How to convert VP8 track with different frame resolution to h264

I have a .webm file with VP8 track, recorded from WebRTC stream by external service (TokBox Archiving). The stream is adaptive, so each frame in track could have different resolution. Most players (in webkit browsers) use video resolution from track description (which is always 640x480) and scale frames to this resolution. Firefox and VLC player uses real frame resolution, changing video resolution respectively.

I want to achieve 2 goals:

1. play this video in Internet Explorer 9+ without additional plugin installation.
2. change frames resolution to one fixed resolution, so the video will look identically in different browsers.

So, my plan is:

• extract frames from source webm file to images with real frame resolution (e.g. PNG or BMP) (how could I do that?)
• find max width and max height of images
• add black padding to images, so smaller frames will be in the center of a new frame (of size MAX_WIDHTxMAX_HEIGHT)
• combine images to h264 track using ffmpeg

Is all correct? How can I achieve this? Can this algorithm be optimized some way?

I tried ffmpeg to extract images, but it does not parse real frame resolution, using resolution from track header. I think some libwebm functions can help me (to parse frame headers and extract images). Maybe someone has some code snippets to do this?

Example .webm (download source, do not play google-converted version): https://drive.google.com/file/d/0BwFZRvYNn9CKcndhMzlVa0psX00/view?usp=sharing

Official description of adaptive stream from TokBox support: https://support.tokbox.com/hc/en-us/community/posts/206241666-Archived-video-resolution-is-supposed-to-be-720x1280-but-reports-as-640x480

If you run

ffprobe -show_entries frame=width,height -of compact=p=0:nk=1 video.webm

you will get an output that looks like this:

1254|800
1058|800
890|800
774|800
672|800

The left column is each frame's actual width and the right column has the height. You can then check the max values in each column, to use for canvas size.

Then run

ffmpeg -i video.webm -vf pad=MAXW:MAXH:(MAXW-iw)/2:(MAXH-ih)/2 out.mp4

where MAXW and MAXH should be replaced with the values you discovered.