Color Primaries

Color primaries say which exact red, green and blue the RGB values refer to, plus the white point. Together they define the gamut: the triangle of colors the signal can express. ffprobe reports them as color_primaries.

Primaries are a separate property from the transfer function and the matrix, but each video system uses a fixed set, so in practice the three tags usually agree: BT.709 everywhere for HD, BT.2020 everywhere for UHD and HDR.

Code points

Code	FFmpeg name	System	Where you see it
1	bt709	BT.709; also the sRGB primaries	HD video, web video, sRGB images
2	unknown	Not specified	Untagged files; the player guesses
4	bt470m	BT.470 System M	Obsolete NTSC
5	bt470bg	BT.470 System B/G, BT.601 625-line	PAL/SECAM SD and DVD
6	smpte170m	SMPTE 170M, BT.601 525-line	NTSC SD and DVD
7	smpte240m	SMPTE 240M; same as 6	Obsolete
8	film	Generic film, illuminant C	Rare
9	bt2020	BT.2020 / BT.2100	UHD SDR, HDR10, HLG, Dolby Vision
10	smpte428	CIE 1931 XYZ (SMPTE ST 428-1)	Digital cinema distribution masters
11	smpte431	DCI-P3, DCI white point	Cinema
12	smpte432	P3 with D65 white (P3-D65)	Display P3 content, Apple devices
22	ebu3213	EBU Tech 3213-E / JEDEC P22	Rare

The chromaticities you need most often, for example when writing HDR mastering metadata:

Primaries	Red (x, y)	Green (x, y)	Blue (x, y)	White (x, y)
BT.709 / sRGB	0.640, 0.330	0.300, 0.600	0.150, 0.060	D65: 0.3127, 0.3290
BT.601 525 (SMPTE 170M)	0.630, 0.340	0.310, 0.595	0.155, 0.070	D65
BT.2020	0.708, 0.292	0.170, 0.797	0.131, 0.046	D65
P3 (DCI and D65)	0.680, 0.320	0.265, 0.690	0.150, 0.060	D65, or DCI 0.314, 0.351

SVT-AV1 calls code 6 bt601 and code 7 smpte240; VapourSynth uses 170m, 2020, st432-1, and so on. The number is the same everywhere.

Practical notes

Untagged SD. NTSC and PAL primaries differ slightly from BT.709. Players guess when the tag is missing — mpv picks BT.709 for anything 1280 wide or over 576 high, PAL for 576 lines and NTSC for 480 or 486 — so tag SD encodes with the source system or convert them when upscaling (see the color guide).

BT.2020 is a container, not the content’s real gamut. Most HDR is graded on a P3-D65 monitor and delivered in BT.2020. The stream is tagged bt2020; the mastering-display metadata records P3. Don’t tag such a stream smpte432 because the grade was P3 — the samples are BT.2020 values.

BT.2020 does not mean HDR. UHD SDR also uses BT.2020 primaries. Whether a stream is HDR is decided by the transfer (smpte2084 or arib-std-b67).

P3 variants. DCI-P3 and P3-D65 share primaries but differ in white point. Apple’s Display P3 is P3-D65 primaries with the sRGB transfer, so it is tagged smpte432 plus iec61966-2-1.

Retagging does not change the gamut. Tagging BT.709 pixels as bt2020 makes every color look oversaturated; tagging BT.2020 pixels as bt709 makes them look dull. Going from a wide gamut to a narrow one is a real conversion that has to decide what to do with colors outside the smaller triangle — use zscale, libplacebo or VapourSynth resize with explicit input and output primaries, and tone-map at the same time if the source is HDR (see tone mapping).