Graphics state

The painting operators take only geometry. S strokes the current path without being told a color or a width: it uses whatever the graphics state holds at that moment. The graphics state is the reader's current settings for transformation, clip, color, line style, font and opacity. Operators like rg, w and cm each change one setting, which stays in effect until something changes it again. The model comes from PostScript, where q and Q are called gsave and grestore.

0 0 1 RG 4 w               % blue, 4 wide, from here on
72 700 m 272 700 l S       % a blue line
q
1 0 0 RG [6 3] 0 d         % red and dashed, until Q
1 0 0 1 0 -40 cm           % and moved down 40
72 700 m 272 700 l S       % a red dashed line, drawn at y = 660
Q
72 620 m 272 620 l S       % blue and solid again, still 4 wide

What it holds

The spec splits the parameters in two. Device-independent ones affect what you see on any output. Device-dependent ones fine-tune a particular printer or screen, and most files never set them.

Names with a slash are keys in an ExtGState dictionary.

The graphics state stack

q pushes a copy of the whole graphics state onto a stack, and Q pops the most recent copy and makes it current again. Pairs nest. They must balance within a content stream, or across a page's /Contents array taken together, and they can't appear inside BT … ET.

A clip can only get smaller, so Q is the only way back to a larger one. cm only ever multiplies onto the CTM, so writers wrap it in q … Q rather than undo it with the inverse matrix.

Parameters without an operator: ExtGState

About half the parameters have no operator of their own. They are set from a graphics state parameter dictionary, a named entry in the /ExtGState resources, applied with gs (PDF 1.2). A dictionary changes only the parameters it has keys for. Some keys repeat an operator: /LW is w, and /Font [font size] is Tf. The rest are the only way to set overprint, transparency, stroke adjustment and the device-dependent parameters.

% Resources: /ExtGState << /GS0 << /LW 2 /LC 1 /OP true /op true >> >>
q
/GS0 gs                    % the same as "2 w 1 J", plus overprint, which no operator sets
72 600 200 100 re S
Q

What starts fresh, and what carries over

• Each page starts from the initial values. Nothing set on one page carries over to the next. Within a page, a /Contents array is read as one stream, so the state carries across the joins.
• A form XObject inherits the state at its Do. Do saves the state as q would, multiplies the form's /Matrix onto the CTM, clips to its /BBox, runs its content stream, then restores the state as Q would. A form that never sets a color paints in the caller's color, and nothing it sets leaks out.
• Text state is part of the graphics state; the text matrix isn't. The font, spacing and rendering mode outlast ET. The text matrix exists only inside a text object and is reset by every BT.
• The current path isn't part of it either. A path has to be painted, or ended with n, before any state operator can run.

Read: 1.4 §4.31.7 §8.4 Form XObjects: 1.4 §4.91.7 §8.10