Re: using cairo surfaces

[Tommi Sakari Uimonen <tuimonen cc hut fi>]

>   You are welcome! I also get some help from people here so its nice to give 
> something in return :).

It does give that warm fuzzy feeling, doesn't it :)

>   Are you sure you need a container and various child widgets?

No I'm not sure. To me it also felt very strange, as I just need place to 
draw with the cairo, but I don't know whether I can pass the expose events 
to other widgets that are not child widgets.

>   Can you instead use only a GtkDrawingArea?

I'll look into that.

> > 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.
>   You are right.
>   Though, I don't understand: the new card and the old card are two cards or 
> are the same card but you just want to modify a portion of it?

I was a bit unclear. If some part of the first card gets dirty, I just 
copy the same part from the 'clean' card and patch it over the first one, 
instead of placing the whole card on top of the dirty one.

>   You can tell Cairo what to draw and what to skip with:
> gdk_cairo_region( myCairo_t, myGdkEventExpose->region );
> cairo_clip( myCairo_t );
>   That speeds things up because Cairo doesn't draw things that it doesn't 
> need, though AFAIK Cairo needs to be more smart about it and optimize things 
> more, but at least it already do some things and likely it will just improve 
> with time :). You can help it though: there are gdk_cairo_* functions to find 
> out if a point is inside the region or not, you can use that for two things: 
> one is to avoid bothering Cairo and another is to avoid doing some unecessary 
> computations in your own application too.

Ok, sounds good. I kind of already know how to decide what needs to be 
done when some area gets dirty.

> > 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.
>   It depends on your performance needs.
>   You can do things smart (and more complex) by getting the contents of the 
> drawable and slide it according to the scroll size and then draw only the 
> remaining part; or you can do things more easily: just draw everything again 
> with the new offsets or different translation in your transformation matrix.

Well, I have most of the contents already stored in the cached surface, so 
the former would be the way to go.

>   I suggest to follow the examples, there is a Clock Widget done using Cairo 
> (http://www.gnomejournal.org/article/34/writing-a-widget-using-cairo-and-gtk28) 
> and is a good example for a lot of things and a good start, though instead of 
> using the cairo_rectangle for clipping just use the gdk_cairo_region for 
> clipping, that gives better performance, thats the only thing that I would 
> change to that tutorial.

:) This is exactly the one I have been following. I even created my 
current test widgets just by modifying the example (and I still have the 
same function names haunting me all over the code, never bothered to 
rename them)


>   Hope this helps.
>   Happy coding! :)

Thanks!

Tommi