[cairo] How to get extents of transformed graphics

[Andreas Falkenhahn]

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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