Is there a way to delete ST-detected picture zones?

[spamsickle]

I'm currently processing a textbook which frequently uses the technique of printing large blocks of text with a gray background. I've made an editorial decision that I prefer the black-on-white version of Scan Tailor's output to the black-on-gray version of the original. Sometimes it happens that these pages also contain a black-on-gray section heading, and in those cases I've decided I prefer to keep the original. The easiest way to accomplish this would seem to be to delete the picture zone for the large blocks, but I haven't figured out how to do this.

I can get rid of both of them by flipping the output mode to black and white (assuming there are no real pictures on the page), but I can't subsequently add back only the pictures zones I want ("Picture zones are only available in mixed mode"). If I switch back to mixed mode, Scan Tailor insists on doing its automatic detection of picture zones before letting me have control, and then the only ability I seem to have is the ability to add, not the ability to subtract. I can delete the zones I add, but even if I draw around an automatically detected zone, my ability to delete seems restricted to my redundant overlay, and doesn't affect the original STD zone.

In cases where there are real pictures on the page, black-and-white mode is not really even an option. It is especially troublesome in cases where (due, no doubt, to variations in illumination in my original snapshots) jagged coastlines of black-on-gray "picture" zone appear in the otherwise black-on-white text. The "thinner" and "thicker" slider just seems to control how fat the font is in the black and white zone. I haven't seen any GUI way to modify how Scan Tailor identifies what is picture and what is not. Am I (hopefully) missing something?

ETA: Never mind; I found it. Draw around the unwanted zone, and in the right-click context menu select "properties" rather than delete. This provides the option of subtracting from the auto zone. Thanks!

[spamsickle]

This does raise another question in my mind, however. I see in the options "Subtract from all layers". How does this option differ from "fill zones"?

[Tulon]

This does raise another question in my mind, however. I see in the options "Subtract from all layers". How does this option differ from "fill zones"?

This one is a stronger eraser. It can be used to make holes in your manual picture zones.