Transformations

These functions are declared in the main Allegro header file:

``#include <allegro5/allegro.h>``

The transformations are combined in the order of the function invocations. Thus to create a transformation that first rotates a point and then translates it, you would (starting with an identity transformation) call al_rotate_transform and then al_translate_transform. This approach is opposite of what OpenGL uses but similar to what Direct3D uses.

For those who known the matrix algebra going behind the scenes, what the transformation functions in Allegro do is "pre-multiply" the successive transformations. So, for example, if you have code that does:

``````al_identity_transform(&T);

al_compose_transform(&T, &T1);
al_compose_transform(&T, &T2);
al_compose_transform(&T, &T3);
al_compose_transform(&T, &T4);``````

The resultant matrix multiplication expression will look like this:

``T4 * T3 * T2 * T1``

Since the point coordinate vector term will go on the right of that sequency of factors, the transformation that is called first, will also be applied first.

This means if you have code like this:

``````al_identity_transform(&T1);
al_scale_transform(&T1, 2, 2);
al_identity_transform(&T2);
al_translate_transform(&T2, 100, 0);

al_identity_transform(&T);

al_compose_transform(&T, &T1);
al_compose_transform(&T, &T2);

al_use_transform(T);``````

it does exactly the same as:

``````al_identity_transform(&T);
al_scale_transform(&T, 2, 2);
al_translate_transform(&T, 100, 0);
al_use_transform(T);``````