ALLEGRO_PRIM_ATTR - Man Page

#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.
• 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

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

al_attach_shader_source(3), ALLEGRO_PRIM_STORAGE(3), ALLEGRO_VERTEX(3), ALLEGRO_VERTEX_ELEMENT(3).