[cairo] cairo_stroke_extents() with transformation matrix

[Andreas Falkenhahn]

On 01.02.2010 at 16:56 Chris Wilson wrote:

>On Mon, 01 Feb 2010 17:35:25 +0100, "Andreas Falkenhahn"
><andreas at airsoftsoftwair.de> wrote:
>> 
>> Hi,
>> 
>> how can I get cairo_stroke_extents() to take the current transformation
>matrix
>> into account? I.e. please consider the following code:
>> 
>> // scale * 2 on both axes
>> cairo_matrix_init(&m, 2, 0, 0, 2, 0, 0);
>> 
>> cairo_set_matrix(cr, &m);
>> cairo_new_path(cr);
>> cairo_rectangle(cr, 0, 0, 320, 240);	
>> cairo_stroke_extents(cr, &x1, &y1, &x2, &y2);
>> 
>> I'd expect that cairo_stroke_extents() would return something in the
>range
>> of roughly 640 for x2 and roughly 480 for y2. Instead, it simply returns
>about
>> 320 for x2 and about 240 for y2-- just as if there was no transformation
>at all.
>> 
>> Is there a way to get cairo_stroke_extents() take the current
>transformation
>> matrix into account?
>
>Ab, but it is. It is transforming the extents of the path from device to
>user space using the current transformation matrix. For your purposes, you
>actually want to know the device extents, and so should set an identity
>transformation when querying the extents.
>
>cairo_save(cr);
>cairo_scale(cr, 2, 2);
>cairo_new_path(cr);
>cairo_rectangle(cr, 0, 0, 320, 240);	
>cairo_restore(cr);
>cairo_stroke_extents(cr, &x1, &y1, &x2, &y2);
>
>Hope this helps, and you have fun using Cairo!

Hmm, now I'm confused. Please consider the following code:

cairo_new_path(cr);
cairo_set_line_width(cr, 10);
cairo_rectangle(cr, 0, 0, 320, 240);
cairo_stroke_extents(cr, &x1, &y1, &x2, &y2);
	
printf("NORMAL: %.14g %.14g %.14g %.14g\n", x1, y1, x2, y2);
	
cairo_save(cr);
cairo_scale(cr, 2, 2);
cairo_new_path(cr);
cairo_set_line_width(cr, 10);
cairo_rectangle(cr, 0, 0, 320, 240);	
cairo_restore(cr);
cairo_stroke_extents(cr, &x1, &y1, &x2, &y2);

printf("TRANSFORMED: %.14g %.14g %.14g %.14g\n", x1, y1, x2, y2);

The first one prints:

NORMAL: -5 -5 325 245

And the second one prints:

TRANSFORMED: -5 -5 645 485

But shouldn't the second one print 

-5 -5 655 495

?? ... I mean, because I'm scaling by two on both axes. It seems as
if cairo_stroke_extents() doesn't scale the line width but when I draw it using
cairo_stroke() the line width has been scaled and is 20px wide/high. Any
ideas?

Thanks,

Andreas
--
"Remember: It's nice to be important but it's more important to be nice!"