PSD descriptors, path records, colours and strings

Several structures appear in many places in a PSD file: strings, colours, the path records of saved paths and vector masks, and the Action descriptor, a typed key and value tree that newer Photoshop features use for almost everything. Numbers are big-endian.

Strings

Kind	Layout
Pascal string	A 1-byte length, then that many bytes of text. Padded to an even total in resource names, and to a multiple of 4 in layer names.
Unicode string	A 4-byte count of UTF-16 code units (not bytes), then the characters, two bytes each, big-endian. The specification says the string ends with a two-byte null; readers should strip a trailing null if the count includes one.
ID (in descriptors)	A 4-byte length, then that many ASCII characters; or, when the length is 0, a 4-byte key such as `Clr` .

Colour

Ten bytes: a 2-byte colour space ID, then four 2-byte components. Unused components are zero.

ID	Colour space	Components
0	RGB	Red, green, blue, each 0 to 65535.
1	HSB	Hue, saturation, brightness, each 0 to 65535.
2	CMYK	Cyan, magenta, yellow, black, each 0 to 65535, where 0 is 100% ink.
3	Pantone	Custom colour; data undocumented.
4	Focoltone	Custom colour; data undocumented.
5	Trumatch	Custom colour; data undocumented.
6	Toyo 88	Custom colour; data undocumented.
7	Lab	Lightness 0 to 10000; a and b −12800 to 12700 (signed).
8	Greyscale	Grey 0 to 10000.
10	HKS	Custom colour; data undocumented.

Path records

Saved paths (image resources 2000 to 2997), the working path (1025) and vector masks (vmsk) are lists of 26-byte records. Each starts with a 2-byte selector.

Selector	Record	Contents
0	Closed subpath length	Number of knot records that follow (2 bytes), then 22 zero bytes.
1	Closed subpath Bézier knot, linked	Three points, below.
2	Closed subpath Bézier knot, unlinked	Three points.
3	Open subpath length	Number of knot records that follow, then zeros.
4	Open subpath Bézier knot, linked	Three points.
5	Open subpath Bézier knot, unlinked	Three points.
6	Path fill rule	The first record of a path. 24 zero bytes. Paths are filled with the even-odd rule.
7	Clipboard	Top, left, bottom, right and a resolution, each a 4-byte fixed-point number, then 4 zero bytes.
8	Initial fill rule	A 2-byte value, 0 or 1; 1 means the fill starts with all pixels. The rest is zero.

A knot holds three points of 8 bytes: the control point for the segment entering the knot, the anchor point, and the control point for the segment leaving it. A point is two signed 32-bit fixed-point numbers, vertical first, with 8 integer bits and 24 fraction bits. They are fractions of the image size: (0, 0) is the top left corner and (1, 1) the bottom right. “Linked” knots are smooth: their two control points move together.

00 01 00 80 00 00 00 40 00 00  00 80 00 00 00 80 00 00
00 80 00 00 00 C0 00 00

A linked closed-path knot: anchor at half height and half width, control points a quarter of the width to each side

Descriptor

A descriptor is a class with a list of keyed items. It is usually preceded by a 4-byte descriptor version, which is 16.

Size	Type	Name	Description
varies	Unicode string	`Name`	Class name, often empty.
varies	ID	`ClassID`	Class ID.
4	u32	`Count`	Number of items.
varies	ID	`Key`	For each item: its key.
4	char[4]	`Type`	For each item: its type, below.
varies		`Value`	For each item: its value.

Item types

Type	Meaning	Value layout
`obj`	Reference	A 4-byte count, then reference items (below).
`Objc`	Descriptor	A nested descriptor.
`GlbO`	Global object	A nested descriptor.
`VlLs`	List	A 4-byte count, then for each item a 4-byte type and its value.
`doub`	Double	8-byte IEEE double.
`UntF`	Unit float	A 4-byte unit key (below), then an 8-byte double.
`TEXT`	String	A Unicode string.
`enum`	Enumerated	A type ID, then an enumeration value ID.
`long`	Integer	4 bytes, signed.
`comp`	Large integer	8 bytes.
`bool`	Boolean	1 byte.
`type`	Class	A Unicode name, then a class ID.
`GlbC`	Class	A Unicode name, then a class ID.
`alis`	Alias	A 4-byte length, then an FSSpec on the Mac or a full path on Windows.
`tdta`	Raw data	A 4-byte length and the bytes. Used, for example, for the text engine data of type layers.

Reference items

Key	Meaning	Layout
`prop`	Property	Unicode name, class ID, key ID.
`Clss`	Class	Unicode name, class ID.
`Enmr`	Enumerated reference	Unicode name, class ID, type ID, enumeration ID.
`rele`	Offset	Unicode name, class ID, 4-byte offset.
`Idnt`	Identifier	Not laid out in the specification.
`indx`	Index	Not laid out in the specification.
`name`	Name	Not laid out in the specification.

Units of unit floats

Key	Unit
`#Ang`	Angle, in degrees.
`#Rsl`	Density (resolution), per inch.
`#Rlt`	Distance, at 72 pixels per inch.
`#Nne`	None; a plain number.
`#Prc`	Percent.
`#Pxl`	Pixels.
`#Pnt`	Points.
`#Mlm`	Millimetres.

Descriptor keys and class names are Photoshop’s scripting identifiers, such as Clr , Opct or textKey. Adobe does not publish a complete list; the names used by each block follow Photoshop’s scripting documentation.

.PSD File Format – Descriptors, paths and strings