Speaking of rubble, that was one of the first things I thought about when considering this project. I realized at once that the best way to do coloring book style drawing would be to rely on line thickness to create depth, much as I do in my other drawings, but that to keep the coloring book motif I'd include much less variation within those line. So, foreground would be thick lines, distant background very thin lines. But no thick-to-thin within them. And rubble depends upon a lot of line variance within the forms.

Of course, rubble also depends a lot on heavy shading, and there wouldn't be any of that, either!!

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Well, now at least one image has to be something like this!!! ;)