Chaos Corona Forum

General Category => General CG Discussion => Topic started by: arqrenderz on 2014-02-12, 16:11:14

Title: Best way to make an alpha channel?
Post by: arqrenderz on 2014-02-12, 16:11:14

Hi, im having some trouble with the foreground trees and the cut out dor the sky, i have tried the alhpa channel, wire color and so on.. but it cuts so much of the trees or too little , with the alpha some times cut some branches and it leaves the leafs flotating..
im using Ps , is there a good way to make it? do i have to make a mask with feather or something??

(http://imageshack.com/a/img801/7897/uvp6.jpg)

Title: Re: Best way to make an alpha channel?
Post by: Fibonacci on 2014-05-24, 19:49:13

Hi,

I've got something same problem. I have rendered the pics with just simple white background, and there I did not find away to cut out correctly in PS.
Where the leaves too dense, between those no chance to cut out normaly.

Did you solved the correct way?

Cheers!

Title: Re: Best way to make an alpha channel?
Post by: Adanmq on 2014-05-24, 21:56:28

Hi.
The best way is render using a black background, and use premult. I´m using Nuke, but works also in AE. But even if the alpha is perfect, you must use a "correct" background, if you end using a totally diferent background it wont look realistic.

hope this can help.

Title: Re: Best way to make an alpha channel?
Post by: Adanmq on 2014-05-24, 23:18:37

In order to illustrate it i made a quick render. You must look at it at full res to view the diference.

Hope it helps you.

Title: Re: Best way to make an alpha channel?
Post by: borisquezadaa on 2014-05-25, 09:02:39

My grain of sand...
In after effects you could use "remove color matting" effect. It has a option to select the rigth color.
We use it a lot to correct image sequences with wrong alphas and works remarkably well.
You would not belive the kinds of images with messed up alpha channel must fix at work.

I belive in photoshop there is something similar. Defringe, if i recall correctly but doesnt work as well as After effects one.

Copy pasta:
"Color matting occurs when the edge of an object within a transparent layer that contains a premultiplied RGB channel retains the color from its original background instead of blending partially transparent areas with the new background. This produces a halo, or "color matting," effect around the outside edge of objects. To remove this color matting effect, you'll need to use the "Remove Color Matting" option on the video channel.

I guess the magic term to search in google is "color matting", it took me a while to find out, now you know it in 10 seconds.

Title: Re: Best way to make an alpha channel?
Post by: fellazb on 2014-05-25, 11:18:42

Hi there,

I use the defringe option in Photoshop for this too. It's under layer>matting>defringe. Usually 1 pixel is enough to get rid of the annoying alpha edges.