magnus919_agent-skills/color-management/references/glossary.md

Color Management Glossary

Term	Definition
BPC	Black Point Compensation. LCMS2 algorithm that maps source black point to destination black point so darkest shadows aren't crushed. Essential for LCD monitors (which can't display true black).
Chromaticity	Color quality defined by hue and saturation, independent of luminance. Expressed as xy coordinates in the CIE xyY chromaticity diagram.
Chromaticity coordinates	The xy values that define a color's position in the chromaticity diagram. Working space primaries are defined by their chromaticity coordinates.
CIELAB (CIE Lab)*	Perceptually uniform reference color space derived from XYZ. L* = lightness, a* = green-red, b* = blue-yellow. Primary reference space for ICC profile conversions.
Clipping	Loss of color information when out-of-gamut colors are forced to the nearest in-gamut values. Occurs during conversions to smaller color spaces.
Color gamut	The subset of all visible colors that a device (monitor, printer, camera) can reproduce or a working space can encode.
Color space	A method of representing color numerically. RGB, CMYK, CIELAB, and xyY are different "color spaces" — they define different coordinate systems for the same colors.
Conversion (ICC profile)	Changing an image from one color space to another. Colors are preserved; RGB numbers change.
D50	Standard illuminant — 5000K daylight. ICC profile connection space (PCS) uses D50 as the reference white point.
D65	Standard illuminant — 6500K daylight. sRGB's white point. Roughly corresponds to daylight on a slightly overcast day.
dcraw	Dave Coffin's open-source raw file decoder. Supports essentially every digital camera ever made. The foundation of most open-source raw workflows.
Display-referred	Image data where RGB values are bounded by 0.0-1.0, where (1,1,1) = maximum display white. Standard for web and print output.
Embedded profile	An ICC profile stored in the image file's metadata. Tells color-managed software how to interpret the image's RGB values.
Gamut	See Color gamut.
Hex quantization	Rounding of ICC profile values during the encoding process because ICC V2 format uses 16-bit integer encoding. Causes minor deviations from mathematical ideal. ArgyllCMS compensates for this; LCMS does not.
Imaginary colors	Colors that can be encoded in a working space but don't correspond to any real visible color. ProPhotoRGB and ACES include many imaginary colors — this is deliberate, to capture all possible real colors.
Input profile	A profile that describes how a device (camera, scanner) captures color. Camera input profiles often require negative tristimulus values.
LCMS	LittleCMS (LCMS2). The open-source color management engine used by GIMP, Krita, digiKam, RawTherapee, and most Linux imaging software.
LCH	Lightness, Chroma, Hue — a polar transform of CIELAB. Allows separate editing of tonality (Lightness) and color (Chroma + Hue).
LUT profile	A profile defined by lookup tables rather than matrix math. Required for perceptual intent. Printer profiles and many monitor profiles are LUT.
Matrix profile	A profile defined by a 3×3 matrix + TRC. Simpler, smaller, but cannot support perceptual or saturation intents. All standard working spaces (sRGB, ProPhotoRGB, etc.) are matrix.
Monitor profile	An ICC profile describing a specific monitor's color behavior. Created by measuring the monitor with a hardware colorimeter + ArgyllCMS or proprietary software.
Negative tristimulus	XYZ or RGB values below zero. Required for accurate camera profiles because camera sensors don't see like human eyes. ICC V2 prohibited them; V4 allows via floating point.
Perceptual uniformity	A property where equal numerical changes produce equal perceptual changes. CIELAB is approximately perceptually uniform; XYZ is not.
Primary	The most intense red, green, or blue color a color space can encode. Working spaces are defined by their primaries' locations in XYZ/xyY.
Profile	See ICC profile.
Profile Connection Space (PCS)	The reference color space (CIELAB or XYZ) used for ICC profile conversions. All profiles define their color gamut relative to the PCS.
Radiometrically correct	Editing that accurately models how light behaves in the real world. Requires linear gamma RGB and appropriate blend modes.
Reference color space	A mathematically defined color space that encompasses all visible colors. XYZ and CIELAB are reference spaces.
Relative colorimetric	Conversion intent that preserves in-gamut colors exactly and clips out-of-gamut colors to the nearest in-gamut equivalent. The standard intent for display.
Scene-referred	Image data proportional to the original scene's light intensities. No upper bound on RGB values. Used for HDR and maximum editing flexibility.
Soft proofing	Previewing how an image will look when converted to a different (usually smaller) color space, before actually doing the conversion. Essential for avoiding unwanted clipping.
sRGB	Color space created by HP and Microsoft in 1996 to match CRT monitor phosphors. The universal standard for web images. Too small for many photographic colors.
TRC	Tone Reproduction Curve. Defines how RGB values map to linear light intensity. Gamma curves, sRGB piecewise curve, and LAB L curves are examples.
Tristimulus values	Three numbers (e.g., RGB, XYZ) that define a color in a three-dimensional color space. Based on the trichromatic theory of human color vision.
Unbounded conversion	ICC profile conversion at 32-bit floating point that allows negative and >1.0 RGB values, preventing gamut clipping. Requires true gamma TRC profiles.
Well-behaved	A working space where R=G=B produces neutral gray (a*=b*=0) and R=G=B=0 produces true black. Only ~9 of 30 surveyed profiles are fully well-behaved.
Working space	A well-behaved color space used for editing images. sRGB, AdobeRGB, and ProPhotoRGB are common working spaces.
xyY	A reference color space that separates chromaticity (xy) from luminance (Y). The xy projection is the familiar "horseshoe" chromaticity diagram.
XYZ	The 1931 CIE standard reference color space. All color management ultimately traces back to XYZ.