Pixel

A pixel is one picture element at an indexed position in a raster image. It can contain a single grayscale value or several colour components. The word can also refer to a physical display element; the indexed element of the stored image is the meaning used here. One stored pixel need not occupy exactly one physical display element when the image is resized. Microsoft's bitmap documentation likewise distinguishes an image's grid and its numerical colour representation.^[ref]

Separate position from value: raster sampling and reconstruction concerns the spatial grid, while sample depth concerns how many bits encode each component. Four times as many pixels can describe a finer grid without improving the quantization of any value. Conversely, more bits per sample can distinguish more shades without locating an edge more precisely.

In the binary example under run-length encoding, one bit per pixel selects black or white. In the grayscale example under lossy block averaging, eight bits per pixel select one of 256 codes. Calling them codes keeps their numerical values distinct from the physical light that a display emits.

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]