Ahhh, that makes so much more sense! I've done something kind of similar but way overcomplicated things and did bitmap changing of an image, hahaha. This is very smart!
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I took a similar wrong turn on the initial world gen lol. Had to rewrite it too. The kraken pathing is still terrible lol. But it works for a jam game... prevent becoming too challenging because I don't have time for difficulty balancing.
The reason the shader ended up like that is that for the first 80% of dev there was no hiding at all, all islands and the 3 characters were fully visible so I could debug issues. So the overlay idea came from that.