Re: using cairo surfaces

[Tommi Sakari Uimonen <tuimonen cc hut fi>]

Hi, thanks for your answer!

> If you have a cairo_t obtained from your expose_event callback and you call 
> it "cr", then on the child widgets you can create a surface with 
> cairo_surface_create_similar( cairo_get_target(cr), CAIRO_CONTENT_COLOR, 
> theWidthOfYourChild, theHeightOfYourChild);.

Yes, this I already did. But I still have problems with gtk regarding what 
kind of widgets to use as the container and child. Currently the child is 
GtkImage. The container is the hard part. I have managed to use GtkHBox, 
but I would like to use something like GtkLayout, or actually I don't know 
what would be best. The container has also some cairo drawing, which draws 
on top of the child widgets.

> Then you can create a cairo_t for that surface with: cairo_t *child_cr = 
> cairo_create( the_cairo_surface_t_you_obtained_from_create_similar);.

This is what I'm doing.

> When you need to draw that child in your expose event, then you just need to 
> do a cairo_rectangle() for the area you want to draw it on, then call 
> cairo_set_source_surface(child_cr, startPosH, startPosV) and then 
> cairo_fill() it :-).

You missed the surface:

cairo_set_source_surface(child_cr, my_stored_surface, startPosH, 
startPosV)

Now, since the stored surface is the same height and width as the whole 
widget, I set startPosH and startPosV to 0, otherwise the rectangle to be 
drawn is not taken from the correct position in the stored surface.

Or am I wrong? I'm not quite sure, maybe the next example lights things 
up:

Let's imagine the surfaces as two identical playing cards. If small area 
on the center of the card needs to be redrawn, I just place the new card 
on top of the old one and draw the needed area from the new card, while 
other areas in the old card remain untouched. If I put nonzero values to 
startPosH/V the cards are no longer aligned and yields to incorrect 
result.

What does the cairo_rectangle actually do? And how does the 
set_cairo_surface restrict to drawing only to the exposed area, or does 
cairo handle the drawing for the exposed area only even if I draw the 
whole surface?

I'm not quite sure how to copy a small part of previously stored surface 
to current surface at the same position, or if I do scrolling I just copy 
most of the stored surface to slightly different position and draw the 
new information.

What happens, no matter what size the exposed event area is, if I draw the 
whole wiget area? Does cairo ignore the draw commands that affect outside 
the exposed area? Does this give bad performace?

It needs some logic to decide what to draw and where, if I'm supposed to 
draw only the exposed area. I know I'm bound to do that at some point, but 
for now I just draw the whole thing. It's nothing very complex, it's just 
text.

> IMPORTANT PERFORMANCE TIP: Carl Worth saw me crying about the bad performance 
> of my application and told me to use absolute values for startPosH and 
> startPosV (that is, instead of using say 54.35 use 54.00), you can do that if 
> it is not noticeable to the human eye in your case (it depends on 
> transformation matrices and scales you use, etc., just try it out and see).

Ok, thanks. I'll try that.

Tommi

> El 02/10/06 08:58, Tommi Sakari Uimonen escribió:
>
> > Hello. I'm trying to create a custom container widget that holds the child 
> > widgets in a hbox (or scrollarea, after I learn how to use it). The child 
> > widgets are using cairo for the drawing, and so is the container widget.
> >
> > I want to optimize the container drawing since the child widgets don't 
> > always need complete redrawing.
> >
> > The child drawing is currently done to the container's cairo surface, which 
> > is passed to each child widget, and childs draw to this surface. Reference 
> > to this surface is stored and I'm using a private variable in the container 
> > widget that keeps track of whether the childs need full redraw or can I use 
> > the stored surface to do quick redraw without the need to call the childs' 
> > drawing functions.
> >
> > Now, I want the child widgets to have their own surfaces and each child 
> > should handle the "smart drawing". So the child widget decides whether it 
> > uses the previously rendered surface or redraws some parts. The container 
> > then just uses set_source_surface to draw the child at desired position.
> >
> > What kind of surface I should create in the child widgets and how do I copy 
> > it to the container surface?
> >
> > I've tried creating image surface, but I guess I was doing something wrong, 
> > because nothing was drawn, or I got segfault. So I still have not found the 
> > right way of doing it. The documentation is lacking examples, so I have 
> > proceeded in "trial and error"-basis, and now I'm kind of stuck.
> >
> >
> > Tommi
> > _______________________________________________
> > gtk-app-devel-list mailing list
> > gtk-app-devel-list gnome org
> > http://mail.gnome.org/mailman/listinfo/gtk-app-devel-list
>
> --
> Ivan Baldo - ibaldo adinet com uy - http://ibaldo.codigolibre.net/
> ICQ 10215364 - Phone/FAX (598) (2) 613 3223.
> Caldas 1781, Malvin, Montevideo, Uruguay, South America.
> We believe that we are free, but in reality we are not! Are we?
> Alternatives: ibaldo codigolibre net - http://go.to/ibaldo