Graphics clipping question

[kanderso%dino.bbn.com@RELAY.CS.NET]

;;; -*- Syntax: Common-Lisp -*-

#||

Symbolics System, FEP1:>enhanced-pci-from-pci.load.1
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I am trying to convert some graph drawing software to use GRAPHICS:
functions rather than window operations, and one thing i'm having
trouble with is clipping.  I need to be able to clip drawing
operations to the rectangular outline of the graph.  My old software
modified the TV:CLIPPING-REGION instance variable of the stream being
drawn on, and that worked fine.  This doesn't work with GRAPHICS:
functions because the stream you think you are drawing on often has
nothing to do with the steam you are actually drawing on because you
are in a maze of twistly little continuations all alike.

So, the question is what should i be using.  I've found
GRAPHICS:WITH-CLIPPING-FROM-OUTPUT, but i don't think it is what i
want. The following software and terminal hystery shows what happens.
It is a very simplified version of a typical situation: your data goes
from 0 <= x <= a zillion, but you are only interested in looking at
0 <= x <= 1000 say.  The function (TRY-CLIP WINDOW LENGTH) draws a
line of length LENGTH clipped to a small region on the window.  As
LENGTH increase, larger and larger TV:ROVING-BITMAPS get created in
the TV:BITMAP-STREAM resource.  For example LENGTH = 50000 produces a
bitmap over 50 screens wide.  Once you've created a bitmap that big,
the system just spends the rest of your life paging the bitmap in and
out until you do (SCL:CLEAR-RESOURCE 'TV:BITMAP-STREAM).

I can clip my own lines, but i hoped i could rely on Symbolics to clip
other arbitrary things like text, circles, etc.

Any suggestions?

||#

(defun try-clip (e length)
  (let ((w 200)
	(h 200))
    (let ((cx (/ w 2))
	  (cy (/ h 2)))
      (send e :expose)
      (graphics:with-room-for-graphics  (e h)
	(setq w (/ w 4) h (/ h 4))
	(graphics:draw-rectangle (- cx w) (+ cy h) (+ cx w) (- cy h)
				 :stream e :filled nil)	; Outline clipping region
	(decf w 1) (decf h 1)
	(graphics:with-clipping-from-output
	  (e (progn 
	       (print e)
	       (graphics:draw-rectangle (- cx w) (+ cy h) (+ cx w) (- cy h)
					:stream e :filled t)))
	  ;; Anything drawn below gets clipped
	  ;; Huge TV:ROVING-BITMAP created here
	  (graphics:draw-line 0 100 length 100 :stream e)
	  (print e))))))


;;; Terminal session:

(try-clip *terminal-io* 500)

#<Roving Bitmap 1  1280,791 x 1 2533256> 
#<Roving Bitmap 2  1280,791 x 1 2533430> 

NIL
(try-clip *terminal-io* 5000)


#<Roving Bitmap 1  1280,791 x 1 2533256> 
#<Roving Bitmap 2  5740,791 x 1 2533430> 

NIL
(try-clip *terminal-io* 50000)


#<Roving Bitmap 1  1280,791 x 1 2533256> 
#<Roving Bitmap 2  52970,791 x 1 2533430> 

NIL