Matrix Coefficients

Matrix coefficients define how gamma-encoded R′G′B′ is turned into one luma (Y′) and two color-difference (Cb, Cr) components, and therefore how a player turns them back. For the common systems the matrix is set by two weights, KR and KB: luma is KR·R′ + (1−KR−KB)·G′ + KB·B′. ffprobe reports the matrix as color_space.

The matrix is independent of primaries, transfer and range, although in practice each video system uses a fixed combination of all four.

Code points

Code	FFmpeg name	Coefficients / meaning	Where you see it
0	gbr	Identity: the planes are G, B, R	RGB video, lossless screen captures, 4:4:4 RGB encodes
1	bt709	KR 0.2126, KB 0.0722	HD and most web video
2	unknown	Not specified	Untagged files; the player guesses
4	fcc	KR 0.30, KB 0.11	Obsolete
5	bt470bg	KR 0.299, KB 0.114 (BT.601)	PAL/SECAM SD; also sYCC and JPEG-style YCbCr
6	smpte170m	Same coefficients as 5	NTSC SD, DVD
7	smpte240m	KR 0.212, KB 0.087	Early 1990s HDTV; obsolete
8	ycgco	YCgCo	Rare
9	bt2020nc	KR 0.2627, KB 0.0593, non-constant luminance	UHD and HDR (HDR10, HLG)
10	bt2020c	BT.2020 constant luminance	Specified but almost never used
11	smpte2085	Y′D′zD′x	Rare
12 / 13	chroma-derived-nc / chroma-derived-c	Coefficients computed from the signalled primaries	Rare
14	ictcp	BT.2100 ICtCp	Rare in delivery
15	ipt-c2	IPT-C2	Dolby Vision profile 5
16 / 17	ycgco-re / ycgco-ro	Reversible YCgCo	Lossless coding

Codes 5 and 6 are the same matrix, so for conversion purposes “BT.601” covers both; tag whichever matches the source system. Other tools use other spellings for the same numbers: SVT-AV1 uses bt601 for 6 and bt2020-ncl for 9, VapourSynth uses 170m and 2020ncl.

The BT.601 versus BT.709 mistake

Decoding with the wrong matrix barely changes grays, but it shifts saturated colors: reds, greens and skin tones are where you notice it. It is easy to miss without the source next to the encode. The usual causes:

• Untagged SD upscaled to HD. The SD source is BT.601. After scaling to 1080p, players that guess by resolution switch to BT.709. mpv, for example, assumes BT.709 when the width is at least 1280 or the height is above 576.
• Tagging instead of converting. Writing bt709 onto BT.601 pixels changes nothing in the pixels; it makes every player decode them wrongly.
• RGB sources converted by FFmpeg. Converting PNG frames, screen captures or other RGB input to YUV uses the BT.601 matrix by default and leaves the output untagged, which an HD player then decodes as BT.709.

When the resolution class changes, convert the matrix at the same time as scaling. Commands for FFmpeg zscale and VapourSynth resize are in the color guide. For RGB input, choose the matrix explicitly and tag the result:

ffmpeg -i input.png -vf scale=out_color_matrix=bt709,format=yuv420p,setparams=color_primaries=bt709:color_trc=bt709 -c:v libx264 output.mkv

BT.2020 constant and non-constant luminance

bt2020nc (9) is the normal choice for UHD SDR, HDR10 and HLG; BT.2100 uses the same non-constant-luminance Y′CbCr. bt2020c (10) is a different signal and needs different maths to decode, so changing the tag from 9 to 10, or the reverse, is never a fix. ICtCp (14) is a separate HDR representation from BT.2100, not a variant of BT.2020 Y′CbCr.