[FFmpeg-trac] #8328(undetermined:new): QSV gives much worse PSNR/SSIM for HEVC transcode than VA-API or MediaSDK

FFmpeg trac at avcodec.org
Thu Nov 7 18:31:11 EET 2019


#8328: QSV gives much worse PSNR/SSIM for HEVC transcode than VA-API or MediaSDK
-------------------------------------+-------------------------------------
             Reporter:  eero-t       |                    Owner:
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------

Comment (by eero-t):

 Recap of the findings:
 * Individual frames contents are fine, checked by converting both files to
 raw and calculating PSNR
 * Forcing frame-rate for the QSV encoded file in PSNR calculation with "-r
 60" also gives good PSNR, although encoded video frame-rate is already 60
 * When calculating PSNR for more frames, PSNR gets worse

 One explanation that I could think for this is:
 * Encoded video includes individual frame times (offsets?) in addition to
 overall video frame rate
 * FFmpeg does PSNR comparison by aligning video frames to a timeline based
 on individual frame timings (unless one specifies -r option), instead of
 comparing frames as-is
 * Timings for individual encoded frames drift (are truncated or rounded
 up?) with QSV when it uses higher B-frame count => bug

 Linjie, could you check my theory; get the individual frame timings for
 both videos out of FFmpeg and check whether they differ?

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Ticket URL: <https://trac.ffmpeg.org/ticket/8328#comment:11>
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