Aliased edges on video playback after downsizing in AVIDemux

[Dave2ic]

I have some 1080p video that I have resized using AVIDemux to 640x360 for uploading to Vimeo. I have used the MPEF4 AVC (x264) codec.

However, I notice that when I playback on my computer and the video is upscaled to my 24" HD monitor, there is harsh aliasing, as shown on the 100% crop below:



It seems to look the same on VLC or Windows Media Player, but I don't notice this on other videos that I have on my computer at similar resolution. It also looks the same converted by Handbrake.

Is it that reducing size in Handbrake or AVIDemux results in output that is too sharp? Or is it the VLC and Windows Media Player are just rendering it too sharp when they upscale?

Whatever it is it looks really distracting, so any help in reducing/eliminating the effect would be appreciated.

[poisondeathray]

If you're playing this locally, you can change the resizing algorithm used in the media player. Typically they use something like "bilinear", which is the lowest for CPU usage, but lower quality . Or you can apply an antialiasing filter during playback.

When viewed in flash in a browser, it's even worse, typically something like "nearest neighbor" resizing.

"Too sharp" usually won't cause aliasing; oversharp resizing will cause halos

Also, why would you upload 640x360 to vimeo ?

but I don't notice this on other videos that I have on my computer at similar resolution

Then maybe there is a problem with your footage in the first place ? Can you upload some samples ?

[Dave2ic]

Thanks for your comments. I think there just seems to be something about how that particular codec is rendered on my PC at a low-ish resolution like 640x360.

I tried resizing it to 1280x720 and it was very smooth. Of course it would be much smoother being a higher resolution, but the jaggies that were showing when I played back upscaled on my full HD monitor were so severe I expected them to start showing up even at 1280x720.

After uploading the 640x360 video to Vimeo it plays back just fine, so there obviously wasn't anything wrong with the encoding but just the playback upscaling in my media player. The funny thing is though it was the same in VLC and Windows Media Player. I can't work that one out as it is my understanding that VLC has its own codecs.

And the answer to your other question - why 640x360 for Vimeo? A couple of reasons:
1. I only have a free account, and I needed to fit it within my 500MB limit.
2. It's just a school performance for parents to view online, and in my part of the world we can't expect everyone to have sufficient bandwidth to stream HD video.