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grid like artifacting with scale operations
This is a strange little issue that has been brought to my attention by colleagues after they had some shots rejected in tech check (when they gamma slammed the heck out of the footage and saw these artifacts, …when you know they are there, you can see them without any crunching of levels).
Basically when doing a scale up or down on a layer (more visible on the smaller scales) you get this grid-like interference pattern happening.
I have tried it on numerous packages. After Effects, Combustion, and Photoshop all had the same grid patterning when scaling up this 1080p footage by 104%. Nuke had it at its default cubic filtering, but could be eliminated by using Parzen or Notch). Shake was the only application that seemed to do the transforms without creating the grid interference (am guessing it uses a different transform filter by default).Here is a link to the image (click on the lined thumbnail to see full image, I have marked little yellow squiggles at the start of the grid lines to help see them, plus the shot has been graded to make the grids obvious).
here goes
From what I can see it is something to do with After Effects (and the other apps) use of bicubic sampling. My questions are, has anyone else come across this, and if so, do you have a fix? My only suggestion to the others was an adjustment layer with a 1 pixel fast blur followed by an unsharp mask filter (at default settings), over the top of the worst clips. Not ideal as the image becomes slightly softer, but at least the grid is dealt to enough to be passable.
Its a real head scratcher as I have never encountered this before in AE, …and it seems to depend on the footage, as other clips seem to scale with no artifacts.
Cheers,
andrew
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3 Replies
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Thanks for the reply, no, its Genesis, DPX sequences, it isn’t a moire pattern in a busy part of the screen or aspect ratio thing, …pretty certain it is a bicubic resizing issue from the tests we have done in various apps (have subsequently found that Shake does do it to a lesser degree, but it really depends on the image, …we started testing using a grey solid with grain applied to it, then applying a transform to it, this worked well for testing Shake, …as I say in practice with the video clips Shake seemed okay, at least compared to AE).
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Just an update, we’re 99.99% positive now, that this is just to do with the bicubic filtering when doing transforms in After Effects.
Since I posted this earlier today, I have found that if you use the resize image command in Photoshop and switch the filtering to ‘smooth bicubic’ the grid becomes softened as to almost disappear, you have to look hard to see any trace of it (to the degree that I think it would pass tech checks). Would be great if After Effects had the option to change what filter you used in a similar fashion (or even had an additional transform filter for these tricky situations, which allowed transform filter changing).
I have been conversing with a friend in Canada who has since done his own tests in Shake using a grey solid, grain, then resized using different tranform filters. He has found it does occur in Shake too (but again this must be image specific because in the example I have posted it doesn’t seem to have the grid problem). His results using different transform filters were:
BOX: Brutal. Moving on…
SINC: Very good, but too sharp
GAUSS, QUAD, TRIANGLE: Slight Gridding & quite soft
DIRAC, IMPULSE: Blergh, visible stretching at seams.
LANCZOS: Slight gridding, but sharper than GAUSS
MITCHELL: Halfway between LANCZOS & GAUSSOne of those strange things where maybe we should not be looking so close at these images (higher scalings are fine, it is the ones within 10% that are an issue).
Still looking for ideas for work arounds if anyone has them?
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Okay, just to conclude this thread, we ended up learning to work around the transform issues (a little blur here or there does wonders too), but also I found this fxguide article which goes over this whole issue, with various compositing appliations and transform algorithms, plus has some tips for the various applications at the end of it (for AE it is basically to limit nesting moves in precomps). Here’s the link:
https://www.fxguide.com/article315.html
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