Transformations in allegro

no-reply@allegro.cc (baskerbill) — Fri, 24 Jun 2016 19:04:23 +0000

I'm trying to port some code from processing, and I've found problems with the PushMatrix and PopMatrix. Example:

pushMatrix();
translate(x,y);
rotate(alfa);
translate(desx,desy);
image(bitmap,0,0);

pushMatrix();
translate(x,y);
popMatrix();

popMatrix();

Transformations in allegro are built as:

ALLEGRO_TRANSFORM t;

al_copy_transform(&old,&t);

al_intentity_transform(&t)
al_translate_transform(x,y);
al_rotate_transform(alfa);
al_use_transform(&t)

al_draw_bitmap(bitmap,0,0);

al_identity_transform(&old);
al_use_transform(&old);

It works perfectly with one bitmap, but I haven't foud a way to concatenate two transforms the same way as with pushMatrix, and I think I don't fully understand the behavior of allegro transformations.

Any tip or help will be wellcome.

Thanks!

no-reply@allegro.cc (Mark Oates) — Fri, 24 Jun 2016 23:41:14 +0000

In AllegroFlare's placement2d, you get these two functions:

#SelectExpand
  1void placement2d::start_transform()
  2{
  3   ALLEGRO_TRANSFORM transform;
  4
  5   al_copy_transform(&previous_transform, al_get_current_transform()); 
  6   al_identity_transform(&transform);
  7
  8   al_translate_transform(&transform, -align.x*size.x, -align.y*size.y);
  9   al_scale_transform(&transform, scale.x, scale.y);
 10   al_translate_transform(&transform, anchor.x, anchor.y);
 11   al_rotate_transform(&transform, rotation);
 12   al_translate_transform(&transform, position.x, position.y);
 13
 14   al_compose_transform(&transform, &previous_transform);
 15
 16   al_use_transform(&transform);
 17}
 18
 19
 20
 21
 22void placement2d::restore_transform()
 23{
 24   al_use_transform(&previous_transform);
 25}

previous_transform is an ALLEGRO_TRANSFORM, member of placement2d

no-reply@allegro.cc (baskerbill) — Sat, 25 Jun 2016 00:35:55 +0000

Thank you very much.

I missed the function al_compose_transform, which is the key to combine the matrix stack.

I should read more carefully the man pages.

Again, thank you for your time.