[cairo] Getting started

[Behdad Esfahbod]

On Sun, 24 Jul 2005, Vladimir Vukicevic wrote:

> On 7/23/05, Behdad Esfahbod <behdad at cs.toronto.edu> wrote:
> > Hi,
> >
> > After Carl's good tutorial yesterday at OLS, I finally got
> > started using Cairo.  As an example, I added Cairo support to
> > dasher, the predictive text input widget.  I found it quite
> > straightforward to convert from Gdk drawing to Cairo.  But I've
> > got a big problem:  Currently I get the two-pixel-wide blurred
> > lines, so I thought I can cairo_translate(cr, -0.5, -0.5) to fix
> > that.  But doing that (looks like fixing the problem) makes the
> > rendering /extremely/ slow, so much that it's unusable.
> >
> > Any way to get around it?  Why is a translate so slow?  My
> > graphics consists of a bunch of rectangles...
>
> Note that in the FAQ entry about smeared lines it says "adjust the
> endpoints by 0.5 in the appropriate direction"  For drawing lines, you
> don't want to offset both the x and the y coordinates by 0.5 -- just
> one, depending on the orientation of your line.  The line is converted
> into a solid region that is then filled, by extending the path by half
> of the line-width to either side.  So, for a horizontal line that you
> want to extend from pixel 2 to pixel 5, you'd want to leave the x
> coordinate as integer values, but add 0.5 to the y coordinate to push
> the line down into the middle of the pixels.  But horizontally, you
> want the line to take up an integer width of pixels.  Note that a line
> going from 2 to 5 will only cover pixels 2,3,4 -- the line itself will
> end at the boundary of pixel 5, and won't actually cover pixel 5.
> You'll have to add 1 to the end of the horizontal line in the x
> direction to make it fill pixels 2,3,4,5.
>
> Reading that over, I realize that the explanation is probably pretty
> confusing -- but grab a piece of graph paper, pretend each square is a
> pixel. Then draw a one-pixel-wide region, with your path running down
> the middle of it.  It should be pretty clear which coordinates need to
> be offset. :)
>
> The reason you're getting a massive slowdown is that because with a
> .5,.5 offset you need to blend the first and last pixel, you end up
> going down a very slow codepath that's otherwise not needed.  (I
> followed this path for a while looking for a similar bug in mozilla;
> there may be a cairo bug involved that's causing additional slowdown,
> but I didn't get that far.)

Thanks Vlad,  but I'm not sure that's quite the reason here.  As
I said, I'm mostly drawing rectangles, and so offsetting .5, .5
makes sense, but I'll play with it more to see wether not drawing
any lines helps.


>     - Vlad

--behdad
http://behdad.org/