some concepts for b_adaptive

: · Concepts

            On Fri, 1 Jul 2005, Tuukka Toivonen wrote:

            I'm trying to understand b_adapt algorithm from the latest
svn, but it's hard to make sense of it.

It looks like it encodes a lowres version of some frames
using different frame types (P/B), computes a score using
SATD and the lowres frame, and does a decision based on
that.

There seems to be two algorithms, first disabled (#if 0).
What means "BFS"?
What is this "path" thing?

            "BFS" = breadth first search

            The idea of the disabled algo is: Given some set of future frames (more than max_b_frames), try all possible combinations of frame types, and use the one with minimum total SATD. As stated that would take exponential time, but that's where BFS comes in: Since I use only 1 past reference frame and I'm not actually encoding the frames, to calculate the cost of frame N I don't need to care about the types before the previous P-frame. A "path" here is a list of frame types. paths[i] is the lowest cost path that ends on frame i (where i is counted starting from the last encoded frame). 
            Example:
Let buffer size = 5
    paths[1] = P
    paths[2] = BP
    paths[3] = BPP
    paths[4] = PBBP

            To compute paths[5], we try paths[1]+BBBP vs paths[2]+BBP vs paths[3]+BP vs paths[4]+P. Say the lowest cost is paths[2]+BBP = BPBBP. We have no more frames to consider, so this is the best estimate of optimal frametypes. So we encode the first bunch (BP), read in the next two input frames, and call x264_slicetype_analyse again. It can then calculate the new paths[1..3] for free, since the frame costs are cached. 

            What the indices are in frame->i_cost_est[16][16]?

            The 1st index is the distance from the current frame to list0ref0. The 2nd index is 0 for P-frames or distance to list1ref0 for B-frames. 

            What are the last three parameters p0,p1,b in x264_slicetype_frame_cost?

            p0 is the frame number of list0ref0, where counting starts with #0 = the last frame coded so far, and 1+ are the frames whose types are to be decided. 
            b is the frame number of the frame currently being analysed.
p1 is the frame number of list1ref0, or equal to b for P-frames.