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vpxenc: prevent wraparound in the --rate-hist ringbuffer

For clips that are near 60fps and have a lot of alt refs, it's possible
that the ring buffer holding the previous frames sizes/pts could wrap
around, leading to a division by zero.

In addition to checking for this condition in the ring buffer loop,
the buffer size is made dependent on the actual frame rate in use,
rather than defaulting to 60, which should improve accuracy at frame
rates >= ~60.

Change-Id: If5a04d6e847316dc5f7504f25c01164cf9332be8

--- a/vpxenc.c

+++ b/vpxenc.c

@@ -1302,11 +1302,16 @@

 static void init_rate_histogram(struct rate_hist          *hist,

-                                const vpx_codec_enc_cfg_t *cfg)

+                                const vpx_codec_enc_cfg_t *cfg,

+                                const vpx_rational_t      *fps)

 {

     int i;

-    hist->samples = cfg->rc_buf_sz * 60 / 1000; // max 60 fps

+    /* Determine the number of samples in the buffer. Use the file's framerate

+     * to determine the number of frames in rc_buf_sz milliseconds, with an

+     * adjustment (5/4) to account for alt-refs

+     */

+    hist->samples = cfg->rc_buf_sz * 5 / 4 * fps->num / fps->den / 1000;

     hist->pts = calloc(hist->samples, sizeof(*hist->pts));

     hist->sz = calloc(hist->samples, sizeof(*hist->sz));

     for(i=0; i<RATE_BINS; i++)

@@ -1343,7 +1348,7 @@

         return;

     /* Sum the size over the past rc_buf_sz ms */

-    for(i = hist->frames; i > 0; i--)

+    for(i = hist->frames; i > 0 && hist->frames - i < hist->samples; i--)

     {

         int i_idx = (i-1) % hist->samples;

@@ -1699,8 +1704,6 @@

     memset(&stats, 0, sizeof(stats));

-    init_rate_histogram(&rate_hist, &cfg);

-

     for (pass = one_pass_only ? one_pass_only - 1 : 0; pass < arg_passes; pass++)

     {

         int frames_in = 0, frames_out = 0;

@@ -1827,6 +1830,8 @@

             else

                 vpx_img_alloc(&raw, arg_use_i420 ? VPX_IMG_FMT_I420 : VPX_IMG_FMT_YV12,

                               cfg.g_w, cfg.g_h, 1);

+

+            init_rate_histogram(&rate_hist, &cfg, &arg_framerate);

         }

         outfile = strcmp(out_fn, "-") ? fopen(out_fn, "wb")

-- 

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