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Core melt Causing Aliasing?
Hey Folks,
I’m beating my head against a wall here trying to eliminate some annoying aliasing appearing in the late stages of a composite shot. Basically, my “dead” actress wasn’t as still as I’d like, so I’m replacing her with a freeze frame. Because the original footage was not completely stable, I’m using CoreMelt’s SimpleTracker to hold her in position. Finally, it’s a rack focus shot. So the freeze frame is in a compound clip that contains a keyframed blur and the draw mask.
Inside the compound clip, everything looks great. The blurs are buttery smooth. As soon as I add the compound clip to the SimpleTracker plugin, it’s suddenly got terrible aliasing.
Here you can see the before and after. This is the window where the clip is being added to core melt. On the left, the compound clip looks great. On the right, the preview shows the same clip with terrible aliasing.
Here’s another shot showing a baked version of the composite, now with color. The aliasing looks terrible.
I suppose I can add another masking layer to the final clip and an extra blur, but surely there’s a better solution?
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18 Replies
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What are you calling aliasing? That would be jagged lines on edges. I’m not seeing that in these stills. Are you referring to something else?
– Oliver
Oliver Peters – oliverpeters.com
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I’m referring to the jagged transition lines between colors on the girls face and head scarf. They look almost like a topographical map. Is there a better term to describe those?
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[Ted Cox] “Is there a better term to describe those?”
That’s banding.
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Michael Hancock
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It’s not aliasing, it’s posterization or gradient banding, which is visible if you double-click on the 2nd image. My first guess is it’s caused by shooting underexposed 8-bit material with a flat profile then trying to pull it in post. For a flat or log profile to work well on 8-bit material it needs to be “exposed to the right” on the histogram.
OTOH when I take a screen cap of this and process it in Photoshop it looks mostly OK. This might indicate Core Melt is degrading it somehow. However I don’t have the full resolution to work with so I can’t be sure.
What codec are you editing in, and how was it exported? As an experiment you could try exporting it as ProRes. If that doesn’t change anything, transcode the clip to optimized media, it to a test project, apply your effects then export as ProRes. Make sure you are not using proxy for the export. If this is 4k use a 4k project not a 1080p project.
In general it’s easy to run out of tonal steps when processing 8-bit color. This is exacerbated when shooting with a flat profile and furthermore when exporting to H264.
As an contingency workaround sometimes you can add some dithering or noise in a mask over the affected region. That sometimes breaks up the banding.
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Also, what are you doing the color correction with?
– Oliver
Oliver Peters – oliverpeters.com
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And have you tried rendering or exporting the composite as ProRes? You could be looking at a preview and not final render.
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Hey Joe,
Thanks for replying. It’s 4K native footage in a 4K project. All the media is ProRes. Most is ProRes 422 LT. For the baked composite I’ve been exporting a master file in ProRes 4444.
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Hey Oliver,
Thanks for replying.
[Oliver Peters] “Also, what are you doing the color correction with?”
The FCP Color Board effect. However, as noted above, the banding appears BEFORE the color correction is added.
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[Jeremy Garchow] “And have you tried rendering or exporting the composite as ProRes? You could be looking at a preview and not final render.”
Yes. The “finished” screenshot is from a ProRes 4444 Master export reimported back into my timeline. (The core melt tools drag my computer to a crawl, so I baked the composite for performance reasons.)
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Just for clarity here’s some detail screenshots.
The source image has no banding:
After adding FCP’s Draw Mask and Gaussian Blur, there’s still no banding:
After putting a Compound Clip of the above into the SimpleTracker plugin, the banding immediately appears:
Don’t get me wrong, I’m not trying to blame CoreMelt. My assumption is that I’m doing something wrong. I’m just not having in luck figuring out what it is. 🙂
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