Scaling a Canvas to fit its scrolled window.

["James K. Wiggs" <wiggs novum-millennium org>]

 Folks,

   This post was already sent to gnome-list and gtk-app-devel-list,
but it was suggested to me via email that gnome-devel-list might
be a more appropriate place.  Hopefully gnome-devel is not just
for questions about development *of* gnome, but also for questions
about development *using* gnome.  So, that said, here is the
original message:


   More questions.  Here's the situation: I'm writing a charting
application.  The basic display is a GnomeCanvas inside a scrolled
window, which is placed in a notebook, which is placed in the top
level window.

   I am trying to scale the GnomeCanvas automatically so that it
always matches the dimension of the scrolled window viewport in
the y-direction, but can be set to various levels of scrolling in
the x-direction.  Of course, the first and most obvious thing is
to get the size of the CanvasWidget to precisely match the size of
the viewport.  To that end, I'm getting the size of the scrolled
window with the following code:

    adjust = gtk_scrolled_window_get_hadjustment(
	GTK_SCROLLED_WINDOW(charting_sw) );
    view_width = adjust->page_size;
    adjust = gtk_scrolled_window_get_vadjustment(
	GTK_SCROLLED_WINDOW(charting_sw) );
    view_height = adjust->page_size;

   The first problem, of course, is that until I actually show
the window with gtk_widget_show(), the page size of the scrolled
window is 0.  Using gtk_widget_realize() instead does not help;
I get completely different numbers after _realize() and _show().
OK, so I suppose I could build the contents of the Canvas, show
the window, get the sizes, re-do the affine transforms, and only
*then* drop the Canvas into the scrolled window and show it, but
it seems like there should be some way to get the whole thing
put together before showing it.

   The next problem is the affine transforms themselves.  As
always, there's the infuriating problem of the y-coordinates
increasing as you go *down* (will we *NEVER* get away from this
old throwback to CRT scanning!?!) which means that there's the
immediate requirement to do a negative-value scaling factor in
the y-direction.  But I need to be able to place multiple
graphs on the chart simultaneously, each positioned by its
item-to-world affine transform so that they are all stacked on
top of each other in the y-direction.  The way to do this, of
course, is to make each chart a GnomeCanvasGroup, with its own
independent transform from item-to-world coordinates (or should
it be the item-to-canvas transform?).  Still, the transforms
that I'm doing don't seem to be having the effects I expect.

   Let's use a for-instance:

   The data to be plotted runs from a minimum x-value of 0.0 to
a maximum of ~20000.  The y-value runs from a minimum of ~25.0
to a maximum of ~215.  So the rational transforming sequence
seems to me to be:

   1. Translate -25.0 in the y-direction.  Call this Aff1.

   2. Scale by a negative factor to "flip" the data upright,
     i.e. increasing y-values move upward on the chart, and
     to get the y-size of the object to match the y-size of
     the scrolled window viewport.  Call this Aff2.

   3. Translate the object back 190.0 in the y-direction to
     get it fully visible in the Canvas.  Call this Aff3.

   I compose Aff1 and Aff2 into a temporary affine "foo"
with the following:

   art_affine_multiply( foo, Aff1, Aff2 );

   Then get the final affine with:

   art_affine_multiply( affine, foo, Aff3 );

   And finally apply the affine to the chart group with:

   gnome_canvas_item_affine_relative( GNOME_CANVAS_ITEM(group), affine );

   Unfortunately, this does *not* have the expected effects.
Or, at least, I don't *think* it does.  One of the problems
I'm having is that I can't "see" all of the pertinent info.
What I'd like to do is get some sort of visible border on the
viewport so I can see its "edge," and also get some sort of
border around the entire Canvas so that I can see *its*
boundaries to determine whether I'm in fact getting things
matched up in size, but not matching up to the right things,
if you get my meaning.  Anyone have any suggestions on how
to improve my debugging capabilities in this respect?

   Well, this post has gotten a lot longer and more involved
than I'd originally planned.  I hope that the people who can
help won't get bored reading it and fail to respond.  I have
to say I'm really looking forward to the day when I know
enough that *I* can start answering the questions posed on
this list instead of always asking them...

best,
Jim Wiggs
wiggs wiggs net