Subpixel

A subpixel is the undrawn fraction of a pixel in a position or speed. It lets things move by less than a pixel per frame, as in a slow walk. The fraction sits beside the whole-pixel position and carries into it when it adds up to a pixel. Its storage and resolution decide which speeds a game can represent.

#In the games the atlas has read

• Super Mario Bros. stores vertical speed as a pixel byte and a fraction byte, in 1/256 px. Each frame, it adds the fraction into a subpixel byte of the position¹. Horizontal speeds are coarser, in 1/16 px/frame².
• Sonic the Hedgehog keeps 32-bit 16.16 positions and shifts 8.8 speeds into them, so its motion also resolves to 1/256 px³.
• Celeste keeps whole-pixel positions plus a float remainder per axis, and moves by the rounded remainder⁴.

All three draw only the whole-pixel part, so the atlas reports jump heights in whole drawn pixels.