How can I scale an object from the array of coordinates?

no-reply@allegro.cc (NathanJ) — Tue, 20 Oct 2020 00:25:38 +0000

I would like to scale up a polygon defined on cords below. How can I do that?

float cords[8] = {
x1,y1,
x2, y2,
x3, y3,
x4, y4
};

no-reply@allegro.cc (Eric Johnson) — Tue, 20 Oct 2020 00:36:58 +0000

I suggest you read-up on linear transformations, but basically you just multiply each element of your array by a scaling matrix.

But I suppose if all you're trying to do is scale the points (or vertices) of a polygon, you could simply iterate through the array and multiply it by a scaling factor...

no-reply@allegro.cc (Edgar Reynaldo) — Wed, 21 Oct 2020 16:13:24 +0000

If you scale the data by itself, it will induce a translation. You need to center it on the origin first, then scale it, and THEN you can translate it to its correct position.

#SelectExpand
  1float cords[8] = {
  2x1,y1,
  3x2, y2,
  4x3, y3,
  5x4, y4
  6}; 
  7const float cx = (x1 + x2 + x3 + x4)/4.0;
  8const float cy = (y1 + y2 + y3 + y4)/4.0;
  9
 10ALLEGRO_TRANSFORM t;
 11al_identity_transform(&t);
 12al_translate_transform(&t , -cx , -cy);
 13al_scale_transform(&t , scalex , scaley);
 14al_translate_transform(&t , posx , posy);
 15
 16al_transform_coordinates(&t , &cords[0] , &cords[1]);
 17al_transform_coordinates(&t , &cords[2] , &cords[3]);
 18al_transform_coordinates(&t , &cords[4] , &cords[5]);
 19al_transform_coordinates(&t , &cords[6] , &cords[7]);
 20
 21/// Our polygon is now centered on posx , posy at a scale of scalex,scaley.