[x264-devel] Possible bug in SPS writing

[Alexander Bokovikov]

Hi, All,

I'm sorry, if it is my misunderstanding, but I can't understand what's
happening. I'm using FFMPEG with libX264 to produce FLV files. My FLV player
obtains movie frame size from the SPS, storeg in the
AVCDecoderConfigurationRecord.

All goes OK, but the only unclear point. Here is my command line (Win32):

ffmpeg -i %1 -f flv -s 272x208 -vcodec libx264 -coder 1 -flags +loop -cmp
+chroma -partitions +parti8x8+parti4x4+partp8x8+partb8x8 -me_method
umh -subq 8 -me_range 16 -g 20 -keyint_min 15 -sc_threshold 40 -i_qfactor
0.71 -b_strategy 2 -qcomp 0.6 -qmin 10 -qmax 51 -qdiff 4 -bf 3 -refs
4 -directpred 3 -trellis 1 -flags2
+wpred+mixed_refs+dct8x8+fastpskip -wpredp 2 -crf 25 -acodec libmp3lame -ab
64000 -ar 22050 -ac 1 -threads 0 -y %2

ffmpeg was built with the latest libx264 SVN.

As you can see, here is libx264-hq profile parameters, sent in the command
line. The problem appears when I set "+dct8x8" in the flags2. Then I have
strange (for me) data in the pic_width_in_mbs_minus1 and 
pic_height_in_map_units_minus1 fields of SPS. The first of them is always 10 
regardless to the actual width of movie (which is defined by -s parameter 
and is reported correctly by libx264 screen log). The second always is equal 
to the _WIDTH_ of frame in macroblocks, i.e. it is width / 16. I tried 
different frame sizes and always get such results.

All is pretty correct, if I set "-dct8x8" in flags2. But most of x264 
profiles have this flag set to "+".

I'm trying to understand if it is my misunderstanding or it is a real bug in 
the libX264. BTW, VLC player shows such files pretty correctly, but I don't 
know how it obtains the frame size data.

Here are some data saved in the SPS record for "+dct8x8" flag being set and 
movie of 272 x 208 pixels in frame size:

pic_width_in_mbs_minus1 + 1 = 10
pic_height_in_map_units_minus1 + 1 = 17
frame_mbs_only_flag = 0
mb_adaptive_frame_field_flag = 0
direct_8x8_inference_flag = 0
frame_cropping_flag = 1
frame_crop_left_offset = 0
frame_crop_right_offset = 2
frame_crop_top_offset = 0
frame_crop_bottom_offset = 1

The same source movie, encoded with all the same parameters but "-dct8x8" 
flag procuses such data:

pic_width_in_mbs_minus1 + 1 = 17
pic_height_in_map_units_minus1 + 1 = 13
frame_mbs_only_flag = 1
mb_adaptive_frame_field_flag = 0
direct_8x8_inference_flag = 1
frame_cropping_flag = 0
frame_crop_left_offset = 0
frame_crop_right_offset = 0
frame_crop_top_offset = 0
frame_crop_bottom_offset = 0

The first unclear thing is why cropping is used in the first case, where no 
cropping is required because of picture size being multiple of 16.

Any help would be appreciated!

Best regards,
Alex