Colour spaces

The two newest DIB headers say what the stored colours mean. BITMAPV4HEADER names a colour space or describes one with primaries and gamma, and BITMAPV5HEADER adds a rendering intent and an ICC colour profile. Older headers carry no colour information, and readers treat their colours as sRGB.

00000046  42 47 52 73

LCS_sRGB as stored: the value 0x73524742, little-endian

Colour space type

The field bV4CSType (or bV5CSType) at offset 56 of the header says which colour space applies. Its values are four letters read as a little-endian number, so sRGB appears in the file as BGRs.

Value	Name	Meaning
0x00000000 (0)	`LCS_CALIBRATED_RGB`	Use the endpoints and gamma in the header.
0x73524742 ('sRGB')	`LCS_sRGB`	The colours are sRGB.
0x57696E20 ('Win')	`LCS_WINDOWS_COLOR_SPACE`	The system’s default colour space, sRGB in practice.
0x4C494E4B ('LINK')	`PROFILE_LINKED`	V5 only: a profile in another file, named in the profile data.
0x4D424544 ('MBED')	`PROFILE_EMBEDDED`	V5 only: an ICC profile stored in this file.

Endpoints and gamma

These fields are used only when the type is LCS_CALIBRATED_RGB; otherwise they are ignored and usually zero. Offsets are from the start of the header.

Offset	Size	Type	Name	Description
60	36	CIEXYZTRIPLE	`bV4Endpoints`	The CIE XYZ coordinates of the red, green and blue primaries, in that order. Each is three 2.30 fixed point numbers: 2 integer bits and 30 fraction bits.
96	4	u32	`bV4GammaRed`	The red tone response, as unsigned 16.16 fixed point.
100	4	u32	`bV4GammaGreen`	The green tone response.
104	4	u32	`bV4GammaBlue`	The blue tone response.

In 16.16 fixed point the top 16 bits are the whole number and the bottom 16 the fraction, so a gamma of 2.2 is stored as 0x00023333.

Rendering intent

bV5Intent, at offset 108 of a V5 header, says how colours outside the output device’s range should be handled.

Value	Name	ICC intent
0x01 (1)	`LCS_GM_BUSINESS`	Saturation
0x02 (2)	`LCS_GM_GRAPHICS`	Relative colorimetric
0x04 (4)	`LCS_GM_IMAGES`	Perceptual
0x08 (8)	`LCS_GM_ABS_COLORIMETRIC`	Absolute colorimetric

ICC profile

When the type is PROFILE_EMBEDDED or PROFILE_LINKED, two V5 fields locate the profile data:

Offset	Size	Type	Name	Description
112	4	u32	`bV5ProfileData`	The offset of the profile data, measured from the start of the V5 header, not the start of the file.
116	4	u32	`bV5ProfileSize`	The size of the profile data in bytes.

In a file the header starts at offset 14, so the profile is at 14 + bV5ProfileData. An embedded profile is a complete ICC profile. A linked profile is a file name, ended by a zero byte, in the Windows-1252 character set. Readers find the profile only through this offset, so it can sit anywhere after the header.