From d72ba1cf31286b55db5a156c8a17c6b1cc2af917 Mon Sep 17 00:00:00 2001
From: Ravenwater <theo@stillwater-sc.com>
Date: Thu, 2 Jan 2025 11:30:12 -0500
Subject: [PATCH] element drawing

---
 src/shaders/vertex.glsl |  1 +
 src/triangle_mesh.cpp   | 30 ++++++++++++++++++++++--------
 src/triangle_mesh.hpp   |  3 +++
 3 files changed, 26 insertions(+), 8 deletions(-)

diff --git a/src/shaders/vertex.glsl b/src/shaders/vertex.glsl
index b4679d8..1a5002a 100644
--- a/src/shaders/vertex.glsl
+++ b/src/shaders/vertex.glsl
@@ -14,4 +14,5 @@ const vec3 colors[3] = vec3[](
 void main() {
     gl_Position = vec4(vertexPos, 1.0);
     fragmentColor = colors[colorIndex];
+	//fragmentColor = colors[gl_VertexID];
 }
\ No newline at end of file
diff --git a/src/triangle_mesh.cpp b/src/triangle_mesh.cpp
index c5ee495..dcd012c 100644
--- a/src/triangle_mesh.cpp
+++ b/src/triangle_mesh.cpp
@@ -4,17 +4,24 @@
 #include "triangle_mesh.hpp"
 
 TriangleMesh::TriangleMesh() {
-    vertex_count = 3;
+    vertex_count = 6;
 
-    std::vector<float> positions = {
-         -1.0f, -1.0f, 0.0f,
-          1.0f, -1.0f, 0.0f,
-         -1.0f,  1.0f, 0.0f
+    std::vector<float> vertexCoordinates = {
+         -1.0f, -1.0f, 0.0f,   // bottom left
+		  1.0f, -1.0f, 0.0f,   // bottom right
+		 -1.0f,  1.0f, 0.0f,   // top left
+		  1.0f,  1.0f, 0.0f,   // top right
     };
-    
+
     std::vector<int> colorIndices = {
 		0, 1, 2
     };
+	// 1st triangle: bottom left, bottom right, top left, 
+    // 2nd triangle: top right, bottom right, top left
+    std::vector<int> elementIndices = {
+        //0, 1, 2, 2, 1, 3
+        0, 1, 2, 3, 1, 2
+    };
 
     glGenVertexArrays(1, &VAO);
     glBindVertexArray(VAO);
@@ -25,7 +32,7 @@ TriangleMesh::TriangleMesh() {
 
     // position
     glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
-    glBufferData(GL_ARRAY_BUFFER, positions.size() * sizeof(float), positions.data(), GL_STATIC_DRAW);
+    glBufferData(GL_ARRAY_BUFFER, vertexCoordinates.size() * sizeof(float), vertexCoordinates.data(), GL_STATIC_DRAW);
   // vertexAttribPointer(index, size, type, normalized, stride, pointer)
     glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 12 /* stride to next attribute */, (void*)0);
     glEnableVertexAttribArray(0);
@@ -37,6 +44,11 @@ TriangleMesh::TriangleMesh() {
     glVertexAttribIPointer(1, 1, GL_INT, 4 /* stride to next attribute */, (void*)0);  // <---- notice the attrib integer pointer function signature
     glEnableVertexAttribArray(1);
 
+    // element buffer
+	glGenBuffers(1, &EBO);
+    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
+    glBufferData(GL_ELEMENT_ARRAY_BUFFER, elementIndices.size() * sizeof(int), elementIndices.data(), GL_STATIC_DRAW);
+
 	// the VBO handles are stored in the VAO
 	// and the specific VBO, that is VBO[0] and VBO[1] are tied to the location 0 and 1 of the vertex shader
     // location=0 is glVertexAttribPointer( -->0<--...
@@ -45,10 +57,12 @@ TriangleMesh::TriangleMesh() {
 
 void TriangleMesh::draw() {
     glBindVertexArray(VAO);
-    glDrawArrays(GL_TRIANGLES, 0, vertex_count);
+	//glDrawArrays(GL_TRIANGLES, 0, vertex_count);  // if you draw using glDrawArrays, you don't need to bind the EBO
+	glDrawElements(GL_TRIANGLES, vertex_count, GL_UNSIGNED_INT, 0);
 }
 
 TriangleMesh::~TriangleMesh() {
     glDeleteVertexArrays(1, &VAO);
     glDeleteBuffers(2, VBO.data());
+	glDeleteBuffers(1, &EBO);
 }
\ No newline at end of file
diff --git a/src/triangle_mesh.hpp b/src/triangle_mesh.hpp
index 06bc622..34e9f88 100644
--- a/src/triangle_mesh.hpp
+++ b/src/triangle_mesh.hpp
@@ -10,6 +10,9 @@ class TriangleMesh {
 private:
 	unsigned int vertex_count;
 
+	// EOB - Element Buffer Object
+	unsigned int EBO;
+
 	// VAO - Vertex Array Object
 	unsigned int VAO;