Framebuffer

A framebuffer is computer memory holding an image's pixel values in a representation usable by graphics hardware or software. A pixel format specifies how stored bits encode those values. The image's dimensions alone do not determine its complete memory layout: rows can include padding, and different layouts can organize components differently.^[ref]

For a concrete packed-row model, let the image be three pixels wide and two rows high, using four bytes per pixel. A row's actual pixel data occupies $3\times 4=12$ bytes. If each row starts 16 bytes after the previous one, the framebuffer stride is 16 bytes and four padding bytes separate consecutive pixel rows. Reserving two complete row slots takes $2\times 16=32$ bytes, although the pixel values themselves occupy only 24 bytes.

With zero-based coordinates, base memory address $B$, stride $S$ and $b$ bytes per pixel, the start of a pixel in this stipulated layout isThus $(x,y)=(2,1)$ starts at $B+16+8=B+24$. Replacing $S$ by the packed width12 would instead address the wrong location. This formula assumes packed, byte-addressable rows; it is not a formula for every possible pixel format.

The Linux interface makes the distinction explicit: line_length reports row length in bytes, separately from resolution and bits_per_pixel. A program should use the layout supplied by the interface when interpreting a framebuffer, rather than infer it from the visible dimensions alone.^[ref]