Overview
IEC 61966-2-1:1999/AMD1:2003 amends the IEC/ISO multimedia colour management standard that defines the default RGB colour space - sRGB. Amendment 1 adds normative and informative annexes that formalize a default YCC (luma–chroma–chroma) encoding for sRGB (called sYCC), an extended-gamut encoding for sRGB (called bg‑sRGB) and its YCC counterpart (bg‑sYCC), and a CIELAB (Lab*) transformation. The amendment is intended for implementers of colour management in multimedia systems and equipment.
Key topics and technical requirements
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sYCC (Annex F - normative)
- Defines bidirectional transformations between non‑linear sR′G′B′ and CIE 1931 XYZ, and the default YCC encoding for sRGB.
- Specifies matrix coefficients (from ITU‑R BT.601) and the sRGB transfer (companding) functions for converting between linear and non‑linear RGB.
- Default quantization and bit depth: 24‑bit encoding (8‑bit/channel) is the default sYCC depth; rules for N‑bit/channel encodings (N>8) are provided.
- Notes retention of out‑of‑range (negative or >1.0) sRGB tristimulus values and calls out mapping needs when converting over‑range 8‑bit sYCC to 8‑bit sRGB.
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bg‑sRGB / bg‑sYCC (Annex G - informative)
- Recommends specific parameters for extended‑gamut encoding (e.g., for 10‑bit/30‑bit total: KDC = 384, WDC = 894) and general N‑bit rules for N>10.
- Defines conversion steps, quantization rules and default 30‑bit (10‑bit/channel) encoding for bg‑sRGB/bg‑sYCC, plus mappings between 8‑bit sRGB and 10‑bit bg‑sRGB.
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Precision and implementation notes
- Recommends using higher‑precision inverse matrix coefficients when supporting >8 or >10 bits per channel.
- Includes both normative transformation equations and practical rounding/quantization formulas.
Applications and who uses this standard
- Colour management implementers: software and middleware that perform colour transforms, ICC profile authors.
- Multimedia application developers: image viewers, editors, video encoders/decoders that need a well‑defined default RGB↔YCC pipeline.
- Hardware designers: display, camera and video‑codec engineers implementing sRGB, extended‑gamut encodings or hardware YCC compression.
- Standards engineers and researchers working with colour fidelity, gamut mapping and digital imaging pipelines.
Practical value: ensures interoperable, unambiguous conversions between sRGB, sYCC/bg‑sYCC and CIE XYZ; provides default quantization/bit‑depth rules to support implementations ranging from 8‑bit consumer content to extended‑gamut encodings.
Related standards
- IEC 61966‑2‑1 (base standard) - sRGB specification
- ITU‑R BT.601 (coefficients used for YCC matrices)
- ISO TC 42 / IEC TC 100 (joint technical committees referenced in the amendment)
Open preview (PDF)Amendment 1 - Multimedia systems and equipment - Colour measurement and management - Part 2-1: Colour management - Default RGB colour space - sRGB
