@TEMPLATE encoder_tmpl.c
Two Pass Encoder
================
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION
This is an example of a two pass encoder loop. It takes an input file in
YV12 format, passes it through the encoder twice, and writes the compressed
frames to disk in IVF format. It builds upon the simple_encoder example.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ INTRODUCTION


Twopass Variables
-----------------
Twopass mode needs to track the current pass number and the buffer of
statistics packets.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_VARS
int                  pass;
vpx_fixed_buf_t      stats = {0};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_VARS


Updating The Configuration
---------------------------------
In two pass mode, the configuration has to be updated on each pass. The
statistics buffer is passed on the last pass.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_LOOP_BEGIN
for(pass=0; pass<2; pass++) {
    frame_cnt = 0;

    if(pass == 0)
        cfg.g_pass = VPX_RC_FIRST_PASS;
    else {
        cfg.g_pass              = VPX_RC_LAST_PASS;
        cfg.rc_twopass_stats_in = stats;
    }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_LOOP_BEGIN


Encoding A Frame
----------------
Encoding a frame in two pass mode is identical to the simple encoder
example, except the deadline is set to VPX_DL_BEST_QUALITY to get the
best quality possible. VPX_DL_GOOD_QUALITY could also be used.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ENCODE_FRAME
frame_avail = read_frame(infile, &raw);
if(vpx_codec_encode(&codec, frame_avail? &raw : NULL, frame_cnt,
                    1, flags, VPX_DL_BEST_QUALITY))
    die_codec(&codec, "Failed to encode frame");
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ENCODE_FRAME


Processing Statistics Packets
-----------------------------
Each packet of type `VPX_CODEC_CX_FRAME_PKT` contains the encoded data
for this frame. We write a IVF frame header, followed by the raw data.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PROCESS_STATS
case VPX_CODEC_STATS_PKT:
    stats.buf = realloc(stats.buf, stats.sz
                        + pkt->data.twopass_stats.sz);
    if(!stats.buf)
        die("Memory reallocation failed.\n");
    memcpy((char*)stats.buf + stats.sz,
           pkt->data.twopass_stats.buf,
           pkt->data.twopass_stats.sz);
    stats.sz +=  pkt->data.twopass_stats.sz;
    break;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ PROCESS_STATS


Pass Progress Reporting
-----------------------------
It's sometimes helpful to see when each pass completes.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_LOOP_END
    printf("Pass %d complete.\n", pass+1);
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ TWOPASS_LOOP_END
