Obtaining a mask from alpha channel in AE...

[JamesTaylor]

not quite as straight forward as it sounds!!!

 

I'd like to be able to apply a stroke effect (which i also want animate "drawing on" as the camera moves) in AE to areas of an image determinded by an Alpha Channel / MatID pass or transparency supporting file format.

 

The problem i face is that when i apply a stroke effect in AE to a render i get no resulting stroke, i can apply the stroke to a mask (path) element and it works... so how can i create a path element from an Alpha channel / MatID or PNG with transparency.

 

At present the only solution i can come up with is to key frame a path manually, which to be honest isn't going to happen! Surely i can extract something using different render passes???

 

Answers on the back of a post card please!!

[Matt Clementson]

I don't know AE so I don't know if this will work but how about this as an idea;

 

Load the file into photoshop. Go to the channels tab and choose the Alpha channel. Click the 'make selection from channel' button. Go to the Paths tab and click the 'make path from selection' button. Then export the path to illustrator.

 

That should give you an AI file that you can import into AE. Possibly you may even be able to load in the PSD with the path saved. Anyway, just an idea.

[JamesTaylor]

yeah not a bad idea, which i'll have a go at next week now... but... my initial thought is this is going to be labour intensive as i need to do it across a series of frames that make up a no. (a few hundred at least) of animation paths... i could try automating the process in p'shop tho so maybe??

 

actually doing it that way is going to create an individual path for each frame which is no good as i need a continual path throughout the animated sequence so i can animate the stroke across the sequence.

 

best idea so far tho!!

[Crazy Homeless Guy]

can you describe a little more what it is that you want to accomplish...

 

from what i am reading, you want to animate a camera that shows a building in wire frame, and while that camera is animating, you want the wire frame model to be ‘draw’ in and complete the wireframe? Yes, No?

[JamesTaylor]

Travis,

 

i have a camera that pans along the facade of a building, the building will be rendered photoreal. As the camera pans along the facade i want to highlight specific section, perhaps a balcony or a window. To highlight the element my plan is to have a stroke "draw" around the object as the camera is panning past it.

 

so, i can create a MatID pass or alpha channel or a file with transpancy such as png for any given object and therefore isolate a given element quite easily in AE. If i was in photoshop i would be able to apply a stroke to the layer and it would be applied around the edges between image and transparency. However, in after effects when i apply a stroke effect to the layer it doesn't produce anything but if i apply the stroke effect to a path or mask it works as i would expect and i can keyframe it to "draw on".

 

What i therefore need to do - i think - is figure out a way that i can obtain a path in AE, from the MatID, Alpha or .png transparency which corresponds to the isolated element. I can then use the stroke effect to draw around the object.

 

Hope that makes sense, i can't post any images to help explain at the min as i'm not near a machine with any apps installed.

 

James

[Crazy Homeless Guy]

i see. i was going to suggest watching this tutorial. it doesn't provide the exact thing you want to do, but perhaps it will give you a different way of looking at it. rather than trying to draw in the elements, what if the elements were drawn in, and you simply revealed them in a drawing manner?

 

http://library.creativecow.net/articles/hansen_jaysen/growing.php

 

.

[JamesTaylor]

Thanks Travis, i'll have a watch of the link...

 

going to have to put it to one side for a few days tho as i work on an alternative section of the project. Back to modelling for a bit i'm afraid!!