Yes it wasn’t really that difficult. A bit hard to explain but I’ll try.
The thing is, since rendering text is awfully slow, every time you see a dialogue box in the game, it’s pre rendered in an image, and then the image is drawn on screen every frame.
So in order to animate the box reveal with the text at the same time, I need to display the image in two passes. First I draw a new mask in the dialogue box image (that mask reveals more and more as the box grows), and then the box is drawn on screen. Initially I was just an imaging a white square polygon and then replaced it with the image with the text once it was fully open. It’s not a big hit on performance but since there’s no gpu on the Playdate you have to be mindful of everything you draw if you want to maintain 50 fps.