I hope you'll be able to help me. The problem may not be a big one, I think it's just a detail I don't know.
Here it is : I want to make a symetrical image, made with the same image twice, on the right and on the left, in symetry.(One is reversed.)
I made adjustments and animation in a first precomp. This precomp is included twice in a comp. One of them is reversed in scale.
In order to have each precomp contained into the right or left part of the global image, I put one mask on each layer, one mask covering the right part, and the other covering the left one.
Problem : despite the 2 masks are meeting just in the middle, I get a thin black line between them.
If I make bigger the mask which is below, making it coming under the other mask, the line is white.
I've tried all the possibilities I saw to fix this problem, but I haven't found the clue yet.
Re: Mask edges by Dave LaRonde on Mar 27, 2008 at 4:06:57 pm
I wouldn't fool with masks in this case. I'd add a comp-sized solid, position it so one vertical edge is at center screen, then use it as a track matte for one of the layers.
Re: Mask edges by Lucia Bastenhof on Mar 27, 2008 at 5:04:12 pm
Thanks for your answer, Dave.
If I understand well, your solution is about the same as changing the precomp size in order to fit with the half of the comp size.
I didn't even think about that ! :-/
But if you know or somebody knows why 2 masks just touching each other still leave a thin line at the boundary, I'd be glad to understand that.
Re: Mask edges by Dave LaRonde on Mar 27, 2008 at 5:17:41 pm
[Lucia Bastenhof]"If I understand well, your solution is about the same as changing the precomp size in order to fit with the half of the comp size. "
That's a misunderstanding. You simply have two precomps: one with footage looking right-to-left, one with footage looking left-to-right. They use NO masks.
The solid acts like a matte in the classical movie-world sense of the word: it isolates one of the layers. Since one edge of the solid is positioned at mid-screen, it hides one half of the screen. By making it a Track Matte, it makes visible the layer that should be on the hidden half of the screen. No masks involved whatsoever.
Re: Mask edges by Lucia Bastenhof on Mar 27, 2008 at 7:09:07 pm
OK, I understand, thanks. And it's certainly the easiest way to do it.
But do you have an idea why -in another situation, it's abstract-, 2 masks touching each other still leave a line at the boundary ? I shouldn't !!
Re: Mask edges by Darby Edelen on Mar 27, 2008 at 9:22:44 pm
[Lucia Bastenhof]"But do you have an idea why -in another situation, it's abstract-, 2 masks touching each other still leave a line at the boundary ? I shouldn't !!"
If the wholly opaque pixels of the two layers actually do overlap, then it's either an issue with a blending mode (are they set to Normal?) or the pixel information in the layers you are masking.
Darby Edelen Designer Left Coast Digital Santa Cruz, CA
Re: Mask edges by Darby Edelen on Mar 27, 2008 at 7:12:39 pm
[Lucia Bastenhof]"But if you know or somebody knows why 2 masks just touching each other still leave a thin line at the boundary, I'd be glad to understand that. "
Because masks are not limited to residing on a whole pixel, they use vectors to describe their boundaries with sub-pixel accuracy. The edge of a mask may run down the center of a pixel, in which case the pixels on that edge will be rendered at 50% opacity in order to reduce aliasing.
If you want to view this phenomenon in action, create a new solid in a new comp, apply a rectangular mask, zoom into a vertex at 3200% and start moving the vertex. Notice that the vertex moves with sub-pixel accuracy, but AE must render the resulting solid in 'whole' pixels (some of which it will make semi-transparent so that they don't appear to be 'whole' pixels).
Darby Edelen Designer Left Coast Digital Santa Cruz, CA
Re: Mask edges by Lucia Bastenhof on Mar 27, 2008 at 7:54:43 pm
I tried what you said, Darby, and indeed, the mask isn't accurate. Trying to fix it, I've noticed that it succeeded by switching the render quality button (with the dotted line).
Glad to find that, I went to try it on the masks (already done) of my symetrical composition.
As a result, the black line becomes white, and of course the quality of the whole photo is suffering.
In this case, it doesn't matter (thanks to Dave ! :-) , but I'm sure the problem will come again in another project. That's why if you have any idea about how to fix it, I'll listen to it thoroughly.
Re: Mask edges by Lucia Bastenhof on Mar 28, 2008 at 4:00:38 pm
Darby, I didn't see your question about blending modes sooner.
The blending mode is "Normal", but indeed, in the precomp, the original image is the matte of another image.
I changed this to see what happened in the symetrical comp : the black line remains. Switching the Render Quality button makes it disappear, but the quality of the whole image becomes bad.
So, the matte in the precomp isn't the source of the problem.
In this case, I don't need masks finally, but it could happen on another project, and I can't imagine there isn't a way to make disappear this line (without damaging the quality of the image).
Re: Mask edges by Lucia Bastenhof on Mar 28, 2008 at 5:07:05 pm
Dave, I first tried your method (a white solid with the edge coming just in the middle of the comp - Track matte) but it appears that the same problem as with the masks occurs !!
Looking at the white solid in solo mode at a big scale, it appears that there is a transparency on 1 pixel !!
But there are pixels remaining everywhere !! ;-) I'm really astonished !
In my case, I found a solution by changing radically the precomp size, in order to make it the half of the symetrical comp. It solves my problem, but this lack of accuracy is frightening for future projects.
Re: Mask edges by Darby Edelen on Mar 28, 2008 at 8:04:05 pm
[Lucia Bastenhof]"It solves my problem, but this lack of accuracy is frightening for future projects. "
It's not really a lack of accuracy, just a byproduct of anti-aliasing. Believe me, if you had no anti-aliasing you'd be complaining a lot more =)
An alternative method to increasing the sharpness along alpha edges would be to apply a Channel > Alpha Levels effect and slam the white point (or black point... or gamma) to an extreme value.
Darby Edelen Designer Left Coast Digital Santa Cruz, CA
Re: Mask edges by Frank Thomas on Mar 28, 2008 at 11:58:42 pm
I might be misunderstanding this, but why do you need a mask on both layers?
Can't you mask only one layer and feather the mask a little?
To my way of thinking, having overlapping areas of transparency/semi-transparency makes no sense, when a seamless result is required.
Re: Mask edges by Lucia Bastenhof on Mar 29, 2008 at 12:37:53 pm
@ Darby : Thanks for the tip ! ;-)
@ Frank : I needed 2 masks because there is an animation of opacity. But the anti-aliasing leaves a line between 2 masks just touching each other.
But finally, I don't need any mask at all ! :-)
