static int mov_read_tkhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
{
    int i;
    int width;
    int height;
    int64_t disp_transform[2];
    int display_matrix[3][2];
    AVStream *st;
    MOVStreamContext *sc;
    int version;

    if (c->fc->nb_streams < 1)
        return 0;
    st = c->fc->streams[c->fc->nb_streams-1];
    sc = st->priv_data;

    version = avio_r8(pb);
    avio_rb24(pb); /* flags */
    /*
    MOV_TRACK_ENABLED 0x0001
    MOV_TRACK_IN_MOVIE 0x0002
    MOV_TRACK_IN_PREVIEW 0x0004
    MOV_TRACK_IN_POSTER 0x0008
    */

    if (version == 1) {
        avio_rb64(pb);
        avio_rb64(pb);
    } else {
        avio_rb32(pb); /* creation time */
        avio_rb32(pb); /* modification time */
    }
    st->id = (int)avio_rb32(pb); /* track id (NOT 0 !)*/
    avio_rb32(pb); /* reserved */

    /* highlevel (considering edits) duration in movie timebase */
    (version == 1) ? avio_rb64(pb) : avio_rb32(pb);
    avio_rb32(pb); /* reserved */
    avio_rb32(pb); /* reserved */

    avio_rb16(pb); /* layer */
    avio_rb16(pb); /* alternate group */
    avio_rb16(pb); /* volume */
    avio_rb16(pb); /* reserved */

    //read in the display matrix (outlined in ISO 14496-12, Section 6.2.2)
    // they're kept in fixed point format through all calculations
    // ignore u,v,z b/c we don't need the scale factor to calc aspect ratio
    for (i = 0; i < 3; i++) {
        display_matrix[i][0] = avio_rb32(pb);   // 16.16 fixed point
        display_matrix[i][1] = avio_rb32(pb);   // 16.16 fixed point
        avio_rb32(pb);           // 2.30 fixed point (not used)
    }

    width = avio_rb32(pb);       // 16.16 fixed point track width
    height = avio_rb32(pb);      // 16.16 fixed point track height
    sc->width = width >> 16;
    sc->height = height >> 16;

    //Assign clockwise rotate values based on transform matrix so that
    //we can compensate for iPhone orientation during capture.

    if (display_matrix[1][0] == -65536 && display_matrix[0][1] == 65536) {
         av_dict_set(&st->metadata, "rotate", "90", 0);
    }

    if (display_matrix[0][0] == -65536 && display_matrix[1][1] == -65536) {
         av_dict_set(&st->metadata, "rotate", "180", 0);
    }

    if (display_matrix[1][0] == 65536 && display_matrix[0][1] == -65536) {
         av_dict_set(&st->metadata, "rotate", "270", 0);
    }

    // transform the display width/height according to the matrix
    // skip this if the display matrix is the default identity matrix
    // or if it is rotating the picture, ex iPhone 3GS
    // to keep the same scale, use [width height 1<<16]
    if (width && height &&
        ((display_matrix[0][0] != 65536  ||
          display_matrix[1][1] != 65536) &&
         !display_matrix[0][1] &&
         !display_matrix[1][0] &&
         !display_matrix[2][0] && !display_matrix[2][1])) {
        for (i = 0; i < 2; i++)
            disp_transform[i] =
                (int64_t)  width  * display_matrix[0][i] +
                (int64_t)  height * display_matrix[1][i] +
                ((int64_t) display_matrix[2][i] << 16);

        //sample aspect ratio is new width/height divided by old width/height
        st->sample_aspect_ratio = av_d2q(
            ((double) disp_transform[0] * height) /
            ((double) disp_transform[1] * width), INT_MAX);
    }
    return 0;
}