[cairo] How to get extents of transformed graphics

[Behdad Esfahbod]

Try flooring x1,y1 before subtracting them from x2,y2 for computing the
size of your surface.

behdad
http://behdad.org/


On Thu, Sep 8, 2022 at 10:21 AM Andreas Falkenhahn <andreas at falkenhahn.com>
wrote:

> Is there really nobody who knows how to solve this? There surely must be a
> way to find out the *exact* extents of transformed graphics? The idea is to
> find out the exact dimensions in order to allocate a Cairo surface that
> *exactly* matches the size of the path.
>
> On 28.08.2022 at 21:47 Andreas Falkenhahn wrote:
>
> > Hi,
>
> > is there any way to get the *exact* extents of transformed
> > graphics? AFAICS, cairo_fill_extents() is in user coordinates, i.e.
> > it doesn't take any transformation settings into account. So I've
> > tried to apply the transformation manually to what I get from
> > cairo_fill_extents() but I don't seem to get it 100% right. It looks
> > like one row is missing at the bottom.
>
> > This is what I've tried:
>
> >         cairo_t *cr;
> >         cairo_surface_t *surface;
> >         cairo_matrix_t cm;
> >         int k;
> >         double tx = 0, ty = 0;
> >         double x1, y1, x2, y2;
> >
> >         surface = cairo_image_surface_create(CAIRO_FORMAT_A8, 1000,
> 1000);
> >         cr = cairo_create(surface);
>
> >         for(k = 0; k < 2; k++) {
> >
> >                 cairo_new_path(cr);
> >                 cairo_move_to(cr, tx, ty);
> >                 cairo_set_font_size(cr, 100);
> >                 cairo_text_path(cr, "C");
> >
> >                 if(!k) {
> >
> >                         double width, height;
> >
> >                         cairo_fill_extents(cr, &x1, &y1, &x2, &y2);
> >
> >                         tx = -x1;
> >                         ty = -y1;
> >
> >                         width = fabs(x2 - x1);
> >                         height = fabs(y2 - y1);
>
> >                         width = ceil(width * 4.6);
> >                         height = ceil(height * 4.6);
> >
> >                         cairo_destroy(cr);
> >                         cairo_surface_destroy(surface);
> >
> >                         surface =
> > cairo_image_surface_create(CAIRO_FORMAT_A8, width, height);
> >                         cr = cairo_create(surface);
> >
> >                         cairo_matrix_init(&cm, 4.6, 0, 0, 4.6, 0, 0);
> >                         cairo_set_matrix(cr, &cm);
> >                 }
> >         }
> >
> >         cairo_fill(cr);
>
> >         cairo_surface_flush(surface);
> >         cairo_surface_write_to_png(surface, "tmp.png");
> >
> >         cairo_destroy(cr);
> >         cairo_surface_destroy(surface);
>
> > I'm attaching the resulting image. As you can see, there's at least
> > one row of the "C" shape missing at the bottom of the image.
>
> > Any ideas how to get this right? Is this some sort of floating
> > point inaccuracy and should I just add 1 to width/height to solve
> > this or am I doing something wrong here and there is a better way?
>
>
>
> --
> Best regards,
>  Andreas Falkenhahn                            mailto:
> andreas at falkenhahn.com
>
>
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