Everyday One Step Forward

Outline

Doc of MB Tree

Process

The x264 decide slice type in a GOP structure unit, which include one reference frame (I,P) and no / several non-reference frame (B). The processed frame will be shift out frame lookahead frame list and put into frame's current list.

if( !h->frames.current[0] )
    x264_lookahead_get_frames( h );

The slice type analyse and decide was first handle in x264_lookahead_get_frames. The function will call x264_slicetype_decide and x264_slicetype_analyse.

x264_stack_align( x264_slicetype_decide, h );
/* For MB-tree and VBV lookahead, we have to perform propagation analysis on I-frames too. */
if( h->lookahead->b_analyse_keyframe && IS_X264_TYPE_I( h->lookahead->last_nonb->i_type ) )
    x264_stack_align( x264_slicetype_analyse, h, shift_frames );

x264_slicetype_decide

1. The lookahead routine accumulate many frame in it's queue for analyse. The frames in list next in lookahead structure are waiting for process. The first part of x264_slicetype_decide is to calculate the duration of each frame in the lookahead list next

2. If user choose 2-pass rate control the slice type has been analysed and decide in the 1st-pass. Thus, at current stage, just need to read the result of the 1st-pass. Otherwise, call x264_slicetype_analyse, which will be analyse in the following section.

   if( h->param.rc.b_stat_read )
       x264_ratecontrol_slice_type();
   else if(...)
       x264_slicetype_analyse();
   

   As stated above, slice type analyse will determine slice type in a gop.

3. Calculate the frame costs ahead of time for x264_rc_analyse_slice while still have lowres. The frame cost will be used for rate control.

x264_slicetype_analyse

The x264_slicetype_analyse focus on the following tasks:

1. Decide the frame type. For non-b setting, actully, the slice type decision is straightforward that keyframe (IDR/I) followed by interframe (P).

   For sequence with B frame, x264 propose two B adaptive algorithm X264_B_ADAPT_FAST and X264_B_ADAPT_TRELLIS and one non-adaptive algorithm X264_B_ADAPT_NONE.

   For X264_B_ADAPT_NONE, the encoder will follow the gop structure constructed by keyframe intervel, bframe number, etc. For example, if user set --bframes 2 and --keyint 30. The encoded gop structure will be IPBBPBBPBB... with IDR picture appear every 30 frames.

   The X264_B_ADAPT_TRELLIS is complicate and slow, we will discuss in the future.