ALLEGRO_PRIM_ATTR(3)

NAME

ALLEGRO_PRIM_ATTR - Allegro 5 API

SYNOPSIS

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

DESCRIPTION

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.

•   ALLEGRO_PRIM_COLOR_ATTR - Color information, stored in an ALLEGRO_COLOR(3). The storage field of ALLEGRO_VERTEX_ELEMENT is ignored

•   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.

•   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

SEE ALSO

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