comping alpha in pshop...

V-Ray V-Ray Common Tutorials, Tips & Tricks
1

Came across this article that may help people comp elements in photoshop…

http://www.digitalartform.com/archives/2005/10/compositing\_pre.html

Basically the method is…

Top layer - rendered object on black background (no transparency or alpha on this layer!). Set blending mode to linear dodge(add).
Middle layer - ‘holdback matte’ - solid black fill using alpha of top layer.
Bottom layer - your background plate.

No black/white/colored outline at the alpha edges - works great!
pshop_alpha.psd (449 KB)

2

Great tip, thanks! Those nasty white edges have always bugged me, not anymore..

cheers

3

From the comments 8)

                                                                                        **[archibald](http://www.ragecg.com/)                                      \| [October 25, 2005  2:58 PM](http://www.digitalartform.com/archives/2005/10/compositing_pre.html#comment-1437)                                      \| [Reply](http://javascript%3Cb%3E%3C/b%3E:void%280%29;)**

Am I wrong in that if one renders premultiplied on either a Black or White, then using photoshops’:
LAYER > MATTING > REMOVE BLACK/WHITE MATTE
will do this same thing in one shot?
I’ve been doing this for 3 years with no issues.
Am I missing something?
Just curious:)
Great site btw!!

                                                                                        **[Joseph Francis](http://www.digitalartform.com/)                                      \| [October 25, 2005  4:43 PM](http://www.digitalartform.com/archives/2005/10/compositing_pre.html#comment-1438)                                      \| Reply**

Nope. You are right.

4

This doesn’t seem to be working correctly for me. The edges still look a little wrong-it only works correctly if I’m in 32 bit mode. Also the “Remove black matte” looks slightly different too. Anyone else?

5

this also works with a *.tga 32bit with “premultiplied alpha” unchecked.

best regards
themaxxer

6

thanks for the helpful link