Context-aware Locator

Question

I'm trying to use a Locator for some image processing tasks. Unfortunately, the images we're using have a high dynamic range of low-frequency black and white features. I've made a custom Appearance for the Locator with some concentric circles that works pretty well, but am wondering about using other compositing operations instead, like, Xor.

The current implementation of Locator seems to composite with a simple Over operator. Meaning that the 'cross hairs' disappear on black patches.

Has anyone successfully created a more dynamic locator appearance or have some thoughts about how one might do that?

Answer 1

I don't think you can change the compositing operator but you can dynamically change the appearance. A simple example:

im = LinearGradientImage[];

DynamicModule[{pt = {10, 10}},
 LocatorPane[Dynamic[pt], im, Appearance -> Graphics[{
     Dynamic @ GrayLevel[1 - PixelValue[im, pt]], Disk[]}, ImageSize -> 20]]]