.PDF File Format – Graphics

Coordinate spaces

A matrix is six numbers [a b c d e f], standing for the 3 × 3 matrix with rows a b 0, c d 0 and e f 1. A point (x, y) goes to (ax + cy + e, bx + dy + f). Rotation by θ is [cos θ sin θ −sin θ cos θ 0 0].

From PDF 1.6 a page can set UserUnit, the size of a default user space unit in multiples of 1/72 inch. Acrobat limits a page to 14,400 units, 200 inches at the default size; with UserUnit 10 the same page box covers ten times as much, for maps and banners. The page’s Rotate turns the whole of user space when it is shown.

Colour spaces

A colour space is a name, for the device spaces and Pattern, or an array whose first element is the family name. It is set with CS or cs, or given to an image, a shading or a group.

• The WhitePoint of the CIE-based spaces is required, as CIE XYZ with Y = 1; BlackPoint defaults to [0 0 0].
• A Separation or DeviceN colourant is a named ink, such as /PANTONE#20185#20C; All means every colourant, and None paints nothing. When the device has no such ink, the tint transform, a function, converts the tints to the alternate space.
• The DeviceN attributes dictionary has Subtype (DeviceN, or NChannel from 1.6, which says that the colourants are described for blending), Colorants (Separation spaces for spot colourants), Process (the process colour space and its components) and MixingHints.
• The initial colour is black in the device and CIE-based spaces, the index 0 in Indexed, and a tint of 1.0 in Separation and DeviceN.
• A page or group can set DefaultGray, DefaultRGB and DefaultCMYK in its ColorSpace resources to replace the device spaces with calibrated ones.

Patterns

A pattern paints an area with something other than a flat colour. It is selected by setting the Pattern colour space and giving its name to scn or SCN: /Pattern cs /P1 scn.

Shadings

A shading, from PDF 1.3, is a smooth change of colour. It is painted either as a shading pattern, or directly with sh over the current clip. Every shading has ShadingType, ColorSpace (required), and may have Background, BBox and AntiAlias.

In the mesh types the coordinates and colours are packed integers, read most-significant bit first and mapped through Decode to real values. When Function is given, each vertex has one colour component, t, which the function turns into a colour.

Functions

Functions map one or more inputs to outputs. Shadings, tint transforms, transfer functions, halftone spot functions and soft masks use them. Each has FunctionType, Domain (required, input ranges) and Range (output ranges, required for types 0 and 4). Inputs and outputs are clipped to them.

The graphics state and ExtGState

Some graphics state parameters can be set only from a graphics state parameter dictionary, named in the ExtGState resources and applied with gs:

Black point compensation, from PDF 2.0, adjusts colour conversion so that the darkest colour of the source maps to the darkest of the destination, keeping detail in shadows. Before PDF 2.0 it was left to the reader.

Transparency

From PDF 1.4 each object painted is blended with the backdrop beneath. Three things take part: the blend mode, the shape and the opacity.

Form XObjects

A form XObject is a content stream that can be drawn many times, with Do, as a reusable piece of a page, an annotation appearance, a pattern cell or a transparency group.