question about over operator for images with no alpha/matte channel

Hi,

I’m new to Nuke but pretty familiar with the capabilities of the program.

I’m trying to learn the merge operations, starting with the Merge(over).

I understand that this operator expects a premultiplied image that is, correct me if I’m wrong, an image with an alpha channel integrated with the file.

Image A 400×400 pixels a picture of a happy face
Image B 400×400 pixels the Nuke Checkerboard

So OK, Image A has NO transparency, you can’t see through it.
I put a transform and translated Image A out of the frame ( I moved it over) and with the merge you can now see the checkerboard under it.

What I want to do is add an alpha channel to Image A to make it a premultiplied image.

Could anyone help me figure this out?

Notice how the checkerboard underneath shows through a little but with the transform it doesn’t show through the image itself, only around the sides where it was moved. Could anyone explain why that is?

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