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How do I chop out regions of a bitmap in Illustrator, revealing the transparency beneath?

I'm starting with a raster graphic imported from Photoshop. The goal is to cut off a precisely-measured triangular section at each corner of the image.

My intuition is to use the pen tool to mark out the exact area I want to cut out, then hit the delete button and watch everything within the path bounds vanish back to transparency. And that's the goal. But it doesn't seem to work. Hitting delete, once I've laid out the path with the pen tool, has no effect. Using the eraser tool inside the path has no effect. I don't see a way to convert the path to a selection.

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Clarifications: The need to precisely measure the triangle is the motivation for using Illustrator. –  baudot Jan 16 '12 at 22:03
    
It is possible to achieve the effect needed by first making the triangle in Illustrator, coloring it a shade that doesn't appear in the raster graphic, saving that to an ai file, and then opening the ai file in photoshop. Then I copy and paste that into the photoshop graphic, position it precisely with the transform dialog boxes, merge the new layer from the paste with the layer I want to cut out, select all by color, and finally delete it. But surely there's an easier way! –  baudot Jan 16 '12 at 22:16
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1 Answer

up vote 4 down vote accepted

Create an object that is the shape of the opaque areas of your bitmap. Make sure it is above your bitmap. Select both objects and hit Ctrl + 7 to mask your bitmap.

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I was hoping to learn the approach by which I could chop bits out at a time, but this solves the current problem yet more elegantly. Thanks. –  baudot Jan 16 '12 at 22:40
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Illustrator doesn't really know of things like "pixels" so you can't "chop bits out at a time" like you would in a raster graphics program. It helps to think of whatever bitmap image you've imported as a blank rectangle (that's how Illustrator thinks of it) that you're removing specific shapes from. –  lawndartcatcher Jan 17 '12 at 13:03
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