Segmentation Fault accessing qpscale_table in AVFrame - c++11

Question

I'm modifying this file slightly: https://gist.github.com/yohhoy/f0444d3fc47f2bb2d0e2
This code decodes a video and makes opencv Mats out of the frame pixels as it goes.
In particular I only want to grab frames that have specific macroblock-related data. I'm attempting to get that data like this:
total_qp = get_total_qp(decframe->qscale_table, mb_width, mb_height, mb_stride);
However, whenever I try to access the data by iterating over that array, I get a segmentation fault:
static float get_total_qp(int8_t *qscale_table, int mb_width, int mb_height, int mb_stride)
{
int mb_count = mb_height * mb_width;
int y, x;
float qp_total = 0.0f;
for (y = 0; y < mb_height; y++) {
for (x = 0; x < mb_width; x++) {
qp_total += qscale_table[x + y * mb_stride]; <-- SEGFAULT here
}
}
return qp_total;
}
I've also tried sending in:
frame->qscale_table
and I've tried populating it, but this own't compile because it can't find that function:
int8_t *qscale_table = av_frame_get_qp_table(decframe->qscale_table, &mb_stride, &qscale_type);
So my question is this:
Given an AVFrame* how do I ensure that the qscale_table is populated and access it?

Answer 1

It turns out that the qpscale_table doesn't get exported onto the decoded frame after the decoding happens in h264dec.c.
In order to retrieve the values I had to modify the finalize_frame method in h264dec to export the qscale_table onto the frame, like so:
static int h264_export_qp_table(H264Context *h, AVFrame *f, H264Picture *p, int qp_type)
{
AVBufferRef *ref = av_buffer_ref(p->qscale_table_buf);
int offset = 2*h->mb_stride + 1;
if(!ref)
return AVERROR(ENOMEM);
av_assert0(ref->size >= offset + h->mb_stride * ((f->height+15)/16));
ref->size -= offset;
ref->data += offset;
return av_frame_set_qp_table(f, ref, h->mb_stride, f->qscale_type);
}
and add in the call into finalize_frame:
...
if (CONFIG_MPEGVIDEO) {
ff_print_debug_info2(h->avctx, dst, NULL,
out->mb_type,
out->qscale_table,
out->motion_val,
NULL,
h->mb_width, h->mb_height, h->mb_stride, 1);
// NT: make the qscale_table accessible!
h264_export_qp_table(h, dst, out, FF_QSCALE_TYPE_H264);
}
...
And then recompile FFmpeg using these instructions: https://trac.ffmpeg.org/wiki/CompilationGuide/Ubuntu