🚀 go-pugleaf

Re <9211120354.AA16255@raisin-nut>
<1992Nov11.231724.13211@dcs.warwick.ac.uk>:

> I thought I understood PostScript VM until I built the following
> example!  It prints the same image twice, on both the HP IIISi, and
> the LW+, but using gs gives me an invalidrestore during the second
> image, on sun and sgi kit.  All data and dimensions are
> inconsequential.  Could someone explain why under ghostscript only the
> first one works?
> 
> %! Here's the program:
> 
> 18 684 translate -90 rotate 540 540 scale
> 
> /srread {
> 	/srread { restore save currentfile 20 string readhexstring pop } def
> 	save
> 	currentfile 20 string readhexstring pop
> } def	
> 
> 10 10 8 [ 10 0 0 -10 0 10 ] 	% /A [ 10 0 0 -10 0 10 ] def   if you want
> {srread} image
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> restore
> showpage	
> 
> % here's a more direct way of printing the image
> % -- except it doesn't work with ghostscript
> 
> 18 684 translate -90 rotate 540 540 scale
> 
> 10 10 8 [ 10 0 0 -10 0 10 ] 
> {restore save currentfile 20 string readhexstring pop} save 6 1 roll image
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> bfbfbfbfc1c3c3c5c567bfbfbfbfc1c3c3c5c567
> restore
> showpage

Ghostscript's implementation of `image' allocates some state
information in VM, deallocating it when the image is complete.  The
`restore' inside the procedure gives an error because this
information cannot be deallocated while control is still inside the
`image' operator.

I may fix this eventually, but almost certainly not until I implement
garbage collection.

As I'm sure you realize, allocating a new string each time around the
loop is not good PostScript programming practice in any case.

L. Peter Deutsch :: Aladdin Enterprises :: P.O. box 60264, Palo Alto, CA 94306
ghost@aladdin.com, ...decwrl!aladdin!ghost ; voice 415-322-0103 ; fax 322-1734
	    "Implementation is the sincerest form of flattery."