.PDF File Format – Graphics

Coordinate spaces

Every position is given in user space and mapped to the device by the current transformation matrix (CTM). Default user space has x to the right, y up, and a unit of 1/72 inch, the PostScript point. The page’s MediaBox gives its corners in this space, usually with (0, 0) at the lower left. A matrix is written as six numbers [a b c d e f], standing for

| a b 0 |
| c d 0 |      x′ = a·x + c·y + e
| e f 1 |      y′ = b·x + d·y + f

cm multiplies a new matrix onto the CTM, so the last one given is applied to points first. Other spaces sit on top of user space:

• text space, set by the text matrix Tm, in which glyphs are placed;
• glyph space, in which a font’s glyphs are drawn, 1,000 units to the em for most fonts, mapped by the font matrix;
• image space, in which an image of w × h samples always fills the unit square of user space; a page draws it at size with w 0 0 h x y cm before Do;
• form space, set by a form XObject’s Matrix;
• pattern space, set by a pattern’s Matrix, which maps to the default coordinate space of the page, not the CTM at the time of use.

A page’s Rotate turns the whole page when shown; it does not change user space.

Colour spaces

A colour space is a name for the device spaces and the Pattern space without parameters, or an array of the family name and its parameters. Content streams name it in the ColorSpace resources.

WhitePoint is required in the CIE spaces, the diffuse white in CIE 1931 XYZ, with Y equal to 1. The Separation names All (every colourant, for registration marks) and None (paint nothing) are special. The ICC profile in the small PDF 1.3 example is << /Length 10 0 R /N 3 /Alternate /DeviceRGB /Filter /FlateDecode >>.

Rendering intents, set by ri or an image’s Intent, say how to map colours a device cannot show: AbsoluteColorimetric, RelativeColorimetric (the default), Saturation or Perceptual.

Patterns

A pattern paints an area with a repeated cell or a smooth shading, used as a colour: /Pattern cs /P1 scn and then a fill. Patterns are in the Pattern resources.

Shadings

From PDF 1.3, seven types of smooth colour transition. Every shading dictionary has these keys:

The mesh types are streams of packed vertices: a flag (types 4, 6 and 7) that says how the next vertex or patch joins the previous one, coordinates, and colour components or a single function input, each in the given number of bits and mapped through Decode.

Functions

Functions turn input numbers into output numbers, for shadings, Separation and DeviceN tint transforms, transfer functions and soft masks. Every function has FunctionType, Domain (the input ranges, required) and Range (output ranges; required for types 0 and 4). Inputs and outputs are clipped to them.

Type 4 programs use only arithmetic, comparison, boolean and stack operators, with if and ifelse; there are no loops, names or strings.

Graphics state parameter dictionaries

From PDF 1.2 the gs operator sets several parameters at once from a dictionary in the ExtGState resources. Parameters that have no operator of their own can be set only this way.

The small PDF 1.3 example uses << /Type /ExtGState /OPM 1 >>.

Form XObjects

A form XObject is a content stream that can be drawn many times with Do, like a PostScript form: a logo on every page, or an annotation’s appearance.

Drawing a form saves the graphics state, multiplies Matrix onto the CTM, clips to BBox, runs the stream and restores the state.

Transparency (PDF 1.4)

PDF 1.4 made every painting operation blend with what is beneath it, using a shape and an opacity. With the defaults (Normal blend mode, opacity 1, no soft mask) the result is the same as opaque painting, so older files look as they did.