Color and linear-light processing

ClipAsm currently has one project color profile:

config {
    video {
        color = sdr_bt709
    }
}

The setting defaults to sdr_bt709, but writing it makes the project intent visible. It is one profile instead of four independent switches. Primaries, transfer, matrix, and range describe different parts of a signal, but arbitrary combinations are not necessarily meaningful or supported.

The SDR BT.709 contract

Project Video uses BT.709 primaries, BT.709 transfer, BT.709 non-constant- luminance Y’CbCr coefficients, and limited range. ClipAsm stores working Video as 10-bit 4:4:4 FFV1 with signed-16-bit FLAC Audio. It publishes 8-bit 4:2:0 H.264 with left-positioned chroma. Both forms carry explicit color metadata, and ClipAsm verifies it after every render step.

Pixel format and color meaning are separate. yuv420p describes component layout and depth. It does not by itself say whether samples are BT.709, BT.2020, full range, limited range, SDR, or HDR.

Source rules

Still images have an authored convention. Opaque untagged RGB images are sRGB. JPEG Y’CbCr is interpreted as full-range BT.601 with centered chroma before it is converted. ClipAsm currently rejects alpha and embedded ICC profiles because correct support needs explicit compositing and ICC conversion policies.

Video files do not have a safe equivalent default. A video(...) source must state BT.709 primaries, transfer, matrix, range, and chroma location when its pixel format is subsampled. Missing metadata is rejected. ClipAsm does not guess from frame size, codec, or file extension.

PQ (smpte2084), HLG (arib-std-b67), and HDR mastering metadata are rejected under the SDR profile. Converting BT.2020 coordinates into BT.709 coordinates is not tone mapping. A future HDR-to-SDR policy must define reference white, nominal or mastering peak, target display, tone-mapping operator, and metadata handling before it can produce predictable results.

Display-linear pixel math

Encoded BT.709 samples are not proportional to displayed light. Averaging two encoded code values therefore does not produce an optical midpoint. ClipAsm converts picture data to full-range floating-point linear BT.709 RGB before:

• source fitting and resize interpolation;
• zoom_in perspective interpolation;
• the white fade in flash_cut;
• Video crossfade blending.

It then converts the result back to the canonical 10-bit working signal. Trim, repeat, concat, and other routing-only operations preserve the canonical samples without a conversion round trip.

“Linear” here is display-linear. For mastered BT.709 Video, zimg applies its BT.1886-style display EOTF. This is not the inverse BT.709 camera OETF and is not scene-linear radiance. ClipAsm fixes nominal peak luminance at 100 cd/m² and disables zimg’s approximate-gamma option so this behavior stays stable and identity-bearing.

Standards basis

• ITU-R BT.709 defines the HDTV primaries, signal transfer, and Y’CbCr coefficients.
• ITU-R BT.1886 defines the reference SDR display EOTF.
• ITU-R BT.2100 defines PQ and HLG HDR television systems.
• ITU-T H.273 defines independent code points for primaries, transfer, matrix coefficients, and range-related video signal metadata.

See From source to published video for the surrounding pipeline and Files and configuration for authored settings.