From 6218601074e7070ccd5ded69a79f313d83a17cf6 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Mon, 29 Aug 2022 07:11:23 +0200
Subject: Add initial working example of instanced text drawing

---
 src/main.c | 30 ++++++++++++++++++++++++++----
 1 file changed, 26 insertions(+), 4 deletions(-)

diff --git a/src/main.c b/src/main.c
index 7241409..2a901a3 100644
--- a/src/main.c
+++ b/src/main.c
@@ -208,6 +208,26 @@ render(Context *ctx) {
     glEnableVertexAttribArray(1);
     glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 4 * sizeof(f32), (void*)(2 * sizeof(f32)));
 
+    // NOTE: This should be a dynamic array of offsets and index into the
+    // texture atlas.
+    struct Letter {
+        f32 x;
+        f32 y;
+        f32 idx;
+    };
+    struct Letter letters[] = {
+        {-0.25, 0.0, 0},
+        {0.0,   0.0, 1},
+        {0.25,  0.0, 2},
+    };
+    u32 offset_vbo;
+    glGenBuffers(1, &offset_vbo);
+    glBindBuffer(GL_ARRAY_BUFFER, offset_vbo);
+    glBufferData(GL_ARRAY_BUFFER, sizeof(letters), letters, GL_STATIC_DRAW);
+    glEnableVertexAttribArray(2);
+    glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(f32), (void*)0);
+    glVertexAttribDivisor(2, 1);
+
     // Element buffer object setup.
     u32 ebo;
     glGenBuffers(1, &ebo);
@@ -243,14 +263,14 @@ render(Context *ctx) {
 "#version 330 core\n"
 "layout (location = 0) in vec2 position;\n"
 "layout (location = 1) in vec2 tex_coords;\n"
-// TODO: layout (location = 2) in u32 index;
+"layout (location = 2) in vec3 offset;\n"
 "out vec2 tex;\n"
 "void main() {\n"
+"    float idx = offset.z;\n"
 "    float N = 3;\n"
-"    float idx = 2;\n"
 "    float m = 1.0 / N;\n"
 "    float k = m * idx;\n"
-"    gl_Position = vec4(position.x, position.y, 0.0, 1.0);\n"
+"    gl_Position = vec4(position / 8 + offset.xy, 0.0, 1.0);\n"
 "    tex = tex_coords * vec2(m, 1.0) + vec2(k, 0.0);\n"
 "}";
 
@@ -288,7 +308,9 @@ render(Context *ctx) {
     // // glBlendEquation(GL_FUNC_ADD);
     //
     // glBindTexture(GL_TEXTURE_2D, text_tex); // TODO: Bind the texture atlas.
-    glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);
+    // TODO: Send the number of instances as a uniform or see if you can query
+    // it from the GLSL for N. In this case N == 3.
+    glDrawElementsInstanced(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0, 3);
     // glDisable(GL_BLEND);
 
     glfwSwapBuffers(ctx->window);
-- 
cgit v1.2.1