 Sample depth

The sample depth $b$ is the number of bits allocated to one stored sample. An unsigned $b$-bit integer has $2^b$ codes, $0,\dots ,2^b-1$. This is per component, not automatically per pixel: three eight-bit components require 24 payload bits per pixel. A raster image with $WH$ pixels and $k$ such components requires $WHkb$ payload bits before data compression, padding or a file header.

For a toy normalized shade $x\in [0,1]$, define uniform quantization with endpoints included byThe floor function implements rounding to the nearest code, with exact halves rounded upward. The reconstruction step is ${\delta }_b=1/(2^b-1)$, and $|{\hat{x}}_b-x|\le {\delta }_b/2$. For $b=2$, the levels are $0,1/3,2/3,1$; for $b=8$ they are $0,1/255,\dots ,1$. Increasing sample depth refines this value grid without changing the spatial grid. These levels are uniform in $x$; if $x$ is encoded sRGB, they are not uniform in relative luminance.

PNG uses a bit-depth field per sample or palette index. Its grayscale type without alpha permits 1, 2, 4, 8 or 16 bits per sample; its truecolour type permits 8 or 16 bits per component. These are format-specific restrictions, not a rule for every raster image.^[ref]