user space coordinate system:
	default user space:
		CropBox per page (visible area of output medium).
		x positive to the right.
		y positive upwards.
		Exception: Rotate entry in Page dict.
		unit: UserUnit or 1/72 inch in the Page dict.
	coordinate transformation transformation operator "cm".
user space -> device space by CTM [graphics state].
text space:
	text space -> user space:
		text matrix
		9.4.2 Text-Positioning Operators.
glyph space:
	glyph space -> text space: font matrix or 1000 glyph space units=1 unit of text space. Font FontMatrix
image space:
	for sampled images.
	image space -> user space: Fixed: image is 1 unit wide, 1 unit high in user space.
form space:
	used by XObject.
	form space -> user space: form matrix.
pattern space:
	?
	pattern space -> user space: pattern matrix.
translation: [1 0 0 1 tx ty]
scaling: [sx 0 0 sy 0 0]
rotation: [(cos q) (sin q) -(sin q) (cos q) 0 0]
skew: [1 (tan a) (tan b) 1 0 0]

[[a b 0]
 [c d 0]
 [e f 1]]

point: [x y 1]
[x' y' 1] = [x y 1]⋅M where x,y are the coordinates in the transformed system.