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Core melt Causing Aliasing?

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Ted Cox
Core melt Causing Aliasing?
on Jul 25, 2017 at 12:42:46 pm

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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Oliver Peters
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 5:40:16 pm

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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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 7:43:45 pm

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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Michael Hancock
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 7:56:40 pm

[Ted Cox] "Is there a better term to describe those?"

That's banding.

----------------
Michael Hancock
Editor


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Joe Marler
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 8:00:43 pm

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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Oliver Peters
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 8:40:12 pm

Also, what are you doing the color correction with?

- Oliver

Oliver Peters - oliverpeters.com


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Jeremy Garchow
Re: Core melt Causing Aliasing?
on Jul 25, 2017 at 11:57:37 pm

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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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 7:37:42 am

[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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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 7:35:47 am

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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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 7:34:08 am

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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Joe Marler
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 7:58:41 am

Thanks. The first frames you posted were in a flat or log profile. The exposure had been "de-flattened" and the histogram stretched. This was done by something besides simple tracker. Do you know for certain that if this was exported without simple tracker it would look OK, or would it also have banding? E.g, what if you experimentally added the same blur to the entire frame? This would help decide if it's the Coremelt tool or simply the combination of color correction and blur step which triggers the banding.

Another test would be leaving the Coremelt tool in but taking out the color correction and exporting in in the original flat exposure.

BTW while FCPX is pretty good at showing in the viewer a representation of the final export for difficult cases like this I always recommend exporting the clip and examining it, often with more than one tool. E.g, VLC and Quicktime.

Simple tracker might be degrading the image by itself, but it's suspicious that I've seen similar things happen without that plugin.

E.g, an underexposed 8-bit image in log format is often on the threshold of banding when color corrected. Performing normally innocuous steps such as video noise reduction, smoothing (or maybe even blur) can precipitate the banding. Those steps are essentially suppressing the normal grain or noise, thereby creating a smoother gradient. That smoother gradient requires more tonal steps to smoothly represent, and with log-format underexposed 8-bit color, you don't have those steps.


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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 8:19:27 am

Thanks, Joe.

[Joe Marler] "The first frames you posted were in a flat or log profile. The exposure had been "de-flattened" and the histogram stretched. This was done by something besides simple tracker."

I'm not sure I understand. When you say the "first frames" I take it you mean the first two screen grabs I posted. One of the viewed while adding my tracked item to the SimpleTracker plugin. In this frame, both images, the before and after are flat. The second is the final image after color correction (using Apple's Color Board). Is that color correction step what you're referencing? Or are you saying it looks like something strange was done with the original flat footage?

[Joe Marler] "Do you know for certain that if this was exported without simple tracker it would look OK, or would it also have banding?"

Now we're on to something! I exported the Compound Clip prior to adding it to SimpleTracker and... it HAS banding. I'm not sure why the viewer shows a higher quality imagine than I get on a final export (I'm sharing as a Master File -> ProRes 422LT in this case). But that's helpful to know.

So that eliminates CoreMelt, or my misusage of CoreMelt as the source of the problem. I can totally accept that the trouble is the source footage breaking down. You suggested adding some noise to try to dither the banding. Any other suggestions? Any neat tricks to get what I'm seeing in the viewer to actually export?


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Joe Marler
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 9:12:39 am

[Ted Cox] "Now we're on to something! I exported the Compound Clip prior to adding it to SimpleTracker and... it HAS banding. I'm not sure why the viewer shows a higher quality imagine than I get on a final export (I'm sharing as a Master File -> ProRes 422LT in this case). But that's helpful to know.

So that eliminates CoreMelt, or my misusage of CoreMelt as the source of the problem. I can totally accept that the trouble is the source footage breaking down. You suggested adding some noise to try to dither the banding. Any other suggestions? Any neat tricks to get what I'm seeing in the viewer to actually export?"


As I suspected it is probably the blur that's causing the problem when combined with the flat exposure being on the left side of the histogram. Re the viewer, if it's set for "best performance" that by itself might be adding noise that prevents the banding. If it's set for "best quality", that when combined with a scaled display might be preventing it. The key point is the viewer isn't doing a better job, it's actually doing a worse job and those imperfections (whether scaling or noise) are by happenstance suppressing the banding.

I think the last time I encountered this I added some noise or film grain to the smooth region and it lessened the problem. I don't remember which. There is a built-in "add noise" effect and I also have the 3rd-party FilmConvert plugin; I used one of those, I think in a mask. You will have to experiment.

Another possibility is you could export it as 1080p. If you have not cropped the 4k, in theory 4k 8-bit 4:2:0 can be transcoded to 1080p 10-bit 4:4:4. It would be interesting if a 1080p ProRes export looked any better: http://www.provideocoalition.com/can-4k-4-2-0-8-bit-become-1080p-4-4-4-10-b...

In general these are common problems when shooting 8-bit color with a flat profile which is not exposed to the right on the histogram.


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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 9:49:44 am

Thanks so much for the help and clarifications, Joe. This has been really enlightening.

I hope your field assignment goes well.


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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 7:52:41 am

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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Joe Marler
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 8:05:00 am

OK that's interesting. What if you export the compound clip as 4k ProRes, then re-import that, then apply the SimpleTracker plugin?

Unfortunately I'm leaving for a field assignment and won't be able to follow up on this for a week.


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Jeremy Garchow
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 1:05:29 pm

How did you make the still frame and what format is it?


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Jeremy Garchow
Re: Core melt Causing Aliasing?
on Jul 26, 2017 at 6:31:47 pm

And by still frame, I mean the still of the woman.


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Ted Cox
Re: Core melt Causing Aliasing?
on Jul 27, 2017 at 8:34:25 am

[Jeremy Garchow] "How did you make the still frame and what format is it?"

I used FCPX's Share "Save Current Frame" on the original 4K clip outputting to TIFF format.

Thanks, everyone for your thoughts and assistance. Joe was quite correct. In the end, it was a function of underexposed 8-bit footage breaking down due to the addition of the blur. Using FCPX's "Add Noise" function, I was able to add 0.02% blue noise and eliminate the banding. ☺ The noise is visible, but not distracting, and the shot looks so much better!


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