If you just want to make visualizations, and not worry about computing whether a point is in an overlapping area, you will probably want to use some sort of transparency. Tk is not very good at transparent objects, but there is an example in the "widget" demo. Tk::Zinc does a better job of transparency, and examples are in the #7 in the OpenGL section of the zinc-demo.

I did notice in the Gnome2-Canvas-1.002 ( the very latest Perl-gtk2 ) demo, that they have transparency. If you run canvas.pl look for the beige oval, and drag it around. You can see through the color transparency. It could be the basis of pretty nice Venn diagram.

I want a venn diagram that is in jpg format so i can use it to show some results over the web

First, is an example where the colors obscure one another. You need the -fill option, in order for the mouse bindings to work.

I thru (sic) in a balloon too, to make it seem like a party. :-)

Hmm...about the balloon. I can't seem to find the Tk::CanvasBalloon module. When I try to run your code, I get "Can't find module...". I thought "hmm...must not have it installed", so I fired up cpanp to pull it down. No dice there, either. Is it part of some larger distribution or something?

thor

Feel the white light, the light within
Be your own disciple, fan the sparks of will
For all of us waiting, your kingdom will come

Yeah, it's one of eserte's(Slaven Rezsic) modules. Its at CanvasBalloon It makes balloons easier on a Canvas.

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I'm not really a human, but I play one on earth. flash japh

Yes.

As almost anything else, you can do it in Perl.

Just in case anyone is looking for answers, here is how the transpparency works in Gtk2. It is from 'muppet', the Perl/Gtk2 master.

Return-path: <scott@asofyet.org>

> I can manipulate the "fill_color_rgba" hex number to
> give me a few different semi-transparent ellipses, but
> can anyone explain how the hex numbering actually works to
> produce the semi-transparent effect?

The RGBA numbers are 32-bit values composed of one-byte-per-channel
for red, green, blue, and alpha (0xrrggbbaa).  The alpha value is the
"opacity", and this is used to describe how much of the source color
is blended with the destination color to create the output color
according to this formula:

    alpha_pct = alpha / 255.0
    destination red' = source red * alpha_pct + dest red * (1.0 -
alpha_pct)
    destination green' = source green * alpha_pct + dest green * (1.0
- alpha_pct)
    destination blue' = source blue * alpha_pct + dest blue * (1.0 -
alpha_pct)

An alpha value of 0xff is 255, which is 100% of the source color,
which is a traditional "source-copy" operation.  An alpha value of
25% means the output color is composed of 25% of the source color and
75% of the color already existing at the destination.  The actual
color you'll get as a result of the opacity value, therefore, depends
on what you're blending it with.  On a light background, a lower
opacity makes the color lighter, on a dark background, a lower
opacity makes the color darker, over red it gets redder, etc.

This is implemented using client-side blitting of microtiles in
libart.  To get proper alpha blending, you need full-resolution color
data, so it must be done on 24-bit RGB values -- dithered colors
don't work --- so it happens on the client side, using XPutImage to
transfer the full RGB to the X server.

> The canvas demo, is set up, to allow you to choose a normal
> (not anti-aliased) example, which does it on the basis of using a
> stiple.
> It's alot uglier, but it is always how I thought it was done,
> coming from
> Tk.

Stippling is a form of halftoning, which is how you approximate
colors on a device that can't actually reproduce them.  By having a
stipple pattern that displays every other dot, you get effectively
50% of the input color, and the background shows through.  No alpha
blending occurs --- it's the equivalent of alpha=100%.  And, yes,
this is uglier than alpha blending, but tends to be much faster, as
it can be performed on the X server.  The Tk canvas pretty much
predates widespread use of alpha blending; old hardware had a hard
time with it, because it requires keeping full-color copies of the
various components to be blended.

As an aside, cairo does everything in an alpha space, and is much
more efficient at it than libart.  Part of the big fuss about the new
compositing-based X servers is that they can do alpha blending on the
server side.
[download]

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I'm not really a human, but I play one on earth. flash japh