Re: Canvas basics

[Arnaldo de Moraes Pereira <egghunt gmail com>]

  Hi Roger,


On Tue, 21 Dec 2004 18:52:20 +0000, Roger Leigh
<rleigh whinlatter ukfsn org> wrote:
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> Is the Canvas on-topic for this list?  If so, I have a few basic
> questions about it, which I hope are simple :)
>
> Firstly, which Canvas implementation should I be using?  I've tried
> libgnomecanvas2, but I see there's also a FooCanvas, and a GtkCanvas
> (GTK+ 1.2 only?).  Is the AA canvas still experimental and not
> recommended?

  It's a matter of what you need the canvas for.
  I've been using gnomecanvas with AA smoothly.

> I've been looking at George Lebl's "Piskvorky" canvas tutorial.
> (ftp://ftp.5z.com/pub/unix/gnome-piskvorky-1.tar.gz)  After
> tweaking the Makefile to use pkg-config, this built fine with
> GTK+ 2.4/GNOME 2.8.  However, I did find a problem: if I resize the
> window, the canvas viewable area becomes centred in the canvas widget,
> but the coordinates of button presses are still relative to the top left
> corner of the canvas widget, rather than the canvas view itself.  How
> should I compensate for this in the code?  Or is this a canvas bug,
> since the events are outside the drawable area?

  Are you talking about canvas or world coordinates ? Canvas
coordinates are relative to the item's parent position, so if the
parent of the button you're clicking is at the top left corder, the
behavior you described is right.
  To avoid centering the canvas viewable area, use
gnome_canvas_set_center_scroll_region().

> What I really want to do with the canvas is create a grid display
> system, similar to Gnumeric, but probably much simpler.  What I want to
> do is achieve the same scroll behaviour as Gnumeric: the row and column
> labels scroll with the canvas, and scrolling is in discrete steps.  How
> should I do this.  I thought of two possible methods:
> - - packing four canvas widgets into viewports in a GtkTable and tying
>   their adjustments to the same scrollbars.
> - - overlaying the row/column headings on top of the canvas relative to
>   the scroll position.
> How does Gnumeric do this?  I had a look, but couldn't find the relevant
> source files.

  I don't know if I understood, but you could put the row and column
labels and the canvas into a scrolledwindow.

> Many thanks,
> Roger
>
> - --
> Roger Leigh
>
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  Take care.

-- 
Arnaldo M. Pereira
egghunt at gmail.com
http://www.ansi-c.org/~arnaldo