WebM Video FPS information is not part of video metadata when bit rates are exceeding 30MBPS

[pawan kumar]

I am part of team that build API's to capture video using Chrome browser. When a file is recorded with webM format and with video bit rate setting is greater than 30MBPS, there is no FPS information as part of metadata. 

If it was using VBR, I believe it should mention. that as part of the metadata. 

Would there be any association of bit rate causing webm encoder to switch from CBR to VBR?? 

[James Zern]

Hi,

The WebCodecs interface has an option for CBR/VBR [1]. How are you capturing and encoding the video?

[1] https://source.chromium.org/chromium/chromium/src/+/main:media/video/vpx_video_encoder.cc;l=114;drc=86223e421af31a52ac959ae84b71405a64a9092b

 

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[pawan kumar]

Hello James,

I think there was a typo in the question. So posting it again -

I am part of team that build API's to capture video using Chrome browser. When a file is recorded with webM format and with video bit rate setting is greater than 30MBPS, there is no FPS information as part of metadata. 

If it was using Variable frame rate, should that be mentioned as part of the video metadata? 

Would there be any association of bit rate setting causing webm encoder to switch from constant frame rate to variable frame rate? 

I think our development team is using chrome API's to capture video and audio from builtin camera and microphone devices and encoding using chrome natively supported webm.

Regards,

-Pavan.

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[James Zern]

On Sun, Mar 27, 2022 at 8:42 PM pawan kumar <pawan...@gmail.com> wrote:

Hello James,

I think there was a typo in the question. So posting it again -

I am part of team that build API's to capture video using Chrome browser. When a file is recorded with webM format and with video bit rate setting is greater than 30MBPS, there is no FPS information as part of metadata. 

The capture may be variable frame rate, the webm file format supports this. Do you have an example you could provide that has the metadata you're looking for? I don't think the bitrate should affect this.

 

If it was using Variable frame rate, should that be mentioned as part of the video metadata? 

Would there be any association of bit rate setting causing webm encoder to switch from constant frame rate to variable frame rate? 

I think our development team is using chrome API's to capture video and audio from builtin camera and microphone devices and encoding using chrome natively supported webm.

So that would be through the Media Stream Recording API [2]? There's nothing in libvpx that would make this switch and the encoder wrapper in MSR expects VBR [3].

[2] https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/mediarecorder/

[3] https://source.chromium.org/chromium/chromium/src/+/refs/tags/99.0.4844.94:third_party/blink/renderer/modules/mediarecorder/vpx_encoder.cc;l=276;drc=53fd6ee4378195e3252cd39f05663e0fc472362d

 

Regards,

-Pavan.

On Sat, Mar 26, 2022 at 4:54 AM 'James Zern' via WebM Discussion <webm-d...@webmproject.org> wrote:

Hi,

On Thu, Mar 24, 2022 at 7:44 PM pawan kumar <pawan...@gmail.com> wrote:

I am part of team that build API's to capture video using Chrome browser. When a file is recorded with webM format and with video bit rate setting is greater than 30MBPS, there is no FPS information as part of metadata. 

If it was using VBR, I believe it should mention. that as part of the metadata. 

Would there be any association of bit rate causing webm encoder to switch from CBR to VBR?? 

The WebCodecs interface has an option for CBR/VBR [1]. How are you capturing and encoding the video?

[1] https://source.chromium.org/chromium/chromium/src/+/main:media/video/vpx_video_encoder.cc;l=114;drc=86223e421af31a52ac959ae84b71405a64a9092b

 

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[pawan kumar]

Hello James,

I am attaching the recorded media with and without FPS information.  Adding in Terence  and Vincent as well from my team to follow the discussion.

 sandbox_30FPS.webm

 sandbox_noFPS.webm

The only difference between them was the video bitrate setting applied. Any setting >30 MBPS would render the output file without the FPS infro.

Regards,

-Pavan.

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[James Zern]

On Tue, Mar 29, 2022 at 3:59 PM pawan kumar <pawan...@gmail.com> wrote:

Hello James,

I am attaching the recorded media with and without FPS information.  Adding in Terence  and Vincent as well from my team to follow the discussion.

 sandbox_30FPS.webm

 sandbox_noFPS.webm

The only difference between them was the video bitrate setting applied. Any setting >30 MBPS would render the output file without the FPS infro.

Thanks for posting the clips. Where do you see the fps reported? I don't see a FrameRate element in the Track header for either [1] and there likely shouldn't be since this is variable rate input, see below.

The noFPS clip actually has timestamps closer to 30fps than the 30FPS clip, at least looking quickly at the beginning of the file.

$ mkvinfo -v sandbox_30FPS.webm |grep 'track number 2' |head
| + Simple block: key, track number 2, 1 frame(s), timestamp 00:00:00.000000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.028000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.061000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.091000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.124000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.157000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.190000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.219000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.251000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.285000000
$ mkvinfo -v sandbox_noFPS.webm |grep 'track number 2' |head
| + Simple block: key, track number 2, 1 frame(s), timestamp 00:00:00.000000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.033000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.067000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.096000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.133000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.163000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.199000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.231000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.267000000
| + Simple block: track number 2, 1 frame(s), timestamp 00:00:00.299000000

[1] https://matroska.org/technical/elements.html

 

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