graphics bug: type="l" (PR#120)

Prof Brian D Ripley <ripley@stats.ox.ac.uk> writes:

On Tue, 16 Feb 1999 Matt Wiener, mcw@ln.nimh.nih.gov wrote:

The following commands illustrate a problem with graphing relatively large
data sets using the "line" option to plot:

temp <- runif(200000)  # or whatever other numbers you like;
			# same thing happens with a sine wave
plot(1:200000, temp)    # everything fine
plot(1:200000, temp, type="l")   # data gets cut off


The number of points isn't crucial.  The cutoff point appears to be about
65,000 points.  (70,000 points demonstrates the cutoff; with 50,000 points
there's no cutoff.) 

This is device-specific (and which device was not mentioned). It does not
happen on Windows (rw0632) nor on a postscript device. It does happen on
the Unix x11() device for me, and my guess is that it is a limitation in
the X library used.  (Not in the X server, for this works in gs
displaying postscript, and in S-PLUS. The lines are drawn in a single Xlib
call.)

Apparently, this is not only device-specific but X-server specific as
the following network experiment shows:

Run on Linux,   display Linux   OK
Run on Solaris, display Linux   OK
Run on Solaris, display Solaris Error
Run on Linux,   display Solaris Error

So it seems that the Solaris OpenWindows X Server cannot handle
requests for drawing lines through more than 64K points. I suppose we
*could* fix this by splitting all XDrawLines into blocks of max 64K.

The man page for XDrawLines and friends have the following blob:

       All x and y members are signed integers.  The width and
       height members are 16-bit unsigned integers.  You should
       be careful not to generate coordinates and sizes out of
       the 16-bit ranges, because the protocol only has 16-bit
       fields for these values.

The "and sizes" bit might be taken to mean that you should do the
above, although it's beyond me how it would work on any platform if
there's only room for 16 bits in the protocol (perhaps the protocol
has changed from X11R5 to R6?). XDrawLines only occurs twice in
devX11.c, so I suppose it's no big deal to fix up.

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907

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