David Wright wrote:

Does the gimp support opacity layers? By opacity layer I mean an layer with a 8-bit numer for each pixel, where instead of being intrepreted as a gray or color intensity value, the number is intrepreted as a pure opacity. When it is 255, the pixel is black; when it is 0, the pixel below shows through perfectly; when it is intermediate, the pixel is an appropriate average of black and the color of the pixel below.

Assuming such a layer exists, how can I transform a grayscale layer into an opacity layer?

This sounds like you want layer masks or layer modes. But you probably won't need them here.

This problem arises because I have scanned penciled text on white paper into a single-layer grayscale image. I want to use the gimp to make that text appear over another image. I have already tried (1) making a single color transparent and (2) using the magic wand to "partially" select the text. These solutions all have the problem that some of the color of the original background "leaks" into the final image near the text. That's why I want to use the intensity values of the original purely to measure opacity, not to communicate any color information at all. How can I do this?

Try Filters->Colors->Color To Alpha

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