Can anyone help in making this clipping mask?

[ItsOscar]

I know a clipping mask lets you keep what is overlapping, but how do you keep the difference? For example, on the attached screenshot, how would I keep the red part of the ellipse? Any suggestions would be greatly appreciated.

[Crapaud]

Copy the blue channel and adjust to make the blue area white and the red area black; I used curves.

On the layers palette, double click the background layer to unlock it, cntl+click the new alpha channel, invert the selection and make it a mask.

The mask might need a touch-up with the burn/dodge tool to tidy it up.

[ItsOscar]

Sorry, maybe I should've been a little more specific. I'm using Illustrator to try to get this shape. I need to actually cut it out so I can layer something under it.

[Crapaud]

It's been a long time since I've used Illustrator, but there's a palette with shapes on it, one of which is unite (not the one you want). There should be one called something like divide or cut. Highlight the 2 shapes, click and delete the parts you don't want.

IIRC there's also a dropdown list with the commands: add/subtract foreground/background.

Make a copy of the file and have a play around.

[ItsOscar]

Got it! In the Effect menu, there's an option called Pathfinder>Subtract. When shapes are grouped, it subtracts whatever is overlapping.

[Crapaud]

That's the one! Couldn't remember, too near bedtime.