I was working on my book at lunch today and discovered an interesting and (as far as I can tell) undocumented requirement when using the Displace filter. The source and map layers must have the same size. Displace provides a menu of potential map layers in two menus (one for X displacement and one for Y displacement). If the sizes of a layer doesn't match the source layer then the layer doesn't show up in the menu.

This becomes a problem when you're trying to map text to some object using displace. Text layers tend to be compact things, not expanded to the full layer size. So to map text to some “object” in another layer the text layer and the object layer must have the same size.

I googled around, dug through the mail archives and bugzilla and didn't find this mentioned anywhere. There isn't even page for this filter in the online documentation nor in the built in Help browser. So maybe by putting it in my blog some other poor fool can find an answer easier than digging through the displace code like I did.