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You can adjust memory allocation upwards in the "performance" options dialog. You may need to restart for this to fully take effect (not sure).

I'm not sure if 32-bit Photoshop will ever make more than 2gb available to a filter. (The field that exposes how much memory it has is signed...) Currently, a 7kx10k image will need almost 3gb to run, so I'm not sure that'll work anyway--we're coming up to hard 32-bit limits here (swap won't help).

Note that the greyc commandline defaults to tiling; to get the same type of filtering, you need to use -tile 0.

If you're hitting runtime errors, it's probably just out of memory; it wasn't handling OOM in a thread correctly. I've fixed this and I'll update it in a bit, as well as setting the PS hint that tells it how much memory it'll need (which may or may not matter; the documentation doesn't actually say exactly what this does).

I might try an overlapped tile version: work on tiles (horizontal slices, actually; simpler), but yank out much bigger pieces than are actually being processed to avoid tiling artifacts. This would be slower, though, as part of the work for each slice would be repeated twice.