how to region select/maintain an alpha channel?

[ericsanpablo]

Hello, I'm a little rusty with sa, and I've been trying to figure a couple of things out....I'm pretty sure they are relatively simple. I've created a bunch of free-flowing particle line effects in maya, exported out as tiff (or is targa better?), brought it into studio artist. I'm assuming the alpha channel is there, i forget how to view it. After that I would like to select the alpha channel so that my brush only paints over the particle, maintaining the alpha. But, I would like their to be some fall off, i.e. soften the alpha, so that the brush can, at times, paint a bit off the alpha. Then, I would like to be able to automate this so I can create a video running a Paint Action Sequence. Lastly, I would like to be able to export it so that i maintains alpha and I can bring the footage into a compositing program like combustion or nuke. Thanks!

[Synthetik]

You would build a paint action sequence (PASeq) of the steps you want to run to process the frames that make up the movie. The you could process a folder of images with a PASeq to a movie (or another folder of frame images).

You can use a menu to set the current selection to the source alpha. Then you could just turn on the mask and your auto drawn paint strokes would be masked by the source alpha channel. If you use path masking you'd get some softness since the complete nib of the brush gets drawn in this case at the edge of the mask. An alternative approach would be to turn off the mask and use Path Start conditions like the Inhibitor set to Select Region Only. So this way your paint paths always start in the alpha selected area but the paint strokes can spill out.

In the Process movie settings dialog you can set what you want the output alpha channel to be generated off of. You could just pass through the source alpha. If you want the paint strokes to generate alpha in the canvas as they draw then you need to make sure the canvas preference that turns on alpha drawing for the paint synth is turned on,and you would want to turn the alpha view on in the layers window, and then you'd want the canvas alpha to be used to generate the embedded alpha channel in the output movie (as opposed to using the selection or the source alpha).

[ericsanpablo]

hi john, thank you for the response!

another question though - studio artist seems to have pre-matted (which I believe also means premultiplied?) the footage, is their any way to export a straight matte? Is their any way to just export the alpha channel?

[ericsanpablo]

oops never mind, i figured it out!