[R] A question on grid - grid.points not spaced properly

[Paul Murrell]

Hi


Saptarshi Guha wrote:
> Hello,
> 	How can i 'fix' the following output.
> 
> 	v=viewport(x=216/2, y=216/2,w=216, h=216,default.units = "bigpts")
> 	pushViewport(v)
> 	x=c(119,130,140,151)
> 	y=c(124,124,124,124)
> 	grid.points(x,y,size=unit(1,"bigpts"),default.units="bigpts",pch=".")
> 	grid.points(x-2,y,size=unit(1,"bigpts"),default.units="bigpts",pch=".")
> 	grid.points(x+2,y,size=unit(1,"bigpts"),default.units="bigpts",pch=".")
> 	grid.points(x,y-2,size=unit(1,"bigpts"),default.units="bigpts",pch=".")
> 	grid.points(x,y+2,size=unit(1,"bigpts"),default.units="bigpts",pch=".")
> 
> 
> 	One would expect to get a 4 figures composed of 5 dots each  - 2  
> vertically spaced and 2 horizontally spaced symmetrically around the  
> center dot.
> 	However i seem to get odd results - e.g on Quartz(OS X) output, the  
> last command, places the dot bang next to the center dot.
> 	On 'jpeg' output with higest quality, this oddity happens with the 'x 
> +2' command.
> 
> 	This doesn't happen to all of the points, only some - and not necc.  
> the edge figures.
> 
> 	Is there anyway i can control this?


I think you are seeing a rasterization effect.  Both on screen and in a
bitmap format you are essentially turning on a single pixel at a time.
The locations you are giving do not necessarily correspond to an exact
pixel location (bigpoints are in 1/72 inches, but your screen might have
a resolution of 96 ppi) so you just get the nearest pixel to that
location.  So the gap you specify of two bigpoints sometimes comes out
as 2 pixels, sometimes as 1 pixel (for example).

For comparison, try running your code on a PDF (or other vector format)
device;  the result is much more what you are expecting I think.

Paul


> 	Thanks
> 	Saptarshi
> 
> Saptarshi Guha | sapsi at pobox.com | http://www.stat.purdue.edu/~sguha
> 
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-- 
Dr Paul Murrell
Department of Statistics
The University of Auckland
Private Bag 92019
Auckland
New Zealand
64 9 3737599 x85392
paul at stat.auckland.ac.nz
http://www.stat.auckland.ac.nz/~paul/