Artifacts in encoded output

[jamrial]

https://0x0.st/zFGa.bz2

rav1e --min-keyint 24 --keyint 24 -b 1000 -l 240 --threads 6 --tiles 4 zFGa.y4m --output zFGa.ivf

Artifacts can be seen around the seven seconds mark. Both keyint arguments are needed to reproduce this.
Tested on a Win10 x86_64 machine using an AVX2 cpu.

[jamrial]

Encoded file
zFGa.zip

Screenshots of the broken frames (including one correct frame before them, plus the only correct frame in between them). They are frames 170, 171 and 173 out of the 240 total frames.

[shssoichiro]

There does appear to be a desync going on here, reproduced locally using (the lazy manual version of) our desync script:

❯ dav1d -i zFGa.ivf -o zFGa_dec.y4m
❯ tail -n+2 zFGa_dec.y4m > /tmp/dec_file
❯ tail -n+2 zFGa_rec.y4m > /tmp/rec_file
❯ cmp /tmp/dec_file /tmp/rec_file || (printf '\e[1;31m%-6s\e[m\n\n' 'Desync detected!!!' && exit 1)
cmp: EOF on /tmp/dec_file after byte 242699040, in line 287
Desync detected!!!

[tdaede]

Looks like the wrong reference frames being used. Maybe something to do with forced keyframes (from scene detection)?

[shssoichiro]

I lied, it's not a desync (my repro was wrong somehow). The glitch is also present in the reconstruction.

[shssoichiro]

I wonder if it's just the single-pass RDO messing up? These frames are being like "Hey, I'd like some bits please" and the encoder is just like "All out, have some skip blocks".

The glitch doesn't appear if I run in qp-based mode (tested at qp 100, which gives a lower overall filesize than 1000 kbps).

[dwbuiten]

Is it possible this is just the same "1-pass ratecontrol is bad" problem that has been known for a long time? I think these could be caused by the qindex dropping to 255 and coding a lot of skip blocks.

[dwbuiten]

Update: Yes it is.

The frames in question:

[AVBSFContext @ 0xbf1840] 78          base_q_idx                                           10010001 = 145
[AVBSFContext @ 0xbf1840] 39          base_q_idx                                           01001111 = 79
[AVBSFContext @ 0xbf1840] 79          base_q_idx                                           11111111 = 255
[AVBSFContext @ 0xbf1840] 79          base_q_idx                                           11111111 = 255
[AVBSFContext @ 0xbf1840] 78          base_q_idx                                           11111111 = 255
[AVBSFContext @ 0xbf1840] 78          base_q_idx                                           10011011 = 155
[AVBSFContext @ 0xbf1840] 79          base_q_idx                                           10000011 = 131
[AVBSFContext @ 0xbf1840] 79          base_q_idx                                           10011110 = 158