Act transitions, part 2: Angel Island Zone

Angel Island Zone's act transition does not happen during the act 1 results screen. Instead, it takes place much earlier in the stage, at the act 1 boss cutscene encounter where the entire level catches on fire.

The cutscene area at the very end of the act 1 layout is interesting in that all the background tiles, including the sky and the horizon line, are actually part of the level's foreground plane. Furthermore, all tiles are set to low priority, as is every sprite that appears throughout the cutscene.

The reason for this should be clear: the game is about to pull some shenanigans using the background plane. Although they're usually drawn behind the foreground plane, by setting the background's tiles to high priority, we can make them render in front of the foreground. This trick is used to display the scrolling flames that cover up the entire screen, during which the act transition silently takes place.

Finally, there's an extra trick that brings the whole cutscene together. As the boss descends from the top of the screen, several more Fire Breath robots scroll across the sky in the background, logically going behind the palmtree on the left side of the screen. But as we know now, the background tiles in this area are all set to low priority, so the robots' sprites should be getting drawn in front of the tree!

The trick here is to introduce an extra sprite where the robots intersect with the tree. This sprite also needs to be set to low priority so it's covered by the background flames, but by giving it a low priority value on its SST, we can ensure it's drawn on top of the robots, but still behind Sonic and everything else.