PDF teasers - how would you handle this stack problem?

This article arose as a result of debugging a customer file which was not displaying properly. There are many PDF files out there which do not actually meet the spec so we spend a lot of time tweaking our library to allow for all these ‘interesting’ cases.

The PDF file format has a stack system so that you can save the current graphics Status, make some changes and restore it later. In the PDF stream you will see this with the Q/q command. Here is an example

q  //save stack
1 0 0 1 130.32 117.601 cm
/X7 Do //draw an image or execute some commands
Q //restore stack

This code saves the stack, makes a change to the co-ordinates, does something and then restores original values.

It can even nest calls so you can have

q //save orig state
//something
q //save new state
//something
Q //restore new state
Q //restore orig state

It is a very powerful feature.

The Do command can also call some code commands including saving and restoring the stack like this

q  //save stack
1 0 0 1 130.32 117.601 cm
/X7 Do //execute these commands

     q //save state
     7.92 0 0 7.92 0 -0.001 cm //move position
     0 0 0 rg //set color
     BI /W 34 /IM true /D [1 0] /BPC 1 /H 34 ID //draw image

     //end of stream of commands (we pushed value onto stack but did not use)

Q //oh dear!!!

What do I do? Do I use value pushed by subroutine or value pushed before the sub-routine

The answer really is whether there is a single, global stack or whether the sub-routine has its own stack….

Update: The answer is to that the sub-routine effectively has its own stack so any values left should be ignored. Did you get it right?