I've uploaded a 4K video containing 2D motion graphics and there are strange encoding artifacts present. (They are not at all present in the video file I am uploading.) These aren't the typical blocks or halos or whatever. There are no artifacts at 4k, but the lower res versions have artifacts (and different artifacts, depending on the resolution.)

There are many, many little glitches and problems, but I'll just highlight a couple:

At 1:24 in this video, viewed at 1080p you can see a series of horizontal lines. Just as the first note appears, you can see the leftmost 20 or 30% of the lines appear to get brighter for 5 or 10 frames before snapping back to normal. Here is a screenshot during the "flash" (bizarrely, this flash is much more apparent in Firefox compared to Chromium, both on Linux; that surprised me...):

http://caseyconnor.org/pub/blender/youtubeissue1.png

Another artifact is visible around 2:00 where the bottom right line appears moderately "dashed" even though it should be perfectly even. Around 2:14 it snaps back to normal. Screenshot:

http://caseyconnor.org/pub/blender/youtubeissue2.png

Those horizontal lines seem to give youtube's encoder a lot of trouble; they pulse and flash different brightness levels throughout the video.

I'm encoding an mp4 with Handbrake on windows 8. I've tried Main and High profiles, with and without "Animation" tuning, and different bitrate targets (although the file always ends up about the same size, presumably due to the solid swaths of identical color.) Again, it looks perfect before I upload it -- the issue is clearly in youtube's transcoding.

I noticed that part 2 of this video series has no such artifacts (that I have yet noticed) despite having the same kind of content. The grey horizontal lines in part 2 are a bit brighter... perhaps youtube's algorithm is someone treating the fainter grey tones as more sacrificial? I'm currently re-rendering part 1 to see if brighter lines will help.

Any tips on how to avoid this? Thanks!