Re: Canvas, Ellipses, and RGBA

[Federico Mena Quintero <federico redhat com>]

>  1.) First of all, how the hell do I come up with  guint32 for the 
>  color_fill_rgba with the GnomeCanvasItem's

You can use the GNOME_CANVAS_COLOR() and GNOME_CANVAS_COLOR_A()
macros.  And those *are* in the reference documentation :-)

>  'fill_color' but I really really want to use the alpha channels. I have a 
>  GnomeColorPicker, but thats gives me _4_ values in various forms, i.e. 
>  double, guint8, guint16, etc?) anyway, how do I combine values that I 
>  gain from the GnomeColorPicker into /ONE/ guint32 for one guint32 RGBA 
>  value. Havoc's book is not very helpful with this, since he directly 
>  writes the RGBA values - Also, it appears to be a 'hex' value (i.e. it's 
>  something like 0x32ab9FFF which I find weird for a guint32). Help!

When expressed in hexadecimal, an RGBA color is 0xRRGGBBAA, that is, 8
bits per channel.  An alpha of 0 is fully transparent, an alpha of 255
is fully opaque.

>  2.) Why on earth to ellipses use bounding boxes? 

Because

	- The Tk canvas works that way.

	- It is more convenient to specify a bounding box than to have
          two values for the semiaxes of an ellipse and a flag that
          specifies whether you want a circle or an ellipse.

>  wouldn't it make /much/ more sense to provide radius, and then x
>  and y coordinates for the center - Thats how I learned how circles
>  to be drawn. This is relatively easy to change on my own (just
>  needs a little bit of math), so I don't really mind.  

X and Gdk also use bounding boxes, btw :-)

>  Also, when i do a gnome_canvas_item_move on an ellipse, what
>  exactly gets moved, the center of the circle? the top-left corner
>  of the boundin box? the center of the bounding box?

All the points that compose the ellipse get moved, just as you would
expect.  If a point (x, y) is on the ellipse, then it will get moved
to (x + dx, y + dy).

  Federico