Overview
SIST ISO 18619:2020 - "Image technology colour management - Black point compensation" specifies a formal, reproducible procedure (including computation) for performing black point compensation (BPC) when converting colours between ICC profiles. The standard defines how to adjust an ICC profile transform to account for differences between the darkest reproducible values (the “black point”) of a source and a destination colour space so shadow detail and black utilization are preserved. It complements the relative colorimetric mechanism that already addresses the white (light) end.
Key technical topics and requirements
- Inputs required: SourceProfile, DestinationProfile (ICC profiles) and RenderingIntent (RelativeColorimetric, Perceptual, or Saturation). BPC is not appropriate for AbsoluteColorimetric.
- Profile support: Profiles conforming to ISO 15076-1 and ICC.1:2001-04 (Version 2 and Version 4) are supported; profile types may be Input, Display, Output, or ColorSpace.
- Core computational outline: Four main steps - compute SourceBlackPoint (4.2.3), compute DestinationBlackPoint (4.2.4/4.2.5), compute mapping between them (4.2.6), and apply the mapping in colour conversion (4.2.7).
- Functions & concepts:
- T(x, Profile1, Profile2, Intent): transforms colour coordinates between profile colour spaces.
- D(Profile, Intent): finds the darkest colour coordinate by testing defined vertex sets of data colour space (Gray, RGB, CMYK).
- LabIdentityProfile: identity transform between CIELAB and profile connection space (PCS).
- LocalBlack, SourceBlackPoint, DestinationBlackPoint: intermediate values based on transforms and clamped L* (commonly capped at L* = 50 for dark-end handling).
- LUT vs non‑LUT destinations: Special handling for LUT-based DestinationProfiles (inRamp/outRamp calculations, BT transform, monotonicity fixes and a mid-range straightness test) vs simpler calculation for non‑LUT profiles (Gray or RGB).
- Safety rules: Data colour spaces allowed include Gray, RGB, CMYK or CIELAB; rendering intents must match for source and destination profiles.
Practical applications and users
ISO 18619 is relevant for:
- Software developers building colour management modules (CMS), image-editing apps, RIPs, and proofing tools that apply ICC profile conversions.
- Prepress and print professionals ensuring consistent shadow detail and black reproduction across devices and media.
- Camera, scanner and display manufacturers implementing colour pipelines.
- Colour scientists and workflow integrators seeking repeatable BPC calculation and consistent cross-application behaviour.
Benefits include preserved shadow detail, consistent black rendering across devices, and efficient reuse of computed transforms for many images.
Related standards
- ISO 15076-1 (ICC-based colour management architecture and profile format)
- ICC.1:2001-04 (File format for color profiles)
Keywords: ISO 18619, black point compensation, BPC, ICC profiles, colour management, color management system, CIELAB, rendering intent, SourceBlackPoint, DestinationBlackPoint, LUT-based profile.
