Things about PDF file - xiangxw/mupdf-qt GitHub Wiki

CTM

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The transformation from user space to device space is defined by the current transformation matrix (CTM).
The transformation between two coordinate systems is represented by a 3-by-3 transformation matrix written as follows:

Because a transformation matrix has only six elements that can be changed, it is usually specified in PDF as the six-element array [ a b c d e f ]. Coordinate transformations are expressed as matrix multiplications(see fz_matrix from Things about Mupdf source code):

(From: pdf_reference_1-7.pdf P201-209)

DPI

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dots(pixels) per inch, the default resolution is 72 dpi