Re: canvas, cairo and/or pixbufs? (was Re: Application Approach)

["Johan Kohler" <johan kohler gmail com>]

I've been learning to use Gnome::Canvas for some time now The gnome
canvas demo app is very useful, but documentation or tutorials are
scarce.

There are some things that puzzle me though, and I apologise for
hi-jacking your thread :-)

Does anyone have any idea how to:

1. Access the items on a canvas after they've been created?
2. Delete items on a canvas after they've been created?

These things are not obvious to me and I really need that
functionality (my app runs out of memory otherwise).  I'll look more
closely at cairo in the meantime.

Thanks ,
Johan


On 05/11/06, Peter Lund <firefly vax64 dk> wrote:
> On Sun, 2006-11-05 at 02:07 -0600, Chris Horlick wrote:
>
> > That is the overview the question that i have is about the overall
> > approach to this project. I am thinking of using a GtkImage to display a
> > GdkPixbuf which actually holds the contents of the
> > workspace(icons,lines,nodes...). The Gtkimage is being displayed inside
> > of a GtkDrawingArea, which catches events from the user.
>
> It's probably better to either use one of the canvases or use Cairo.
>
> There are several different canvases floating around in the GTK+/Gnome
> universe -- they take care of handling input/selection events, redraw,
> scrolling, and to a certain extent also accessibility.
>
> The theory is that you just say that you want an icon widget or a label
> widget or whatever at position (x,y) and the canvas takes care of the
> rest.  Some of the canvases can also handle arbitrary BÃzier drawing
> (would be nice for the connections between the icons) with antialiasing.
>
> I personally think, however, that it is much easier to implement it
> without a canvas, and do the event handling/scrolling/drawing oneself,
> just like you are intending to do.
>
> If you go that route you will be VERY pleased with cairo -- you can do
> antialiased drawing, BÃzier paths, alpha blending, rotated and masked
> images, etc...  It is surprisingly easy to write something that looks
> gorgeous.
>
> If you are (very) lucky it will even be accelerated on today's
> hardware/software.
>
> On the other hand, using a pixbuf will also work and it will probably be
> faster to implement because you already know the concepts involved.
>
> > First i would like to know if this is the correct approach according to
> > the official GTK way. I am curious how to add one of my icons to the
> > back pixbuf. Also, what are my options to actually allow the user to
> > move the icons. Currently i am looking at using my own set of functions
> > that check to see if the user clicked one of the icons already in the
> > workspace then take appropriate action.
>
> That's how you do it (unless you use one of the canvases).
>
> As you can probably guess, I kinda like Cairo, but maybe you should wait
> a bit before using it.  I don't know.  It depends on your ambition,
> skill level, learning speed, and on how much time you are willing to
> spend.
>
> If you use Cairo, the best way to handle the mouse seems to me to be:
>
> 0) have a high-level data structure that describes the layout.
>
> 1) have a hit test data structure that describes the "hot" areas in the
> window in window coordinates.  Each area points to an icon (or perhaps a
> line) or something else that's interesting.
>
> 2) the mouse event handler(s) throws coordinates into the data structure
> in one end and sees what comes out the other end (nothing or a
> prioritized list of icons, perhaps).
>
> 3) the data structure starts out being empty.  It gets filled in by the
> expose handler.  This means that there is always a one-to-one
> correspondence between what's on the screen and what the "hot" areas are
> (or as close as one can get).  The expose handler has to know how to go
> from the high-level data structure to window coordinates anyway.
>
> You can optimize the updating of the hit test data structure later, if
> necessary.  The hit test data structure can be a simple list of
> rectangles or a simple tree or (if you get fancy) an R*-tree.  I'd go
> for the simple list...
>
> This strategy decouples the high-level data structure from the mundane
> details of the layout, it ensures that knowledge of how to translate
> between the high-level data structure and the coordinates is in one
> place, it ensures that the mouse handler doesn't have to know anything
> about the high-level data structure, and it makes it easy to add extra
> little doodahs to your window (such as transparent overlays with extra
> information about whatever is closest to the mouse, animations and size
> changes when the mouse is over an icon, fish eye lenses, etc.).
>
> http://cairographics.org/
> http://cairographics.org/samples/
>
> (note that these samples change the coordinate system before drawing:
> "The user space is the unit square ( (0,0) - (1, 1) ), this user space
> is initialized byt [sic] the snippet_normalize function, which also sets
> a line_width of 0.04. The snippets are meant to be short, and easy to
> understand" -- you probably don't want to do that.)
>
> http://macslow.thepimp.net/?page_id=23
>
> (if you try macslow's code you probably won't get the proper transparent
> windows effect because that requires using a compositing window manager
> -- but the screen shots are beautiful)
>
> http://gnomejournal.org/article/34/writing-a-widget-using-cairo-and-gtk28
> http://gnomejournal.org/article/36/writing-a-widget-using-cairo-and-gtk28-part-2
>
> (how to build a relatively simple widget from scratch that draws with
> cairo -- implemented in C)
>
> http://www.pygtk.org/articles/cairo-pygtk-widgets/cairo-pygtk-widgets.htm
> http://www.pygtk.org/articles/cairo-pygtk-widgets/cairo-pygtk-widgets2.htm
> (the same widget example, this time in Python)
>
> If you decide to use cairo AND you create your own widget AND you want
> to implement scrolling, please call for help on the list again.  There
> are some non-obvious things to handle there.  Actually, scrolling
> involves a lot of non-intuitive stuff in GTK+ in any case, unless you
> are just using prebuilt widgets.
>
> -Peter
>
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