Macintosh Transperancy (PR#11511)

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