How to: Scale alpha matt

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Old 01 January 2010   #1
How to: Scale alpha matt

I've got a problem with my alpha showing through around the edge of my image, the image being a sequence of Maya volumetric fluids.

How I can 'choke' it in a bit so I can get rid of most of this?

Old 01 January 2010   #2
Try a dilate/erode node.

Sounds more like a premult alpha issue though.
Old 01 January 2010   #3
is your background color white? Did you premultiply the color by the alpha?
Old 01 January 2010   #4
Yeah it is premultiplied. Ill try with the background white, see how I go.

Thanks guys.
Old 01 January 2010   #5
sorry, I didn't explain myself. I was asking if it was white because that would be an issue with giving funky edges. It should be black.
Old 01 January 2010   #6
mmmm funny that because I know not premultiplying it gives funky edges but the alpha seems to be okay and by making the background white creates a snow blizzard where I cant see the edges but again the alpha seems fine too. Ill fire off a batch render later on and get back to yous with some results.
Old 01 January 2010   #7
Okay my findings are these.

Setting background colour to white, as we know, doesnt work. Initially I thought it had potential because in the render window it rendered with alpha but in batch render mode it doesnt as one would expect.

Secondly, un-premultiplying is also not a viable option as this 'hardens' the edge and with volumetrics, this makes it look worse than an image that has been premultiplied.

What has worked is rendering the fluid with pre-multiplcation on and then using the dilate/erode node in Shake to crush the edge in therefore getting rid of the fuzzy gray line around everything.

Thanks for your help.
Old 01 January 2010   #8
why can't you use a black bg?
Old 01 January 2010   #9
The final test is with a black background with pre-mult on and then brought into Shake to erode alpha inwards to get rid of black ring.
Old 01 January 2010   #10
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