FindAVCDimensions

// Determine video dimensions from the sequence parameterset.
void FindAVCDimensions(
        const sp<ABuffer> &seqParamSet, int32_t *width, int32_t *height) {
    ABitReader br(seqParamSet->data() + 1, seqParamSet->size() - 1);

    unsigned profile_idc = br.getBits(8);
    br.skipBits(16);
    parseUE(&br);  // seq_parameter_set_id

    unsigned chroma_format_idc = 1;  // 4:2:0 chroma format

    if (profile_idc == 100 || profile_idc == 110
            || profile_idc == 122 || profile_idc == 244
            || profile_idc == 44 || profile_idc == 83 || profile_idc == 86) {
        chroma_format_idc = parseUE(&br);
        if (chroma_format_idc == 3) {
            br.skipBits(1);  // residual_colour_transform_flag
        }
        parseUE(&br);  // bit_depth_luma_minus8
        parseUE(&br);  // bit_depth_chroma_minus8
        br.skipBits(1);  // qpprime_y_zero_transform_bypass_flag
        CHECK_EQ(br.getBits(1), 0u);  // seq_scaling_matrix_present_flag
    }

    parseUE(&br);  // log2_max_frame_num_minus4
    unsigned pic_order_cnt_type = parseUE(&br);

    if (pic_order_cnt_type == 0) {
        parseUE(&br);  // log2_max_pic_order_cnt_lsb_minus4
    } else if (pic_order_cnt_type == 1) {
        // offset_for_non_ref_pic, offset_for_top_to_bottom_field and
        // offset_for_ref_frame are technically se(v), but since we are
        // just skipping over them the midpoint does not matter.

        br.getBits(1);  // delta_pic_order_always_zero_flag
        parseUE(&br);  // offset_for_non_ref_pic
        parseUE(&br);  // offset_for_top_to_bottom_field

        unsigned num_ref_frames_in_pic_order_cnt_cycle = parseUE(&br);
        for (unsigned i = 0; i < num_ref_frames_in_pic_order_cnt_cycle; ++i) {
            parseUE(&br);  // offset_for_ref_frame
        }
    }

    parseUE(&br);  // num_ref_frames
    br.getBits(1);  // gaps_in_frame_num_value_allowed_flag

    unsigned pic_width_in_mbs_minus1 = parseUE(&br);
    unsigned pic_height_in_map_units_minus1 = parseUE(&br);
    unsigned frame_mbs_only_flag = br.getBits(1);

    *width = pic_width_in_mbs_minus1 * 16 + 16;

    *height = (2 - frame_mbs_only_flag)
        * (pic_height_in_map_units_minus1 * 16 + 16);

    if (!frame_mbs_only_flag) {
        br.getBits(1);  // mb_adaptive_frame_field_flag
    }

    br.getBits(1);  // direct_8x8_inference_flag

    if (br.getBits(1)) {  // frame_cropping_flag
        unsigned frame_crop_left_offset = parseUE(&br);
        unsigned frame_crop_right_offset = parseUE(&br);
        unsigned frame_crop_top_offset = parseUE(&br);
        unsigned frame_crop_bottom_offset = parseUE(&br);

        unsigned cropUnitX, cropUnitY;
        if (chroma_format_idc == 0  /* monochrome */) {
            cropUnitX = 1;
            cropUnitY = 2 - frame_mbs_only_flag;
        } else {
            unsigned subWidthC = (chroma_format_idc == 3) ? 1 : 2;
            unsigned subHeightC = (chroma_format_idc == 1) ? 2 : 1;

            cropUnitX = subWidthC;
            cropUnitY = subHeightC * (2 - frame_mbs_only_flag);
        }

       SLOGV("frame_crop = (%u, %u, %u, %u), cropUnitX = %u, cropUnitY = %u",
             frame_crop_left_offset, frame_crop_right_offset,
             frame_crop_top_offset, frame_crop_bottom_offset,
             cropUnitX, cropUnitY);

        *width -=
            (frame_crop_left_offset + frame_crop_right_offset) * cropUnitX;
        *height -=
            (frame_crop_top_offset + frame_crop_bottom_offset) * cropUnitY;
    }
}