Re: [g-a-devel] gnome-mag: I don't understand some variables in zoom_region_calculate_scroll_rects

[Bill Haneman <Bill Haneman Sun COM>]

Carlos Eduardo Rodrigues Diogenes wrote:

> Hi Bill,
>
> Can you explain what data the variables *scroll_rect*, *expose_rect_h* 
> and *expose_rect_v *hold? And where/why these variables are used?

As you see in the code, these are GdkRectangles. 

Consider the general case of "scrolling" a viewport or window; in the 
general case, where dx and dy are the relative motion in two dimensions, 
the affected region can be divided into three parts.  One part 
(scroll_rect) is the rectangular region which was "already" onscreen in 
the previous view, and is still onscreen in the new view; in other 
words, the part of the viewport that gets "scrolled" or moved.  The 
other two parts (expose_rect_h and expose_rect_v) represent the vertical 
and horizontal bands which have "moved into" the new view, which were 
not part of the old view.

In order to place the "newly-exposed" areas on screen, an "expose" event 
must be emitted to tell Gdk to repaint those onscreen areas.  The 
"scrolled" area, which was already onscreen, can be moved in one of two 
ways.  It can either have its own "expose" event emitted, just as with 
the newly-exposed areas (via gdk_window_invalidate_rect), or the old 
pixels can be copied from their old location to a new location, via 
gdk_window_scroll.  Since Xlib doesn't support 'scrolling' of pixmaps, 
the Xlib call 'XCopyArea' must be used in the implementation inside 
Gdk.  Although you might think that "moving" the area already onscreen 
should be much faster than invalidating the whole window, in practice it 
doesn't really make much difference, and using "expose" may give a 
little smoother result.  That's why there are two different 
implementations of "scroll" in zoom-region.c, zoom_region_scroll_fast 
and zoom_region_scroll_smooth.  Depending on the zoom region and 
magnifier user preferences, either "smooth" or "fast" scrolling is 
used.  As I said, the "fast" method isn't currently much faster than the 
"smooth" one, so there is little advantage to it.

regards

Bill

> Thanks,
> Carlos.
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