_check_dimensions(avctx, w, h) < 0)
        return -1;
    x = bytestream_get_le16(&buf);
    y = bytestream_get_le16(&buf);
    // skip bottom right position, it gives no new information
    bytestream_get_le16(&buf);
    bytestream_get_le16(&buf);
    rlelen = bytestream_get_le16(&buf);

    // allocate sub and set values
    if (!sub->rects) {
        sub->rects = av_mallocz(sizeof(AVSubtitleRect));
        sub->num_rects = 1;
    }
    av_freep(sub->rects[0].bitmap);
    sub->rects[0].x = x; sub->rects[0].y = y;
    sub->rects[0].w = w; sub->rects[0].h = h;
    sub->rects[0].linesize = w;
    sub->rects[0].bitmap = av_malloc(w * h);
    sub->rects[0].nb_colors = 4;
    sub->rects[0].rgba_palette = av_malloc(sub->rects[0].nb_colors * 4);

    // read palette
    for (i = 0; i < sub->rects[0].nb_colors; i++)
        sub->rects[0].rgba_palette[i] = bytestream_get_be24(&buf);

    // process RLE-compressed data
    rlelen = FFMIN(rlelen, buf_end - buf);
    init_get_bits(&gb, buf, rlelen * 8);
    bitmap = sub->rects[0].bitmap;
    for (y = 0; y < h; y++) {
        for (x = 0; x < w; ) {
            int log2 = ff_log2_tab[show_bits(&gb, 8)];
            int run = get_bits(&gb, runbits[log2]);
            int colour = get_bits(&gb, 2);
            run = FFMIN(run, w - x);
            // run length 0 means till end of row
            if (!run) run = w - x;
            memset(bitmap, colour, run);
            bitmap += run;
            x += run;
        }
        align_get_bits(&gb);
    }
    *data_size = 1;
    return buf_size;
}

AVCodec xsub_decoder = {
    "xsub",
    CODEC_TYPE_SUBTITLE,
    CODEC_ID_XSUB,
    0,
    decode_init,
    NULL,
    NULL,
    decode_frame,
};