How sprite sheets work
A sprite sheet (or atlas) is one image that holds many smaller frames. The game or the CSS keeps a map of each frame’s x, y, width and height, and draws only that rectangle. One texture means fewer uploads on the GPU and fewer HTTP requests on the web.
Sheetpack uses a simple shelf packer: frames are sorted tallest-first and laid left to right, wrapping to a new shelf when the row is full, with two pixels of padding so filters do not bleed. It does not rotate frames on the free plan. TexturePacker and other paid packers can pack tighter with rotation and polygon meshes; if you have a large commercial set, those licences exist for a reason. This page is for a small atlas you can build without sending art to a server.