解析slice header来判断当前slice的帧类型的代码实现

下面是一个基于H.264标准的slice header解析代码示例，可以用来判断当前slice的帧类型：

typedef struct
{
    int first_mb_in_slice;
    int slice_type;
    int pic_parameter_set_id;
    int frame_num;
    int field_pic_flag;
    int bottom_field_flag;
    int idr_pic_id;
    int pic_order_cnt_lsb;
    int delta_pic_order_cnt_bottom;
    int delta_pic_order_cnt0;
    int delta_pic_order_cnt1;
    int redundant_pic_cnt;
    int direct_spatial_mv_pred_flag;
    int num_ref_idx_active_override_flag;
    int num_ref_idx_l0_active_minus1;
    int num_ref_idx_l1_active_minus1;
    int cabac_init_idc;
    int slice_qp_delta;
    int sp_for_switch_flag;
    int slice_qs_delta;
    int disable_deblocking_filter_idc;
    int slice_alpha_c0_offset_div2;
    int slice_beta_offset_div2;
    int slice_group_change_cycle;
} slice_header_t;

int get_slice_type(slice_header_t *slice_header)
{
    int slice_type = slice_header->slice_type;
    if (slice_type == 0 || slice_type == 5)
    {
        return I_SLICE;
    }
    else if (slice_type == 1 || slice_type == 6)
    {
        return P_SLICE;
    }
    else if (slice_type == 2 || slice_type == 7)
    {
        return B_SLICE;
    }
    else if (slice_type == 3 || slice_type == 8)
    {
        return SP_SLICE;
    }
    else if (slice_type == 4 || slice_type == 9)
    {
        return SI_SLICE;
    }
    else
    {
        return -1;
    }
}

int parse_slice_header(uint8_t *data, slice_header_t *slice_header)
{
    bitstream_t bs;
    init_bitstream(&bs, data);

    slice_header->first_mb_in_slice = read_ue(&bs);
    slice_header->slice_type = read_ue(&bs);
    slice_header->pic_parameter_set_id = read_ue(&bs);
    slice_header->frame_num = read_bits(&bs, 16);
    slice_header->field_pic_flag = read_bits(&bs, 1);
    if (slice_header->field_pic_flag)
    {
        slice_header->bottom_field_flag = read_bits(&bs, 1);
    }
    if (slice_header->nal_unit_type == NAL_IDR_SLICE)
    {
        slice_header->idr_pic_id = read_ue(&bs);
    }
    if (slice_header->pic_order_cnt_type == 0)
    {
        slice_header->pic_order_cnt_lsb = read_bits(&bs, slice_header->log2_max_pic_order_cnt_lsb_minus4 + 4);
        if (slice_header->delta_pic_order_always_zero_flag)
        {
            slice_header->delta_pic_order_cnt_bottom = 0;
            slice_header->delta_pic_order_cnt0 = 0;
            slice_header->delta_pic_order_cnt1 = 0;
        }
        else
        {
            if (slice_header->delta_pic_order_cnt_bottom_exist)
            {
                slice_header->delta_pic_order_cnt_bottom = read_se(&bs);
            }
            if (slice_header->delta_pic_order_cnt0_exist)
            {
                slice_header->delta_pic_order_cnt0 = read_se(&bs);
            }
            if (slice_header->delta_pic_order_cnt1_exist)
            {
                slice_header->delta_pic_order_cnt1 = read_se(&bs);
            }
        }
    }
    else if (slice_header->pic_order_cnt_type == 1)
    {
        // TODO: implement
    }

    slice_header->redundant_pic_cnt = read_ue(&bs);
    if (is_slice_type_B(slice_header->slice_type))
    {
        slice_header->direct_spatial_mv_pred_flag = read_bits(&bs, 1);
    }
    if (is_slice_type_P(slice_header->slice_type) || is_slice_type_SP(slice_header->slice_type) || is_slice_type_B(slice_header->slice_type))
    {
        slice_header->num_ref_idx_active_override_flag = read_bits(&bs, 1);
        if (slice_header->num_ref_idx_active_override_flag)
        {
            slice_header->num_ref_idx_l0_active_minus1 = read_ue(&bs);
            if (is_slice_type_B(slice_header->slice_type))
            {
                slice_header->num_ref_idx_l1_active_minus1 = read_ue(&bs);
            }
        }
    }
    if (slice_header->slice_type == 2 || slice_header->slice_type == 4 || slice_header->slice_type == 7 || slice_header->slice_type == 9)
    {
        slice_header->sp_for_switch_flag = read_bits(&bs, 1);
    }
    if (slice_header->slice_type == 1 || slice_header->slice_type == 6 || slice_header->slice_type == 3 || slice_header->slice_type == 8)
    {
        slice_header->slice_qs_delta = read_se(&bs);
    }
    if (slice_header->use_default_scaling_matrix_flag)
    {
        // TODO: implement
    }
    else
    {
        // TODO: implement
    }
    if (slice_header->disable_deblocking_filter_idc != 1)
    {
        slice_header->slice_alpha_c0_offset_div2 = read_se(&bs);
        slice_header->slice_beta_offset_div2 = read_se(&bs);
    }
    if (slice_header->slice_group_map_type >= 3 && slice_header->slice_group_map_type <= 5)
    {
        // TODO: implement
    }
    if (slice_header->slice_group_change_direction_flag)
    {
        slice_header->slice_group_change_rate_minus1 = read_ue(&bs);
    }
    if (slice_header->slice_group_change_direction_flag || (slice_header->slice_group_change_cycle != 0 && slice_header->pic_parameter_set_id == 0))
    {
        // TODO: implement
    }
    return 0;
}

在这个代码示例中，我们定义了一个slice_header_t结构体来保存slice header中的各个参数。然后我们实现了一个parse_slice_header函数，用来解析slice header中各个参数的值。最后，我们定义了一个get_slice_type函数来根据slice_header中的slice_type参数来判断当前slice的帧类型。