What are "subpixels" in Super Mario Bros on the NES? How does it relate to wall clipping?
At 4:42 in the following video, the notion is mentioned in the context of how it relates to "wall clipping":
My own reasoning:
Subpixels is a term used to describe Mario's x,y coordinates as a float. This float increments (in "subpixels") as a function of time multiplied by some gravity constant (y axis), player input (x and y), etc. Eventually the increment is enough for a full pixel of movement. This allows for more smooth acceleration of the movement of Mario in the game.
It appears the collision detection is not perfect in this game. Not every pixel of Mario is checked for collision with every pixel of solid sprites such as pipes and bricks.
This is probably due to optimizations, but exactly how the collision detection in Super Mario Bros works eludes me, and I believe it is key to understand the collision detection in order to understand the wall clipping.