Scaling and Aliasing

MarcS wrote on 12/15/2004, 6:40 AM

I've mentioned this in the forum before, but it came up again during a large DVD project I created, so here is my observation again.

I like to vary color, size, rhythm of my video clips to make them interesting. It seems that scaling the video down, even when the scaling is proportional, induces a noticable alias artifact on diagnol edge lines. It is fairly noticable and it bugs me.

I realize that any "manipulation" to the raw video data induces artifact, but are there any ways to scale down video size without having this occur. I use best rendering settings by the way.

Anyone else notice this problem?

- Marc

Comments

riredale wrote on 12/15/2004, 8:58 AM

Any time you down-sample you will get aliasing artifacts if there is high-frequency information in the original material. This is true for video as well as audio. The answer is to prefilter that high-frequency information out beforehand. For audio, you filter out the highs; for video, you soften the image slightly.

Do a couple of experiments with the Blur plugin and you will find just how much is necessary to clean the smaller image up. Make sure you do the blur before you do the size reduction.

You can also keyframe blur. In one of my more-elaborate menus last year, I showed video full-frame, then shrank it down considerably to make it part of the moving menu. I had to keyframe the blur so that it was sharp at full-frame, yet watchable when small. It worked great.

epirb wrote on 12/15/2004, 9:14 AM

Good info Riredale, thanks that answered a problem I had .

Erk wrote on 12/15/2004, 10:36 AM

riredale -

>Any time you down-sample you will get aliasing artifacts if there is high-frequency information in the original material. This is true for video as well as audio. The answer is to prefilter that high-frequency information out beforehand. For audio, you filter out the highs; for video, you soften the image slightly.<

Thanks! I've never come across a simple, general explanation of this that applies to both the audio and video worlds. Makes a lot of sense.

MarcS wrote on 12/15/2004, 11:03 AM

Thanks for the information. I presumed that the edge aliasing w scaling down images was due to the inherent artifacts caused by sampling, but I've never thought of correcting it by applying a blur.

One problem is that you can't detect the potential alias distortions until the project is rendered as it is undetectable while editing it in Vegas.

I have cropped/scaled/rotated lots of clips in After Effects and don't seem to recall the amount of edge aliasing seen with Vegas's scaling. Does AE transform the clips at a finer sub-pixel level?

Thanks,

Marc