Re: Coordinate from device to user space without a context

Donn <[email protected]> Fri, 08 Nov 2013 10:15:09 +0200

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

On 07/11/2013 20:22, Bryce W. Harrington wrote:
> If I were you I'd probably start afresh with just
> the formulas I gave you, and make a little toy app that *just* does the
> mouse -> userspace conversion, without any other matrix calls.

Bryce, I spend some time making a toy app as you suggested. It's giving 
confusing results, but I hope it will help the list help me! Attached.

System info:
valac 0.20.1
cairo114.vapi

On my system I compile it with:
valac --pkg cairo -X -w mailing_list_matrix_testbed.vala -o test
And run:
./test yes #will do the cersion with the formulas
./test #will use device_to_user
then view the output png:
feh img.png

It's confusing because *both* versions do the same wrong thing! I was 
not expecting that. The odd oscillation in the numbers is at work.


> Btw, when you make cr.translate(), cr.rotate(), and cr.scale() calls,
> those are really just helper routines for modifying the internal
> matrix.  So be aware of those too.
Thanks, I will dispense with these calls in favour of a direct 
set_matrix in the future, when I can tell the trees from the woods.


Thanks for all the patience so far, I worry that I'm not comprehending 
fast enough!
\d

-- 
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mailing_list_matrix_testbed.vala (text/x-vala, 2.4 KB)

using Cairo;
/*
valac --pkg cairo -X -w mailing_list_matrix_testbed.vala -o test
And run:
./test yes #will do the version with the formulas
./test #will use device_to_user
then view the output png:
feh img.png
*/
void main(string[] args) {

  bool flag = false;
  if (args.length > 0) {
    flag = (args[1] == "yes");
  }
  if (flag) stdout.printf("RUNNING WITH FORMULA\n");
  if (!flag) stdout.printf("RUNNING WITH CONTEXT METHOD\n");

  Cairo.Matrix mymatrix;

	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 with -x left and +x right. Range(-250,250)
  var matrix = Cairo.Matrix (1, 0, 0, 1, 250,250 );
  matrix.scale (1, 1); 
	cr.transform(matrix);
  cr.save();

  double user_x=0; double user_y=0;
  double dev_y=0;
  double tmp_d_x; double tmp_d_y;

  cr.save(); 
    cr.translate(-100,0); //This is my "parent" scope. [A]

    cr.arc (0.0, 0.0, 80.0, 0, 2*Math.PI);
    cr.set_source_rgb(1,0,0);
    cr.fill();

    //Draw many children in scope [A] - this loop is device coords, like a mouse.
    for (double dev_x=0; dev_x<250; dev_x+=10) {
    cr.save(); //Preserve [A]

        //Position relative to parent scope [A]
        cr.translate(user_x,user_y); cr.scale(1,1); //this is the child scope.[B]

        mymatrix = cr.get_matrix(); //For formula version of run.
          //Draw a green dot.
          cr.arc (0.0, 0.0, 30.0, 0, 2*Math.PI);
          cr.set_source_rgb(0,1,0);
          cr.fill();

        //Formula run - changing device coords mapped into scope [B].
        if (flag) {
          //stdout.printf("mymatrix:  %g|%g|%g|%g|%g|%g\n",mymatrix.xx ,mymatrix.yx ,mymatrix.xy ,mymatrix.yy ,mymatrix.x0 ,mymatrix.y0 );
          user_x = (dev_x - mymatrix.x0)/mymatrix.xx;
          user_y = (dev_y - mymatrix.y0)/mymatrix.yy;
        } else {
          //Device_to_user run - changing device coords into scope [B]
          tmp_d_x = dev_x; tmp_d_y = dev_y;
          cr.device_to_user(ref tmp_d_x, ref tmp_d_y);
          user_x = tmp_d_x; user_y = tmp_d_y;
        }

        stdout.printf("dev_x dev_y: %G|%G becomes user_x user_y: %G|%G\n",dev_x,dev_y, user_x, user_y);

    cr.restore(); //End scope [B], restore to [A]
    } //draw next child

  cr.restore(); //Ends scope [A]

  cr.restore(); //Ends axes centering
	surface.write_to_png ("img.png");
}