Why does the edges change?

[chagchag]

Hi.

I'm new to fusion (just got the learning edition), and new to this forum as well.

I've barely got to play around a little, and have come across my first nag:
If I merge a background with a keyed or masked foreground, and add a color corrector to the foregound (before the merge, of course) and adjust the contrast, I get really bad results:

If I lower the contrast, the background is affected as well (going white), and if I increase the contrast, the edges of the matte is ruined by getting dark and evident.

Why is this? What am I missing? How do I avoid this?
This does not happen if I do the same simple stuff in AE.

[mike beckman]

It sounds like you're affecting the alpha channel of your keyed image as well.
Do you have premultiplied checked on?

[chagchag]

Thanks for the tip, but it turned out the fix was in the color-corrector tool. I discovered the magic of the "pre-divide/post-multiply" checkbox on the color-corrector's Options-tab. Et voila: Smooth edges!

[alive]

You might well have given the free official courseware too. The exact same problem is troubleshooted there!

[niic]

Just reading through the threads and was hoping someone could perhaps elaborate on this Pre-div/post/mult check for the CC node!

In the help files:
Pre-Divide/Post-Multiply
Selecting the Pre-Divide/Post-Multiply checkbox will cause the image pixel values to be divided by the alpha values prior to the color correction, and then re-multiplied by the alpha value after the correction.

This helps to prevent the creation of illegally additive images, particularly around the edges of a blue/green key or when working with 3D rendered objects.

--
So does this mean it takes a pixel and then divides it by the opacity of the alpaha channel prior to CC then multiplies the alpha value back again after CC? I totally don't get it!?

[Splash]

Color correct the images unpremultipled, but you need to check the operators before the color correction( for instance if your keyer premultiply your image, you need to umpremultiply the image to make the color correction, after this operation you "re" multiply the image). To text this theory, you can multply one image (wihtout alpha channel) for a grayscale image and multiply the background image for the inverse of matte and add the two results (its a over operation)