al_transform_...3d_projective(3alleg5)

✨ al_transform_coordinates_3d_projective ✨

okay bestie, welcome to the Allegro 5 API reference manual 💖 (yes, that's the whole manual, we're fancy like that). Sit down, grab a strawberry 🍓, and let's talk about the most underrated little math goblin in the whole graphics stack.

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🌸 NAME 🌸

al_transform_coordinates_3d_projective — Allegro 5 API, and it does exactly what the name screams at you. periodt.

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🦋 SYNOPSIS 🦋

#include <allegro5/allegro.h>

void al_transform_coordinates_3d_projective(const ALLEGRO_TRANSFORM *trans,
   float *x, float *y, float *z)

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👑 DESCRIPTION 👑

so what does this icon actually do?? It transforms x, y, z as homogeneous coordinates. no cap.

It's literally the same as calling al_transform_coordinates_4d(3alleg5) with the w coordinate set to 1, and then dividing x, y, z by the resulting w. That's it, that's the whole trick. 💅

And why do we care?? Because this gives you the same normalized coordinates Allegro will actually draw to when a projective transform is in effect — the kind you set with al_use_projection_transform(3alleg5). So you get to peek at what Allegro is about to do before it does it. main character energy honestly.

Now — those normalized coordinates are cute and all, but they're not pixels yet. To get the actual pixel coordinates, you translate and scale like so (w and h are the pixel dimensions of the target bitmap):

x = w / 2 + x * w / 2
y = h / 2 - y * h / 2

(yeah the y one is minus, because screen space y goes down, we don't make the rules 😭)

Parameters:

• trans — the transformation you wanna use 🎀
• x, y, z — pointers to the coordinates. pointers!! not values!! don't forget the & or the compiler will simply end you 🫶

Example:

ALLEGRO_TRANSFORM t2;
al_copy_transform(&t2, al_get_current_transform());
al_compose_transform(&t2, al_get_current_projection_transform());

ALLEGRO_TRANSFORM t3;
al_identity_transform(&t3);
al_scale_transform(&t3, 0.5, -0.5);
al_translate_transform(&t3, 0.5, 0.5);
al_scale_transform(&t3, al_get_bitmap_width(al_get_target_bitmap()),
                   al_get_bitmap_height(al_get_target_bitmap()));

al_transform_coordinates_3d_projective(&t2, &x, &y, &z);
// x, y now contain normalized coordinates
al_transform_coordinates(&t3, &x, &y);
// x, y now contain pixel coordinates

look at that. normalized first, pixel second. the two-step glow-up. ✨

Since 5.2.4 — so she's been around a minute, but she still ate. 💅

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🎀 SEE ALSO 🎀

• al_use_transform(3alleg5)
• al_transform_coordinates(3alleg5)
• al_transform_coordinates_3d(3alleg5)
• al_use_projection_transform(3alleg5)

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and that's the whole page bestie!! 🌈 straight from the Allegro reference manual, filed under the very cursed-looking name al_transform_...3d_projective(3alleg5) (the ellipsis is doing a lot of heavy lifting, iykyk 💀). go forth, transform some projective coordinates, and slay. 🦋💖✨