al_perspective_transform(3alleg5)

✨ al_perspective_transform ✨

okay bestie, welcome to the allegro5 manual, page al_perspective_transform(3alleg5), and babes this one is a whole mood 🎀

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💅 NAME 💅

al_perspective_transform — it's the Allegro 5 API, and it's giving perspective projection, no cap

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🌸 SYNOPSIS 🌸

#include <allegro5/allegro.h>

void al_perspective_transform(ALLEGRO_TRANSFORM *trans,
   float left, float top, float n,
   float right, float bottom, float f)

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🦋 DESCRIPTION 🦋

so like, this is al_orthographic_transform(3alleg5)'s cooler sibling — same energy, but it honors perspective ✨. and here's the tea: if literally everything is sitting at a z-position of -near, it will look exactly the same as it would with an orthographic transformation. like, identical. periodt.

now, if you wanna use a specific horizontal field of view, you can use this relation:

tan(hfov / 2) = (right - left) / 2 / near

and therefore:

near = (right - left) / 2 / tan(hfov / 2)

iykyk 😌

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🍓 Example 1:

float w = 800, h = 450; // assume our display is 800 x 450
float fov = tan(90 * ALLEGRO_PI / 180 / 2); // 90 degree field of view

// Our projection goes from 0/0 to w/h with the near parameter set
// for a 90 degree horizontal viewing angle.
ALLEGRO_TRANSFORM projection;
al_identity_transform(&projection);
al_perspective_transform(&projection, 0, 0,
 w / 2 / fov,
 w, h,
 2000);
al_use_projection_transform(&projection);

// Set the camera z to +400 (which is exactly the near distance)
ALLEGRO_TRANSFORM camera;
al_build_camera_transform(&camera, 0, 0, 400, 0, 0, 0, 0, 1, 0);
al_use_transform(&camera);

// This will draw two rectangles at the left and right edge of the
// screen and vertically centered. The x distance between them is 800
// units, but the camera transform moves them 400 along z, so with
// a 90° viewing angle both are visible.
al_draw_filled_rectangle(0, 200, 50, 250, red;
al_draw_filled_rectangle(750, 200, 800, 250, red);

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🧚 Example 2:

float w = 800, h = 450; // assume our display is 800 x 450
float fov = tan(90 * ALLEGRO_PI / 180 / 2); // 90 degree field of view
float aspect = h / w;
float zoom = 2; // enlarge x 2

// This projection is very different from the one in the first example.
// Here we map the left and right edge of the screen to -1 and +1. And
// the y axis goes from -1 at the bottom to +1 at the top, scaled by
// the aspect ratio. We also add a zoom parameter so we can control
// the visible portion of the scene independent of the field of view.
ALLEGRO_TRANSFORM projection;
al_identity_transform(&projection);
al_perspective_transform(&projection,
   -1 / zoom, aspect / zoom,
   1 / fov,
   1 / zoom, -aspect / zoom,
   2000);
al_use_projection_transform(&projection);

// Moves everything by -4 in the z direction.
ALLEGRO_TRANSFORM camera;
al_build_camera_transform(&camera, 0, 0, 4, 0, 0, 0, 0, 1, 0);
al_use_transform(&camera);

// At a z distance of 4 with a 90° hfov everything would be scaled
// down to 25%. However we also zoom 2-fold, so the final scaling is
// 50%. This rectangle therefore will appear at a size of 400 x 225
// pixel (assuming the display is 800 x 450).
al_draw_filled_rectangle(-1, -1, 1, 1, red);

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⭐ SINCE ⭐

5.1.3 — she's been iconic since way back, fr fr 💜

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🌈 SEE ALSO 🌈

• al_use_projection_transform(3alleg5)
• al_orthographic_transform(3alleg5)

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and that's the whole page bestie!! 🫶 this was the Allegro reference manual's al_perspective_transform(3alleg5) page — go forth and project things with correct perspective, you absolute main character 👑💖