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