42+58=100, but not when it comes to opacity

Forum|Forum|7 years ago
February 15, 2019
2 replies
692 views

I've come up on this before, and I'm looking at it again today. Sometimes when I'm making cross fades in After Effects, especially when dealing with layers that use some transparency already, I find that it's really difficult to get an even crossfade. If one clip fades out-to-in with exactly the same time that the other fades in-to-out, the image still darkens (loses opacity against the background) during the transition. But if I keep the underlying layer at full opacity, any partial transparency parts gets brighter as the new clip fades in and layers over the full strength under layer. If I have 50% of one image and 50% of another image, shouldn't I just have a blended image at full opacity though? Why is my image darker when one layer is at 42% and the other at 58%? What do the rest of y'all do to get around this?

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Rick Gerard

Community Expert

Try changing the blend mode to Alpha Add to get a more film-like transition. Just fading opacity is like a video crossfade.

When you combine alpha channels all your are doing is combining the alpha. The luminance values do not change at the same rate. hanging the blend mode to Alpha Add applies a little different curve to the math so things look more like they would if you were creating your fades optically.

It is also usually best not to crossfade opacity if there are big luminance differences in a layer. Just increase the opacity of the top layer. This is problematic if you have transparency in the underlying layers.

If you are trying to get an even crossfade with multiple layers that have multiple opacities it is often a good idea to pre-compose those layers before fading them.

You can also get slightly different results using the Transform effect. If I had two layers at 60%, an underlying layer at 40%, and a bottom solid layer, and I wanted to the two at 60 out and the one at 40 in I would add Transform to all of those layers and animate the crossfade there. You'll get different curves and different midpoints.

If you are putting in the fades to edit a sequence then you probably shouldn't be editing in AE. Editing with transitions is easily handled in Premiere Pro where the curves for several different types of crossfades are already setup and rendering time will be far less.

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sarapixelsAuthor

Inspiring

I'm not seeing any difference at all using Alpha Add. Maybe I'd be better off taking the extra steps to move this project to Premiere just for the fades, but that seems a little excessive.