Hopefully someone can help me out here. I am rendering an image that I am going to composite together in layers and I rendered the foreground part with the Alpha channel rendered. When I composite it onto a plane color background I end up with a white fringe around the image. My question is what is the best way to render an alpha channel so you dont get that fringe as in the attached image. The simple test I ran before doing the larger image worked fine but when I did my foreground layer I get this fringe. Any help would be appreciated. Mike

What format image file are you using? JPG's sometimes do this because of the compresion. Use PNGs instead, ot TIFs.

I am using a tiff as the file type, PNG and jpegs don't allow you to save Alpha channels when you render . I did figure out a solution. My image was using the realistic sky and Skylite rendering option to light my scene. What I found is if you disable the realistic sky and put an image in the Background portion of the Scene effects tab of the properties tray and use this to light the scene instead of the realistic sky the fringe disappears. Thanks

It sounds like your alpha is not pure black and white. Try converting it with the threshold tool in PS to get rid of any not quite pure blacks and whites.

PNGs do allow for alpha channels, I have used it with a Carrara file before. I agree with sailor ed that for your use you need pure black and white for the alpha map.

Bluetone, I tried in C4 and PNG is not an option, in C5 the Alpha channel is greyed out. I will try the threshold tool too,I played with a bunch of things in corel's mask outline settings with moderated success. I am very happy with the results of the preliminary render I did get , I will post the final image soon as I have to be done the image in a week. Mike

Did you have any kind of behavior or effects? I agree with the suggestions. I have seen this happen tho more so when doing animation.Also meant to mention antialias upped, but that is obvious and try no background also or the black or white as mentioned.

Message edited on: 11/28/2005 17:10

A good way to handle this is by using a Targa (.tar) file with an alpha mask in the color channel. Carrara automatically reads the alpha and applies it without the need to apply a seperate alpha in either the Alpha or Transparency channels. Mark

I use Targa or Tiff to render an alpha (RGBA) using C4Pro. Either work well for still or motion compositing.

Depends on the target application. As Mark mentioned Targas carry the alpha information over well. Not just in Carrara but also when I use them in Lightwave. One thing that would be nice is to have the option of seperate alphas. In Lightwave I can save individual RGB frames in a number of different formats, I also have the option of saving the alpha information as seperate frames. That way I dont have to worry about the target application, I just select the alpha's to use as a mask. The masks always come out clean when I do it this way. One drawback is that it creates more files though.

If you can pre-multipy the alpha and color channels. That has always worked for me.

I tend to render sequenced photoshop files with premultiplied alpha (sequenced, of course, only if it's an animation). From time to time I'll get this kind of fringe effect, but I get rid of it in AfterEffects with a simple choker. You could use the defringe effect in Photoshop, as well, I suspect.