VC-1 for screen recordings (New Expression Encoder encodes up)

Posted By: Ben Waggoner | Jan 15th @ 11:02 AM

A few months ago I posted links to my Expression Encoder training. However, I didn't encode the original clips, and being a little obsessive I wanted to see wha I could pull off myself with VC-1 for screen shots.

You can see the results of my work here.

Some tips for encoding screen shots:

I-Frame DQuant. While I normally don't recommend DQuant for low bitrates with the VC-1 Encoder SDK, it makes sense to apply it to I-frames for screen recordings, since we don't need very many of them, and with so little motion, that really helps improve the quality of all frames based on the reference frame.
Long GOPs (distance between keyframes) can really help efficiency, since keyframes can take up the majority of bits in the files.
Use B-frames. >1 can pay off a lot in improved efficiency. And more B-frames improve random access when using long GOPs, since B-frames don't need to be decoded when jumping to a particular frame (just the previous I-frame and all P-frames between that and the current frame, plus the following P-frame for a B-Frame). I think I used 3 for these clips.
Chroma Search! A full precision chroma search can pay off for colorful content like screen shots.
In-Loop, but no Overlap filter. The Ovelap filter softens the image some, which is better than getting blocky artifacts at low bitrates with natural images, but looks weird with crisp static content like screen shots. The In-Loop filter is okay since it only kicks on when needed.
2-pass VBR. Since the complexity of screen recordings varies so much, doing an analysis pass as well as letting the codec distribute bits on its own allows for big momentary spikes for quality, while letting the average bitrate stay low.
Quality 90. Using Quality 90, the codec can dip down to QP (Quantization Parameter) 1, letting our reference frames be really crisp, reducing the bits later frames need. A good thing. In the VC-1 Encoder SDK, we can manually set the min and max QP as different parameters, but that feature was added after I did these clips.
Don't be afraid of low bitrates. Using the above, you can need a lot fewer bits/pixel for screen recordings than typical motion content.