ALLEGRO_PRIM_ATTR(3)

🎀 ALLEGRO_PRIM_ATTR(3) 🎀

okay so this is ALLEGRO_PRIM_ATTR(3), coming at you from the Library Functions Manual / Allegro reference manual, and it is literally the enum that tells you what kinds of vertex attributes a custom vertex can have. bestie, it’s giving “small enum, big shader energy” and we’re obsessed 💅✨

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🌸 NAME 🌸

ALLEGRO_PRIM_ATTR — the Allegro 5 API enum for custom vertex attribute types. periodt 💖

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✨ SYNOPSIS ✨

              #include <allegro5/allegro_primitives.h>

              typedef enum ALLEGRO_PRIM_ATTR

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🦋 DESCRIPTION 🦋

This enum lists the types of vertex attributes that a custom vertex may have. no cap, every member matters:

• ALLEGRO_PRIM_POSITION — position information, and it can be stored only in ALLEGRO_PRIM_SHORT_2, ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_FLOAT_3. that’s the whole lineup, don’t try to sneak in anything else 🫶
• ALLEGRO_PRIM_COLOR_ATTR — color information, stored in an ALLEGRO_COLOR(3). the storage field of ALLEGRO_VERTEX_ELEMENT is ignored here, bestie 💅
• ALLEGRO_PRIM_TEX_COORD — texture coordinate information, and it 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). yes, that bottom-right corner is main character energy 🌈
• ALLEGRO_PRIM_TEX_COORD_PIXEL — texture coordinate information, and it can be stored only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. these coordinates are measured in pixels, so think pixel-precise, not normalized 🎀
• 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. the cap is real, bestie, ten custom user attrs max 👑

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. HLSL besties stay fed too 🍓

Since: 5.1.6 ✨

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🔥 SEE ALSO 🔥

• ALLEGRO_VERTEX_DECL(3)
• ALLEGRO_PRIM_STORAGE(3)
• al_attach_shader_source(3)

these are the linked besties, go check them out when you’re done here 💜

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source/version: Allegro reference manual · date: not listed on this page, bestie — so here’s your sparkly sign-off instead of that footer line. 💖🎀