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

[Saptarshi Guha]

Hi,
    Thank you for the explanation. I have one further question - should 
i wish to plot to the screen, which units (apart from bigpts) for exact 
plotting.
Essentially i wish to create my own plotting character - hence the 
pch="." and the surrounding dots - so it would be nice if i could place 
the surrounding dots exactly.
    Thank you
    Saptarshi Guha


Paul Murrell wrote:
> 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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>>     
>
>