[Rd] Macintosh Transperancy (PR#11511)

Sun May 25 05:28:40 CEST 2008

Bradley,

On May 24, 2008, at 9:07 PM, Bradley Vance wrote:

> 	The bug is that the point that's being plotted is a single point,  
> not a combination of two points, one that's just the outline, and  
> one that's just the fill.  It also doesn't look like we'd expect  
> when you're debugging a plot.  For example, let's say I create a  
> plot, and notice that I can't see some of the points (due to other  
> points being drawn on top of them).  So I decide to make them  
> transparent, so you can see where points over-lay each other, and  
> the relative size/colors/etc of the over-layed points.  Now, rather  
> than just changing the opacity of the points/colors I see, I'm  
> adding in another color to each point where outline and background  
> meet, making the plot more confusing than it needs to be...
> 	In looking at the documentation for bg in points (plot says to look  
> at points for bg) it says
> bgbackground (fill) color for the open plot symbols given by  
> pch=21:25.
> and the way it is now, the color is not a fill (ie - inside the  
> lines) - it's super-imposed over part of the line.

I think you're still missing the point - it is the fill. I suspect  
you're misunderstanding the effect of using semi-transparent strokes.  
Moreover I get *exactly* the same result using X11 (both Mac *and*  
Linux) as well as other devices (PDF etc.). If you see anything else  
then *that* is a bug.

A point is represented by a shape (path) which is filled and then  
drawn (line stroke). As I was trying to explain earlier the effect is  
that half of the line is in the filled area. Imagine a rectangle (0,0)- 
(1,1). When you fill it, you're filling strictly the area between 0  
and 1 (in both x and y). However, when you strike let's say the line  
(0,0)-(1,0) with the width 0.25, you are filling the area (0,-0.25)- 
(1,+0.25) which means that half the line is outside (y<0) the filled  
are and another half is inside (y>=0). Given the alpha-blending  
mechanics [target=alpha*color + (1-alpha)*old] it implies that if the  
fill color is different from the background color and the line is not  
opaque, you get different color outside and inside the rectangle. You  
can verify that this is true for all devices that support alpha  
blending. This is how it works and is supposed to be, that's not a bug.

> 	Also, there's the fact that it does something different on  
> different systems.  When I do the same thing at work on a Linux box  
> (we don't have R.2.7 yet, we use mostly R2.4 but I've tried it on  
> R2.6,

Well, 2.6 didn't have alpha support in X11 AFAIR, so I have no idea  
what you are trying to compare there ...

> and we don't have Quartz, but I did output to PDF), it looks like a  
> single point whose colors were made transparent, and not a point  
> made of two overlapping transparent parts.
> 	As I implied in my original ticket, I'd like to be able to choose  
> whether to have it look the way it does now on the mac, and the way  
> it does on Linux.  This might actually be something that needs to be  
> addressed further by the core team, as that opacity is planning to  
> be added for X11() for the next revision (and hopefully, will work  
> into PNG and other device types that can support it)...
> 	Perhaps the best way to implement the options/choices/etc is to add  
> a separate alpha/opacity/whatever options that would control the  
> opacity of each object it plots.  Thus, if you define an opacity to  
> either col or bg, it would work the way it does on the Mac (ie - the  
> colors are opaque and can thus you can see the other point-color  
> through the top one).  However, if you have a point-opacity defined,  
> the entire point will get the opacity, the way it is currently done  
> on Linux.
> 	Or, you can just say that in order to get it look the way that it  
> currently does on the mac, you need to plot each point twice, once  
> with no bg, and once with no line...
>
> 	Also, I don't think this is a critical bug.  It's just something I  
> noticed that behaves differently on what I have at work (Linux) and  
> what I have at home (mac).  I'm impressed with R, and am thinking of  
> getting more involved in the r-project community, so figured I'd  
> start with something I noticed was different across platforms...
>

Again, I think you are missing something - it behaves consistently on  
Linux and Mac in the latest release and it's not a bug.

Cheers,
Simon

> On May 24, 2008, at 6:17 PM, Simon Urbanek wrote:
>
>> Brad,
>>
>> can you, please, let us know what is the "bug" here? If you paint  
>> semi-transparent line over the same filled shape (same vertices),  
>> clearly half of the line will be inside and and half outside of the  
>> shape, so each part will have a different color - that is the whole  
>> point of semi-transparency. Therefore I fail to see any bug or  
>> unexpected behavior here.
>>
>> Cheers,
>> Simon
>>
>>
>> On May 24, 2008, at 10:55 AM, brdvance at gmail.com wrote:
>>
>>> Full_Name: Brad Vance
>>> Version: 2.7.0
>>> OS: 10.5.2
>>> Submission from: (NULL) (71.123.195.202)
>>>
>>>
>>> Problem : When drawing transperant points with lines thick- 
>>> lines(lwd>1) + fill
>>> (pch=21-25), it looks like the line is transperantly overlayed  
>>> over the fill,
>>> making it look like 2-lines surround the fill (each a different  
>>> color).  I
>>> actually think that this is a nice OPTIONAL way to draw the points  
>>> (even as
>>> default), but since I could not discover how to turn it off, I  
>>> decided to issue
>>> the ticket.  In this option can see on the default-quartz device  
>>> as well as
>>> PDF-device.
>>>
>>> Solution : Have the entire point-object designed/drawn as if there  
>>> is
>>> no-transparency, then make the composite object transparent.    
>>> Optionally figure
>>> out an option to determine whether to apply transperancy as a  
>>> group (to the
>>> entire object) or separately (as it is now).
>>>
>>> Code :
>>>> df <- data.frame(X=rnorm(50),Y=rnorm(50))
>>>> plot 
>>>> (X 
>>>> ~ 
>>>> Y 
>>>> ,data 
>>>> = 
>>>> df 
>>>> ,pch=c(21,22,23,24,25),cex=5,col="#00FF0055",bg="#FF000055",lwd=6)
>>>
>>> Thanks,
>>> -Brad Vance
>>>
>>> ______________________________________________
>>> R-devel at r-project.org mailing list
>>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>>
>>>
>>
>