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Seeing something through an Alpha add seam

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Paul Berlin
Seeing something through an Alpha add seam
on Nov 10, 2018 at 5:23:47 pm



I don't understand this behavior:

The yellow triangle is a shape layer. It is on top of a precomp that is made up of the same yellow triangle, on top of an orange rectangle shape layer. In the precomp, the yellow triangle has been applied as an alpha-inverted track matte to the orange rectangle, so the rectangle has a triangle-shaped hole in it.

In the main comp, I place the yellow triangle on top of the orange rectangle precomp and set the blending mode of the yellow triangle to Alpha Add, so that there is no visible seam between the triangle and the rectangle precomp. And when I move my mouse over the seam, the A value in the info panel is always at 255.

Now I put a green rectangle shape layer underneath both of these layers, and I can see some of the green peeking through the seam. Just for illustration, I set the background color of the comp to the same value green, and moved the green rectangle so that it's only under the right side of the triangle-rectangle seam. And you can see that the green only peeks through the right side of the seam, where it's from the green rectangle, and not through the left side of the seam, where it's from the background color of the comp.

I thought that if a pixel has an alpha value of 255, you can't see anything underneath it. So why am I seeing the green of the rectangle?

All the other layers, besides the yellow triangle, are in Normal blending mode.


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Kalleheikki Kannisto
Re: Seeing something through an Alpha add seam
on Nov 10, 2018 at 7:55:22 pm

As you can see in the image, the pixels are not only 0 or 255, due to antialiasing, the effort of the software to smooth the edges visually. While you can read the values as 255 or 0 based on whether they are inside or outside the resolution-independent VECTOR mask, once you get to the comp pixel resolution BITMAPPED representation, it will be a blend between the two anywhere parts of the pixel are inside and parts of the pixel are outside the vector area. Thus you're really getting in-between values like 52, 89, 167, 214 etc. that get blended together to create a bitmap image for the screen. So in reality, AE is blending transparent pixels at such edges.

There's a "layer image quality" switch in the timeline (the switch with a diagonal line). If you turn that to "draft" you get an aliased (no smoothing) output. You can try that on your track matte layers to see how the difference between anti-aliased and aliased looks.

Kalleheikki Kannisto
Senior Graphic Designer


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Paul Berlin
Re: Seeing something through an Alpha add seam
on Nov 10, 2018 at 11:30:49 pm

Thanks for your reply.

Using Draft quality: does that mean that I can't get this behavior to go away and still have a smooth-edged, antialiased triangle?

If I keep the layers in Best quality, I know that in the main comp, I could just move the yellow triangle down a pixel or two, and it would cover up the green. But this seems like an extra step I shouldn't have to take.

Also, why does this blending of the values only happen on the right side of the triangle, and not the left? The green on the right side is from a shape layer, the green on the left is from the comp's background color. How does that account for the different behaviors?


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Kalleheikki Kannisto
Re: Seeing something through an Alpha add seam
on Nov 11, 2018 at 9:28:30 am

To get rid of this behavior and keep your smooth edges, you just need to change the strategy so that the edges do not coincide. Instead of having two edges exactly in the same place. In your example, you would be better off drawing a red rectangle (without the triangular notch in it) then a yellow triangle on top of it. Or you could just expand the lower shape by one pixel, if you don't want to rebuild.

Kalleheikki Kannisto
Senior Graphic Designer


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Paul Berlin
Re: Seeing something through an Alpha add seam
on Nov 11, 2018 at 2:38:58 pm

The red (supposed to be orange) rectangle has to have that notch in it, so the yellow triangle can move to fit into it, so I can't follow your first suggestion.

If I combine the red rectangle precomp and the triangle shape into another precomp, the behavior goes away, which might be the fastest solution.

Your second suggestion: if I use the Simple Choker effect on the triangle shape in the rectangle precomp, I shrink the size of the notch slightly, and this hides the behavior. Is that what you meant by "expand the lower shape"?

Thanks for your time.


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Kalleheikki Kannisto
Re: Seeing something through an Alpha add seam
on Nov 11, 2018 at 10:32:24 pm

Yes, you just need a tiny amount of overlap to get rid of the problem. Even half a pixel would likely take care of it. You can adjust the track matte by simple choker or offset the shape ever so slightly.

Kalleheikki Kannisto
Senior Graphic Designer


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Paul Berlin
Re: Seeing something through an Alpha add seam
on Nov 12, 2018 at 11:48:29 am

Thanks for all your help.


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