View Full Version : alpha antialiasing problem

10-03-2004, 10:57 AM
i have a scene of a glass lantern on a background, i only need the background to get reflections and refractions on the glass of the lantern (that will eventually be a layer in photoshop).

i put the lantern on a render layer and the background plane on another render layer, turned off the background render layer to render only the lantern...

everythings cool, except the antialiasing of the edge of the lantern is showing up when i take the image into photoshop and take out the alpha channel. it leaves a slight black rim around the lantern.

i tried fixing this by changing the environment color of the camera im rendering with to yellow, (the color of the background im going to put the lantern on in photoshop). that created another problem, now the metallic areas of the lantern are reflecting yellow... if i make the environment white, then the reflections are nice but i get the same initial problem but with white instead of black....

how can i fix both

10-03-2004, 12:48 PM

a typically premultiply issue - means that the anti-aliased edges of your rendered image contain black, because you rendered on black. they will contain yellow if you render on yellow. don't render on yellow, that's dirty :)
there are two ways.

1.) turn off "premultiply" in the render globals (for mental ray, this is under the "framebuffer attributes" tab). this way your RGB channels don't get aliased. just the alpha, which fits with photoshop's "default" behavior about that :)

2.) render your image (on black) as you did already. go to photoshop, apply the alpha to get your transparency. then apply the "remove black matte" function. it's under the layer menu in the submenu "matting" (or so) at the very bottom.


10-03-2004, 01:01 PM
ok. im using maya software render... when i turn OFF premultiply in the render globals maya software tab under color/compositing i get another option which is: premultiply threshold... do i do anything here?

is it also called ""premultiply" threshold" to confuse me? i thought i turned it off...

10-03-2004, 01:16 PM
ah look, maya renderer special fun. leave it at 0.

"... set Premultiply Threshold to 1 for smooth matte edges, with no jagged edges. The default value is 0."

We want shockingly jagged edges! ;)

10-03-2004, 01:19 PM
thanks zuao... i guess i should start reading more from the maya docs...

thanks for ur help... appreciate it

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