Image glossary

Every entry says what the thing is, where it matters, and where it stops mattering. Where a term has more than one common name, all of them are listed — “colour profile” and “ICC” are the same page.

Alpha channelalso: alpha, transparency channel

An alpha channel is a per-pixel transparency map stored alongside the colour channels, recording how much of each pixel is opaque.

Aspect ratioalso: ratio, image proportions

Aspect ratio is the proportional relationship between an image's width and height, expressed as two numbers such as 16:9, and it is what determines whether a platform crops your image.

AVIFalso: AV1 Image File Format, .avif

AVIF is an image format derived from the AV1 video codec that compresses roughly 40-55% smaller than JPEG at matched visual quality, at the cost of much slower encoding.

Bandingalso: posterization, colour banding

Banding is the appearance of visible steps in what should be a smooth gradient, caused by too few distinct values being available to represent a gradual change.

Bicubic interpolationalso: bicubic, image interpolation, resampling

Bicubic interpolation is a resampling method that estimates each new pixel from a weighted cubic function of the sixteen surrounding source pixels, producing smoother results than simpler methods without adding any real detail.

Bit depthalso: colour depth, bits per channel

Bit depth is the number of bits used to store each colour channel of a pixel, determining how many distinct levels are available per channel.

Bleedalso: print bleed, full bleed

Bleed is extra artwork extended beyond the trim line of a printed page so that colour still reaches the edge after the paper is cut.

Chroma subsamplingalso: 4:2:0, 4:4:4, chroma subsampling ratio

Chroma subsampling stores colour information at lower resolution than brightness information, exploiting the eye's poor sensitivity to fine colour detail to remove data before compression even begins.

Display P3also: P3, wide gamut

Display P3 is a wide-gamut colour space used by modern Apple devices and many recent screens, covering roughly 25% more visible colour than sRGB, particularly in reds and greens.

DPIalso: dots per inch

DPI is the number of ink dots a printer places per inch, and in everyday use it is the ratio that converts an image's pixel dimensions into a physical printed size.

EXIFalso: Exchangeable image file format, image metadata

EXIF is metadata embedded in an image file by cameras and phones, recording capture settings, device information, orientation and frequently GPS coordinates.

ICC profilealso: colour profile, color profile, ICC

An ICC profile is a standardised description of how a device or file's colour numbers map to actual perceived colours, allowing the same image to look consistent across screens and printers.

JPEGalso: JPG, .jpg, .jpeg

JPEG is a lossy raster image format from 1992 that remains the most universally readable image format in existence, with no support for transparency.

Lossless compressionalso: lossless

Lossless compression makes a file smaller by encoding redundancy more efficiently, and decompression returns data bit-for-bit identical to the input.

Lossy compressionalso: lossy

Lossy compression permanently discards image information judged imperceptible in order to make files dramatically smaller, and the discarded data cannot be recovered.

OCRalso: optical character recognition, image to text

OCR is the process of detecting and transcribing text from an image into machine-readable characters.

PDF/Aalso: PDF/A-1, PDF/A-2, archival PDF

PDF/A is an ISO-standardised subset of PDF designed for long-term archiving, requiring a document to be fully self-contained and forbidding anything whose rendering depends on external resources.

PNGalso: Portable Network Graphics, .png

PNG is a lossless raster image format with full alpha transparency, excellent on screenshots and line art and very inefficient on photographs.

PPIalso: pixels per inch

PPI is the number of pixels per inch in an image or on a display, and it is the measurement people usually mean when they say DPI.

Quantizationalso: quantisation, quantization table

Quantization is the step in lossy image compression that rounds transformed image data to coarser values, and it is the point at which information is permanently lost.

Raster vs vectoralso: bitmap vs vector, raster graphics, vector graphics

A raster image stores a fixed grid of pixels and degrades when enlarged; a vector image stores shapes as mathematical descriptions and renders sharply at any size.

sRGBalso: standard RGB

sRGB is the default colour space of the web, defining a specific set of red, green and blue primaries and a transfer curve that browsers assume when an image carries no colour profile.

Super-resolutionalso: AI upscaling, neural upscaling

Super-resolution is the use of a trained model to increase an image's resolution by generating plausible detail consistent with its training data, rather than by interpolating between existing pixels.

WebPalso: WebP format, .webp

WebP is an image format developed by Google that supports both lossy and lossless compression with transparency, and produces files roughly 25-35% smaller than JPEG at matched visual quality.