View Full Version : Aliasing on Hor/Vert lines

CC Rider
05-27-2008, 11:26 AM
Is there a Guru out there who can help?
I'm rendering seqences that contain several television monitors.
almost all of the horizontal and verticle lines seem to "crawl" when played back.
I've heard that this is a common problem, especially with lines that are almost horizontal or almost verticle where the line has to be drawn on just a couple of pixels width...
Does anyone have a method or magic setting to help combat this problem?
Anti-Aliasing settings up to "Extreme Enhanced" don't seem to give much better (if any) results that that "Medium Enhanced".
In the image, the circled area really "crawls" when the camera starts to move.

Scene Info:

There are no image maps/prodedurals on the offending areas...just solid colors.
Using classic camera - other cameras render faster but make the crawl even worse!
Same results with all multipass AA settings.
Motion Blur is set to "Dither"
Camera movement is very slow

Let me know if you need more scene info...

I'm sure I'm not the first one to deal with this...Anyone have a magic bullet for me?

05-27-2008, 11:36 AM
Hi CC rider,

Sounds like you're using the classic camera. You should really switch to perspective camera, it's usually a lot faster.
Read up on the details of the LW AA system in the tutorial in my signature. Using a right combination of base AA level and oversampling, combined with a correct AS tolerance level will solve most of these problems very easily.

I'd start with something like base AA of 8, AS 0.03, OS 0.075, fixed sampling pattern and classic reconstruction filter.

CC Rider
05-27-2008, 11:48 AM
Thanks for the quick response!
I am using the classic camera...I can just never get the perspective camera to produce a final render that looks as good.
I'll try using your recommended settings as a starting point and revisit that approach.
I don't recall ever using the OS control...maybe that is the piece I've been missing.
I sure hope it works...they do render a lot faster.

Thanks again!


CC Rider
05-27-2008, 12:33 PM
Thanks for the comments Exception but that's not where the problem lies...
I tried using both the perspective camera and real lens camera and got the same results I got in the past. Just not as good as with Classic.
I started with the settings you recommended and even took them to extremes with higher levels of AA and OS and tighter AS and all the combinations gave the same results.
Here are two images.
The first uses the Perspective camera, AA 9, AS 0.001, OS 0.2 and MB is set to Photoreal 3 passes.
The next image uses Classic camera with AA Enhanced High, AS 0.01

The edges of the monitors are actually much cleaner using the Classic Camera
but still problematic.
It's not an AA issue...there has to be another way to combat this.
Any other thoughts on the subject?


05-27-2008, 02:24 PM
It's not an AA issue...there has to be another way to combat this.

The short answer is: render Enanched at an higher resolution and scale down.

Seems prehistoric, but it works and will fix your issue.

The longer answer is why this happens. Classic camera, in Enanched AA mode, employs a triangle filter and reconstructs a pixel's value from an area which is slightly larger than a pixel according to some sort of sub-pixel coverage (think "smaller grid"). However, as you noticed, it only goes so far and increasing passess improves motion blur, but not sub-pixel movements.

Perspective camera(s), as Exception noted, use the oversample control to obtain similar results, throwing rays around the Oversample radius (think "looser grid"). This is great for a filmic, soft, look but not as good as Enanched AA with thin details like those in your images. Expecially since you mention a slow camera movement and this benefits from a larger sub-pixel resolution.

By rendering, say, 2x and scaling down, you allow pixel center's to move twice as far before stepping up and down between scanlines.

The nicest thing is that there is an undocumented feature in LW since 8.5 or so that allows to render at higher resolution with classic camera without having to suffer 4x the render time. Setup any enanched AA mode, then turn on Adaptive Sampling! Up to LW 8, Adaptive Sampling was ignored in Enanched modes. Now, it shades only half pixels in X, while rendering the full geometry. This halves the extra render time. (Please note I haven't tried this in a long time, but it used to work pretty well).

Just one more thing: in the render comparison between Classic and Persp camera the Classic reconstruction filter was used for both? If so, try the Mitchell filter, it behaves better with Persp camera's oversample.

Hope this helps!

05-27-2008, 02:34 PM
Yes that's a good suggestion by vfxwizard.

That said, I see no problem in any of your images.

also, remember AS in classic cam is a totally different beast than AS in perspective cam. It's reductive rather than additive in sampling, so to increase classic cam's quality you want to turn it OFF while in perspective cam you want it ON.

also, if double res rendering isn't your fancy, try Mitchell or Guassian as a pixel filter. They're more forgiving, while also being softer.

CC Rider
05-27-2008, 02:51 PM
, I see no problem in any of your images.

It doesn't really jump out at you on a still shot like this, but when the sequence is playing it's really distracting.
However -
This is probably a stoopid Noob thing but I discovered that having the "Use Global" box checked in the motion blur section of the camera properties disables everything we've been talking about.
I could set the Oversample to 20.0 and not see a difference in setting it to .02...
I knew there had to be something that I was missing...
I unchecked the box with a "Forest Gump"ish blind luck and backed in to the the answer to my problem!
Now all the suggestions that Exception made in the first reply magically work!
Looks like I have some more tests to do, but this changes everything for me.
Thanks for all your help guys!


CC Rider
05-28-2008, 02:02 PM
Gotta say thanks again to Exception and vfxwizard for the help!
I knew this level of rendering was "do"able in LW but I just couldn't wrap my head around it for some reason.
Your enlightenment has brought the quality of my work up tremendously!
Renders with DOF effects have benefited as well.
I'll post the finished sequence when its done...

Thanks again guys!!