PNG colour and transparency chunks

These ancillary chunks say how the samples relate to real colours and how transparent they are. All come before PLTE and the image data, except tRNS and bKGD, which come after PLTE when there is one (see chunk order).

Which colour chunk wins

A file may hold more than one way of describing its colours. The chunk with the lowest number that a decoder understands takes precedence, and the others should be ignored.

Priority	Chunk
1	cICP
2	iCCP
3	sRGB
4	cHRM and gAMA

tRNS, transparency

Transparency without a full alpha channel. It may not appear with colour types 4 and 6, which have an alpha channel of their own.

Colour type	Size	Field	Meaning
0	2	Grey	The grey sample value to treat as fully transparent.
2	2 + 2 + 2	Red, green, blue	The colour to treat as fully transparent.
3	1 per entry	Alpha	An alpha value for each palette entry, in order, 0 transparent to 255 opaque. It may be shorter than the palette: the rest count as 255. It may not be longer.

For types 0 and 2 the sample is always stored in two bytes. Only the low bits matter below 16-bit depth, and every other pixel is opaque. At 16-bit depth a pixel must match all 16 bits to be transparent.

gAMA, image gamma

Offset	Size	Field	Meaning
0	4	Gamma	The gamma times 100,000. Zero is not allowed.

It relates the stored samples to the desired output light:

sample = light ^ gamma

with sample and light both in the range 0 to 1, so 45455 means a gamma of 1 / 2.2. It assumes the sRGB reference viewing conditions, and it applies to colour samples only; alpha is linear.

cHRM, primary chromaticities

Eight 4-byte values, each a CIE 1931 chromaticity coordinate times 100,000, in this order.

Offset	Size	Field
0	4	White point x
4	4	White point y
8	4	Red x
12	4	Red y
16	4	Green x
20	4	Green y
24	4	Blue x
28	4	Blue y

sRGB, the sRGB colour space

One byte. Its presence says the samples are in the sRGB colour space (IEC 61966-2-1). Encoders are recommended to write matching gAMA and cHRM too (gamma 45455, white 31270, 32900, red 64000, 33000, green 30000, 60000, blue 15000, 6000).

Value	Rendering intent
0	Perceptual
1	Relative colorimetric
2	Saturation
3	Absolute colorimetric

iCCP, ICC profile

An embedded ICC colour profile. It should not appear with sRGB.

Field	Size	Meaning
Profile name	1 to 79 bytes	Latin-1, the same rules as a keyword, case-sensitive.
Null separator	1
Compression method	1	0 is the only value: zlib Deflate.
Compressed profile	The rest	A zlib stream that inflates to an ICC profile.

The profile must be RGB for colour types 2, 3 and 6 and greyscale for types 0 and 4. In the explorer the profile’s header and tags are read for you, after it has been inflated.

sBIT, significant bits

How many bits of each sample were significant in the original, so that a reader can recover the original values exactly. Each value is 1 to the sample depth, or 1 to 8 for a palette image.

Colour type	Fields, one byte each
0	Grey
2, 3	Red, green, blue
4	Grey, alpha
6	Red, green, blue, alpha

cICP, coding-independent code points

Identifies the colour space by the code points of ITU-T H.273, as video does. It is the first choice of the colour chunks when a reader understands it, and the way a PNG states HDR (such as BT.2100 PQ).

Offset	Size	Field	Meaning
0	1	Colour primaries	See below.
1	1	Transfer function	See below.
2	1	Matrix coefficients	Must be 0, identity (RGB): PNG holds RGB samples.
3	1	Video full range flag	1 full range; 0 narrow range.

Colour primaries (others are reserved)
Code	Primaries
1	BT.709, sRGB
2	Unspecified
4	BT.470 System M
5	BT.470 System B, G; BT.601 625-line
6	BT.601 525-line; SMPTE 170M
7	SMPTE 240M
8	Generic film, illuminant C
9	BT.2020, BT.2100
10	SMPTE ST 428-1 (CIE XYZ)
11	SMPTE RP 431-2 (DCI-P3)
12	SMPTE EG 432-1 (Display P3)
22	EBU Tech 3213-E

Transfer functions (others are reserved)
Code	Transfer function
1	BT.709
2	Unspecified
4	Gamma 2.2 (BT.470 System M)
5	Gamma 2.8 (BT.470 System B, G)
6	BT.601, SMPTE 170M
7	SMPTE 240M
8	Linear
9	Logarithmic, 100 : 1
10	Logarithmic, 100√10 : 1
11	IEC 61966-2-4 (xvYCC)
12	BT.1361 extended
13	sRGB (IEC 61966-2-1)
14	BT.2020, 10-bit
15	BT.2020, 12-bit
16	SMPTE ST 2084, BT.2100 PQ
17	SMPTE ST 428-1
18	BT.2100 HLG (ARIB STD-B67)

Examples from the specification: 09 10 00 01 is BT.2100 PQ at full range; 09 12 00 01 is BT.2100 HLG; 0C 0D 00 01 is Display P3.

mDCV, mastering display colour volume

Describes the display an HDR image was mastered on. It is supplemental to cICP’s colour description and is used for tone-mapping decisions.

Offset	Size	Field	Divisor
0	2	Red x	50,000
2	2	Red y	50,000
4	2	Green x	50,000
6	2	Green y	50,000
8	2	Blue x	50,000
10	2	Blue y	50,000
12	2	White point x	50,000
14	2	White point y	50,000
16	4	Maximum luminance	10,000 (units of 0.0001 cd/m²)
20	4	Minimum luminance	10,000 (units of 0.0001 cd/m²)

The primaries appear in the order with the largest x first, then the largest y, then the other: for RGB, red, green, blue.

cLLI, content light level information

Offset	Size	Field	Meaning
0	4	MaxCLL	The brightest single pixel, in units of 0.0001 cd/m². 0 means unknown.
4	4	MaxFALL	The highest frame-average light level, in units of 0.0001 cd/m². 0 means unknown.

A live stream may write 0 at first and update these once it ends.

bKGD, background colour

A suggested background to show the image against. It takes its form from the colour type.

Colour type	Size	Field
0, 4	2	Grey, in 0 to 2^depth − 1
2, 6	2 + 2 + 2	Red, green, blue, each in the same range
3	1	A palette index

.PNG File Format – Colour and transparency