Re: Extents under rotation

Uli Schlachter <[email protected]> Sun, 08 Dec 2013 17:50:29 +0100

Newsgroups	gmane.comp.lib.cairo
Message-ID	<[email protected]>

On 08.12.2013 16:19, Donn wrote:
> On 07/12/2013 20:22, Uli Schlachter wrote:
>> Uhm. So the above code should do just this, right?
> Uli,thanks for your input again.
>
> I put together this code (in Vala) using your idea:
[...]
>             cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
>             cr.set_source_rgb(1,1,0);
>             cr.fill_preserve();
>
>             cr.save();
>             cr.identity_matrix();
>             cr.path_extents(out x1, out y1, out x2, out y2);
>
>               //Draw this device rect in green
>               cr.new_path();
>               cr.rectangle(x1,y1,(x2-x1),(y2-y1));
>               cr.set_source_rgb(0,1,0);
>               cr.set_line_width(3);
>               cr.stroke();
>
>             cr.restore();
>
>             //Go from device into user
>             cr.device_to_user(ref x1, ref y1);
>             cr.device_to_user(ref x2, ref y2);
>
>             //Draw this user rect in white.
>             cr.new_path();
>             cr.move_to(x1,y1);
>             cr.line_to(x2,y1);
>             cr.line_to(x2,y2);
>             cr.line_to(x1,y2);
>             cr.close_path();
>
[...]

That's not what I suggested. You cannot just transform the top-left and 
bottom-right corner of the extents and assume that the other ones can be deduced 
from that (this is not an axis-aligned rectangle in user space!).

Attached is a working version. The diff to your version is also attached.

Cheers,
Uli

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test.patch (text/x-patch, 955 B)

--- test.orig	2013-12-08 17:36:24.634988805 +0100
+++ test.vala	2013-12-08 17:48:36.827865336 +0100
@@ -2,6 +2,12 @@
 // View the img.png file after running "test"
 
 using Cairo;
+
+void foo(ref Cairo.Context cr, double x, double y) {
+    cr.device_to_user(ref x, ref y);
+    cr.line_to(x, y);
+}
+
 void main(string[] args) {
 
    double x1;
@@ -70,16 +76,13 @@ void main(string[] args) {
 
            cr.restore();
 
-           //Go from device into user
-           cr.device_to_user(ref x1, ref y1);
-           cr.device_to_user(ref x2, ref y2);
 
            //Draw this user rect in white.
            cr.new_path();
-           cr.move_to(x1,y1);
-           cr.line_to(x2,y1);
-           cr.line_to(x2,y2);
-           cr.line_to(x1,y2);
+           foo(ref cr, x1, y1);
+           foo(ref cr, x2, y1);
+           foo(ref cr, x2, y2);
+           foo(ref cr, x1, y2);
            cr.close_path();
 
            cr.set_source_rgb(1,1,1);

test.vala (text/x-vala, 2.5 KB)

// valac --pkg cairo test_extents_under_rotation.vala -o test
// View the img.png file after running "test"

using Cairo;

void foo(ref Cairo.Context cr, double x, double y) {
    cr.device_to_user(ref x, ref y);
    cr.line_to(x, y);
}

void main(string[] args) {

   double x1;
   double x2;
   double y1;
   double y2;

	Cairo.ImageSurface surface = new Cairo.ImageSurface 
(Cairo.Format.ARGB32, 500, 500);
	Cairo.Context cr = new Cairo.Context (surface);

   //centre the axes - 0,0 in middle
   var matrix = Cairo.Matrix (1, 0, 0, 1, 250,250 );
   matrix.scale (1, 1);
	cr.transform(matrix);


     cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
     cr.set_source_rgb(1,0,0);
     cr.fill_preserve();

     cr.path_extents (out x1, out y1, out x2, out y2);
     stdout.printf("Extents: %G|%G|%G|%G\n", x1,y1,x2,y2);
     //Draw the bbox as calculated by extents
     cr.rectangle(x1,y1,(x2-x1),(y2-y1));
     cr.set_source_rgb(0,0,0);
     cr.stroke();

     //Now, rotate that circle 45 degrees (and move it over a bit)
     cr.translate(90,0);
     cr.rotate((Math.PI/180.0)*45); //45 degree rot

       cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
       cr.set_source_rgb(1,0,0);
       cr.fill_preserve();

       cr.path_extents (out x1, out y1, out x2, out y2);
       stdout.printf("Extents: %G|%G|%G|%G\n", x1,y1,x2,y2);
       //Draw the bbox as calculated by extents -- this is wrong.
       //Why is the rect so much bigger?
       //Is there any way to get the proper user-space coords?
       cr.rectangle(x1,y1,(x2-x1),(y2-y1));
       cr.set_source_rgb(0,0,0);
       cr.stroke();


     //Test Uli's idea
     cr.translate(0,120);
     cr.rotate((Math.PI/180.0)*67);
     //Unusual rot. 45 degrees sometimes disguises things.

           cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
           cr.set_source_rgb(1,1,0);
           cr.fill_preserve();

           cr.save();
           cr.identity_matrix();
           cr.path_extents(out x1, out y1, out x2, out y2);

             //Draw this device rect in green
             cr.new_path();
             cr.rectangle(x1,y1,(x2-x1),(y2-y1));
             cr.set_source_rgb(0,1,0);
             cr.set_line_width(3);
             cr.stroke();

           cr.restore();


           //Draw this user rect in white.
           cr.new_path();
           foo(ref cr, x1, y1);
           foo(ref cr, x2, y1);
           foo(ref cr, x2, y2);
           foo(ref cr, x1, y2);
           cr.close_path();

           cr.set_source_rgb(1,1,1);
           cr.stroke();


	surface.write_to_png ("img.png");

}