Drawing a cube with OpenGL

So let’s draw a cube in OpenGL

When you use OpenGL to draw a cube, you actually draw multiple triangles, and the triangles end up being stitched together into a cube.

#include "glew.h"
#include <glfw3.h>
#include "common/loadShader.h"
#include "glm.hpp"
#include "ext.hpp"

int main(void)
{
	GLFWwindow* window;

	/* Initialize the library */
	if (!glfwInit())
		return -1;

	/* Create a windowed mode window and its OpenGL context */
	window = glfwCreateWindow(480, 320, "Hello World", NULL, NULL);
	if (!window)
	{
		glfwTerminate();
		return -1;
	}

	/* Make the window's context current */
	glfwMakeContextCurrent(window);

	// Needed in core profile
	if( glewInit() != GLEW_OK)
	{
		glfwTerminate();
		return -1;
	}

	// Our vertices. Tree consecutive floats give a 3D vertex; Three consecutive vertices give a triangle.
	// A cube has 6 faces with 2 triangles each, so this makes 6*2=12 triangles, and 12*3 vertices
	static const GLfloat g_vertex_buffer_data[] = {
		-1.0f,-1.0f,-1.0f, //triangle 1 : begin
		-1.0f,-1.0f,1.0f,
		-1.0f,1.0f,1.0f,  //triangle 1 : end
		1.0f, 1.0f,-1.0f, // triangle 2 : begin
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f, // triangle 2 : end
		1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		1.0f,-1.0f,-1.0f,
		1.0f, 1.0f,-1.0f,
		1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		1.0f,-1.0f, 1.0f,
		1.0f, 1.0f, 1.0f,
		1.0f,-1.0f,-1.0f,
		1.0f, 1.0f,-1.0f,
		1.0f,-1.0f,-1.0f,
		1.0f, 1.0f, 1.0f,
		1.0f,-1.0f, 1.0f,
		1.0f, 1.0f, 1.0f,
		1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f,
		1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f, 1.0f,
		1.0f,-1.0f, 1.0f
	};

	
	//This will identify our vertex buffer
	GLuint vertexbuffer;

	
	//Generate 1 buffer,put the resulting identifier in vertexbuffer
	glGenBuffers(1,&vertexbuffer);		

	//The following commands will talk about our 'vertexbuffer' buffer
	glBindBuffer(GL_ARRAY_BUFFER,vertexbuffer);

	//Give our vertices to OpenGL.
	glBufferData(GL_ARRAY_BUFFER,sizeof(g_vertex_buffer_data),g_vertex_buffer_data,GL_STATIC_DRAW);



	// One color for each vertex. They were generated randomly.
	static const GLfloat g_color_buffer_data[] = {
		0.583f, 0.771f, 0.014f,
		0.609f, 0.115f, 0.436f,
		0.327f, 0.483f, 0.844f,
		0.822f, 0.569f, 0.201f,
		0.435f, 0.602f, 0.223f,
		0.310f, 0.747f, 0.185f,
		0.597f, 0.770f, 0.761f,
		0.559f, 0.436f, 0.730f,
		0.359f, 0.583f, 0.152f,
		0.483f, 0.596f, 0.789f,
		0.559f, 0.861f, 0.639f,
		0.195f, 0.548f, 0.859f,
		0.014f, 0.184f, 0.576f,
		0.771f, 0.328f, 0.970f,
		0.406f, 0.615f, 0.116f,
		0.676f, 0.977f, 0.133f,
		0.971f, 0.572f, 0.833f,
		0.140f, 0.616f, 0.489f,
		0.997f, 0.513f, 0.064f,
		0.945f, 0.719f, 0.592f,
		0.543f, 0.021f, 0.978f,
		0.279f, 0.317f, 0.505f,
		0.167f, 0.620f, 0.077f,
		0.347f, 0.857f, 0.137f,
		0.055f, 0.953f, 0.042f,
		0.714f, 0.505f, 0.345f,
		0.783f, 0.290f, 0.734f,
		0.722f, 0.645f, 0.174f,
		0.302f, 0.455f, 0.848f,
		0.225f, 0.587f, 0.040f,
		0.517f, 0.713f, 0.338f,
		0.053f, 0.959f, 0.120f,
		0.393f, 0.621f, 0.362f,
		0.673f, 0.211f, 0.457f,
		0.820f, 0.883f, 0.371f,
		0.982f, 0.099f, 0.879f
	};
	GLuint colorbuffer;
	glGenBuffers(1,&colorbuffer);
	glBindBuffer(GL_ARRAY_BUFFER,colorbuffer);
	glBufferData(GL_ARRAY_BUFFER,sizeof(g_color_buffer_data),g_color_buffer_data,GL_STATIC_DRAW);


	GLuint programID = LoadShaders("./shader/vertex.shader","./shader/fragment.shader");
	glUseProgram(programID);
	glClearColor(0.0f, 0.0f, 0.4f, 0.0f);
	/* Loop until the user closes the window */

	// Projection matrix : 45° Field of View, 4:3 ratio, display range : 0.1 unit  100 units
	//glm::mat4 Projection = glm::ortho(-4.0f/3.0f, 4.0f/3.0f, -1.0f, 1.0f, 0.1f, 100.0f);
	glm::mat4 Projection = glm::perspective(45.0f,4.0f/3.0f,0.1f,100.f);
	glm::mat4 View = glm::lookAt(
		glm::vec3(4,3,-3), // Camera is at (4,3,3), in World Space
		glm::vec3(0,0,0), // and looks at the origin
		glm::vec3(0,1,0) // Head is up (set to 0,-1,0 to look upside-down)		
		);

	//Model matrix : an identity matrix (model will be at the origin)
		glm::mat4 Model = glm::mat4(1.0f);
		
		// Our ModelViewProjection : multiplication of our 3 matrices
		glm::mat4 MVP = Projection * View * Model;// Remember, matrix multiplication is the other way around
	
		
	
		// Get a handle for our "MVP" uniform.
		// Only at initialisation time.
		GLuint MatrixID = glGetUniformLocation(programID,"MVP");
	
		// Send our transformation to the currently bound shader,
		// in the "MVP" uniform
		// For each model you render, since the MVP will be different (at least the M part)
		glUniformMatrix4fv(MatrixID,1,GL_FALSE,&MVP[0][0]);



	while (!glfwWindowShouldClose(window))
	{
		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

		

		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER,vertexbuffer);
		glVertexAttribPointer(
			0,			// attribute 0. No particular reason for 0, but must match the layout in the shader.
			3,			// size
			GL_FLOAT,	// type
			GL_FALSE,	// normalized?
			0,			// stride
			(void*)0	// array buffer offset
			);

		glEnableVertexAttribArray(1);
		glBindBuffer(GL_ARRAY_BUFFER,colorbuffer);	
		glVertexAttribPointer(
			1,			// attribute 0. No particular reason for 0, but must match the layout in the shader.
			3,			// size
			GL_FLOAT,	// type
			GL_FALSE,	// normalized?
			0,			// stride
			(void*)0	// array buffer offset
			);
		
		glDrawArrays(GL_TRIANGLES,0,12*3);// Starting from vertex 0; 3 vertices total -> 1 triangle
		glEnable(GL_DEPTH_TEST);
		glDepthFunc(GL_LESS);
			
		

		
		

		
		glDisableVertexAttribArray(0);
		glDisableVertexAttribArray(1);
		/* Swap front and back buffers */
		glfwSwapBuffers(window);

		/* Poll for and process events */
		glfwPollEvents();
	}

	glfwTerminate();
	return 0;
}

vertex.shader

version 330 core

layout(location = 0) in vec3 vertexPosition_modelspace;
layout(location = 1) in vec3 vertexColor;

uniform mat4 MVP;
out vec3 fragmentColor;

void main(){
	vec4 v = vec4(vertexPosition_modelspace,1);
	gl_Position = MVP * v;
	// The color of each vertex will be interpolated
    // to produce the color of each fragment
	fragmentColor = vertexColor;
	
}

fragment.shader

out vec3 color;
in vec3 fragmentColor;

void main(){
	color = fragmentColor;
}

First, you set the vertex position, then the vertex color, glvertexattribute passes the data to the shader, and vertex.shader exports the vertex color to the fragment.shader. Note that glEnable (GL_DEPTH_TEST); glDepthFunc(GL_LESS); With the depth test turned on, the final triangle will be occluded based on the Z-axis position. If not, the occluded triangle will be determined based on the sequence of the drawings.


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