x264源码分析三：x264_slices_write和x264_slice_write函数分析

下面将分析x264编码的核心算法部分：

首先先了解一下x264中比特流的层次结构：每个比特都隶属于某个句法元素，句法元素被组织成有层次的结构，分别描述各个层次的信息。如下图所示：

x264分层结构由五层组成，分别是序列参数集、图像参数集、片（Slice）、和宏块和子块。参数集是一个独立的数据单位，不依赖于参数集外的其它句法元素。下图描述了参数集与参数集外的句法元素之间的关系。

而本文所要分析的就是对应的片编码。下面看源代码：

static void *x264_slices_write( x264_t *h )
{
    int i_slice_num = 0;
    int last_thread_mb = h->sh.i_last_mb;

    /* init stats */
    //初始化一些状态
    memset( &h->stat.frame, 0, sizeof(h->stat.frame) );
    h->mb.b_reencode_mb = 0;
    //循环每一个slice
    while( h->sh.i_first_mb + SLICE_MBAFF*h->mb.i_mb_stride <= last_thread_mb )
    {
        h->sh.i_last_mb = last_thread_mb;
        if( !i_slice_num || !x264_frame_new_slice( h, h->fdec ) )
        {
            if( h->param.i_slice_max_mbs )
            {
                if( SLICE_MBAFF )
                {
                    // convert first to mbaff form, add slice-max-mbs, then convert back to normal form
                    int last_mbaff = 2*(h->sh.i_first_mb % h->mb.i_mb_width)
                        + h->mb.i_mb_width*(h->sh.i_first_mb / h->mb.i_mb_width)
                        + h->param.i_slice_max_mbs - 1;
                    int last_x = (last_mbaff % (2*h->mb.i_mb_width))/2;
                    int last_y = (last_mbaff / (2*h->mb.i_mb_width))*2 + 1;
                    h->sh.i_last_mb = last_x + h->mb.i_mb_stride*last_y;
                }
                else
                {
                    h->sh.i_last_mb = h->sh.i_first_mb + h->param.i_slice_max_mbs - 1;
                    if( h->sh.i_last_mb < last_thread_mb && last_thread_mb - h->sh.i_last_mb < h->param.i_slice_min_mbs )
                        h->sh.i_last_mb = last_thread_mb - h->param.i_slice_min_mbs;
                }
                i_slice_num++;
            }
            else if( h->param.i_slice_count && !h->param.b_sliced_threads )
            {
                int height = h->mb.i_mb_height >> PARAM_INTERLACED;
                int width = h->mb.i_mb_width << PARAM_INTERLACED;
                i_slice_num++;
                h->sh.i_last_mb = (height * i_slice_num + h->param.i_slice_count/2) / h->param.i_slice_count * width - 1;
            }
        }
        h->sh.i_last_mb = X264_MIN( h->sh.i_last_mb, last_thread_mb );
        //利用函数指针进行具体的块编码操作
        if( x264_stack_align( x264_slice_write, h ) )
            goto fail;
        h->sh.i_first_mb = h->sh.i_last_mb + 1;
        // if i_first_mb is not the last mb in a row then go to the next mb in MBAFF order
        if( SLICE_MBAFF && h->sh.i_first_mb % h->mb.i_mb_width )
            h->sh.i_first_mb -= h->mb.i_mb_stride;
    }

    return (void *)0;

fail:
    /* Tell other threads we're done, so they wouldn't wait for it */
    if( h->param.b_sliced_threads )
        x264_threadslice_cond_broadcast( h, 2 );
    return (void *)-1;
}

下面分析具体的块编码函数源代码：

static intptr_t x264_slice_write( x264_t *h )
{
    int i_skip;
    int mb_xy, i_mb_x, i_mb_y;
    /* NALUs other than the first use a 3-byte startcode.
     * Add one extra byte for the rbsp, and one more for the final CABAC putbyt