ALLEGRO_PRIM_ATTR(3alleg5) ALLEGRO_PRIM_ATTR(3alleg5)

ALLEGRO_PRIM_ATTR - Allegro 5 API

#include <allegro5/allegro_primitives.h>
typedef enum ALLEGRO_PRIM_ATTR

Enumerates the types of vertex attributes that a custom vertex may have.

•
ALLEGRO_PRIM_POSITION - Position information, can be stored only in ALLEGRO_PRIM_SHORT_2, ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_FLOAT_3.
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ALLEGRO_PRIM_COLOR_ATTR - Color information, stored in an ALLEGRO_COLOR(3alleg5). The storage field of ALLEGRO_VERTEX_ELEMENT is ignored
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ALLEGRO_PRIM_TEX_COORD - Texture coordinate information, can be stored only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are normalized by the width and height of the texture, meaning that the bottom-right corner has texture coordinates of (1, 1).
•
ALLEGRO_PRIM_TEX_COORD_PIXEL - Texture coordinate information, can be stored only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are measured in pixels.
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ALLEGRO_PRIM_USER_ATTR - A user specified attribute. You can use any storage for this attribute. You may have at most ALLEGRO_PRIM_MAX_USER_ATTR (currently 10) of these that you can specify by adding an index to the value of ALLEGRO_PRIM_USER_ATTR, e.g. the first user attribute is ALLEGRO_PRIM_USER_ATTR + 0, the second is ALLEGRO_PRIM_USER_ATTR + 1 and so on.

To access these custom attributes from GLSL shaders you need to declare attributes that follow this nomenclature: al_user_attr_# where # is the index of the attribute.

For example to have a position and a normal vector for each vertex you could declare it like this:

ALLEGRO_VERTEX_ELEMENT elements[3] = {
    {ALLEGRO_PRIM_POSITION, ALLEGRO_PRIM_FLOAT_3, 0},
    {ALLEGRO_PRIM_USER_ATTR + 0, ALLEGRO_PRIM_FLOAT_3, 12},
    {0, 0, 0}};

And then in your vertex shader access it like this:

attribute vec3 al_pos; // ALLEGRO_PRIM_POSITION
attribute vec3 al_user_attr_0; // ALLEGRO_PRIM_USER_ATTR + 0
varying float light;
const vec3 light_direction = vec3(0, 0, 1);
void main() {
    light = dot(al_user_attr_0, light_direction);
    gl_Position = al_pos;
}

To access these custom attributes from HLSL you need to declare a parameter with the following semantics: TEXCOORD{# + 2} where # is the index of the attribute. E.g. the first attribute can be accessed via TEXCOORD2, second via TEXCOORD3 and so on.

Since: 5.1.6

ALLEGRO_VERTEX_DECL(3alleg5), ALLEGRO_PRIM_STORAGE(3alleg5), al_attach_shader_source(3alleg5)

Allegro reference manual