A Simple OpenGL Triangle Demo

Started by blobjim, April 27, 2016, 01:48:35

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To learn OpenGL, I've made a simple-ish little demo program with short comments on each method call. I just wanted to post it in case anyone else is able to find it of use. When run, the program produces a full screen window with a white background and a "swimming" triangle with changing colors. However, it's definitely not perfect since I'm still just learning OpenGL.  :P

Application Code:
package main;

//import the OpenGL classes, Buffer utilities, and GLFW classes
import org.lwjgl.opengl.*;
import org.lwjgl.BufferUtils;
import static org.lwjgl.glfw.GLFW.*;

//import all of the necessary OpenGL functions statically so they can be easily accessed
import static org.lwjgl.opengl.GL11.*;
import static org.lwjgl.opengl.GL15.*;
import static org.lwjgl.opengl.GL20.*;
import static org.lwjgl.opengl.GL30.*;

//these are some extra imports for handling the shader files and Buffers
import java.io.File;
import java.io.FileNotFoundException;
import java.nio.ByteBuffer;
import java.util.Scanner;

public class Main {
	//a variable to hold the id of the GLFW window
	static long window;
	public static void main(String[] args) {
		//start GLFW
		//get the screen resolution of the user's monitor
		final int screenWidth = glfwGetVideoMode(glfwGetPrimaryMonitor()).width();
		final int screenHeight = glfwGetVideoMode(glfwGetPrimaryMonitor()).height();
		//set the window to use opengl version 4.5
		//use the core opengl profile, instead of the compatibility one, to make sure you're only using current features
		//make this program work with newer versions of opengl
		//turn on window resizing
		//create a GLFW window and store its id in the window variable
		window = glfwCreateWindow(screenWidth, screenHeight, "GLFW OpenGL Window", glfwGetPrimaryMonitor(), 0);
		//enables opengl
		//create GLCapabilities instance because it's required (stupid, I know) and use it to print out if OpenGL 4.5 is supported
		System.out.println("OpenGL 4.5 Supported: " + GL.createCapabilities().OpenGL45);
		//make the opengl screen 1600 pixels wide and 900 pixels tall.
		glViewport(0, 0, screenWidth, screenHeight);
		//show the window
		//the vertex data (x and y)
		double[] triangle = {
				0.0,	 0.5, //first x and y
			   -0.5,	-0.5, //second x and y
				0.5,	-0.5, //third x and y
		//the color data (red, green, and blue)
		double[] color = {
				0.0, 1.0, 0.0, //first vertex color
				1.0, 0.0, 0.0, //second vertex color
				0.0, 0.0, 1.0, //third vertex color
		//the order to render the vertices
		int[] index = {
		//convert the vertex data arrays into ByteBuffers using a method I created down below
		ByteBuffer vertices = storeArrayInBuffer(triangle);
		ByteBuffer colors = storeArrayInBuffer(color);
		ByteBuffer indices = storeArrayInBuffer(index);
		//VAO: stores pointers to all of the vbos to keep 'em organized
		//VBO: stores data (vertex coordinates, colors, indices, etc.) and a header that contains information about their format
		//tell the GPU to make a single vertex array and store the returned id into the VBO int
		int vao = glGenVertexArrays();
		//set the current vertex array object
			//tell the gpu to make a VBO and store its ID in the 'coordVBO' varabile
			int coordVBO = glGenBuffers();
			//bind the 'coordVBO' VBO for use
			glBindBuffer(GL_ARRAY_BUFFER, coordVBO);
				//we are currently inside the vertex array so this VBO is associated with 'coordVBO'
				//uploads VBO data (in this case, coords) to the GPU, tells some information about the VBO so that it can work as efficiently as possible
				//we are using STATIC_DRAW because "The data store contents will be specified once by the application...
				//...and used many times as the source for GL drawing and image specification commands."
				glBufferData(GL_ARRAY_BUFFER, vertices.capacity(), vertices, GL_STATIC_DRAW);
				//specifies information about the format of the VBO (number of values per vertex, data type, etc.)
				glVertexAttribPointer(0, 2, GL_DOUBLE, false, 0, 0);
				//enable vertex attribute array 0
			//create a second VBO for the colors
			int colorVBO = glGenBuffers();
			//bind the 'colorVBO' VBO for use
			glBindBuffer(GL_ARRAY_BUFFER, colorVBO);
				//uploads VBO data (in this case, colors) to the GPU
				glBufferData(GL_ARRAY_BUFFER, colors.capacity(), colors, GL_STATIC_DRAW);
				//specifies information about the format of the VBO (number of values per vertex, data type, etc.)
				glVertexAttribPointer(1, 3, GL_DOUBLE, false, 0, 0);
				//enable vertex attribute array 1
			//create a third VBO for the indices (tells the GPU which vertices to render and when)
			int indicesVBO = glGenBuffers();
			//bind the 'indicesVBO' for use
			glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, indicesVBO);
			//uploads VBO data (in this case, colors) to the GPU
				glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.capacity(), indices, GL_STATIC_DRAW);
			//unbind the last bound VBO
			glBindBuffer(GL_ARRAY_BUFFER, 0);

		//unbind the currently bound VAO
		//load the vertex shader from the file using a method I wrote down below
		int vertexShader = loadShader(new File("src/main/vertexShader"), GL_VERTEX_SHADER);
		//load the fragment shader from the file using a method I wrote down below
		int fragmentShader = loadShader(new File("src/main/fragmentShader"), GL_FRAGMENT_SHADER);
		//create a program object and store its ID in the 'program' variable
		int program = glCreateProgram();
		//these method calls link shader program variables to attribute locations so that they can be modified in Java code
		glBindAttribLocation(program, 0, "position");
		glBindAttribLocation(program, 1, "color");
		//attach the vertex and fragment shaders to the program
		glAttachShader(program, vertexShader);
		glAttachShader(program, fragmentShader);

		//link the program (whatever that does)
		//validate the program to make sure it wont blow up the program
		//check for compilation errors
		System.out.println("Vertex Shader Compiled: " 		+ glGetShaderi(vertexShader, 	GL_COMPILE_STATUS));
		System.out.println("Fragment Shader Compiled: " 	+ glGetShaderi(fragmentShader, 	GL_COMPILE_STATUS));
		System.out.println("Program Linked: " 				+ glGetProgrami(program, 		GL_LINK_STATUS));
		System.out.println("Program Validated: " 			+ glGetProgrami(program, 		GL_VALIDATE_STATUS));
		//check for general OpenGL errors
		int error = glGetError();
		while(error != 0) {
			System.out.println("OpenGL Error: " + error);
			error = glGetError();
		//sets the background clear color to white
		glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
		//get the 'colorMod and 'positionMod' variables so I can change them while drawing to create the animation
		int colorMod = glGetUniformLocation(program, "colorMod");
		int positionMod = glGetUniformLocation(program, "positionMod");
		float i = 0.0f;
		//set the current program
		while(glfwWindowShouldClose(window) == GLFW_FALSE) {
			//Animate the triangle by using sinusoids
			if(i > Math.PI * 2) {
				i = 0.0f;
			else {
				i += 0.1f;
			//use the colorMod and positionMod variables to modify the fragment colors and positions to animate the triangle
			glUniform4f(colorMod, (float)((Math.cos(i) + 1)/2), (float)((Math.sin(i) + 1)/2), (float)((Math.cos(i) + 1)/2), (float)((Math.sin(i) + 1)/2));
			glUniform2f(positionMod, (float)((Math.cos(i) + 1)/2), (float)((Math.sin(i) + 1)/2));
			//clear the window
			//set the current vao to the one we made earlier with all of the data
			//draw the current bound VAO/VBO using an index buffer
			glDrawElements(GL_TRIANGLES, index.length, GL_UNSIGNED_INT, 0);
			//unbind the vao if there's another one that will be used, just to get rid of any conflicts
			//swap the frame to show the rendered image
			//poll for window events (resize, close, button presses, etc.)
		System.out.println("Window closed");
		//disable the vertex attribute arrays
		//delete the vbos and vao
		//detach the shaders from the program object
		glDetachShader(program, vertexShader);
		glDetachShader(program, fragmentShader);
		//delete the shaders now that they are deatched
		//stop using the shader program
		//delete the program now that the shaders are detached and the program isn't being used
		//check for general OpenGL errors again to make sure the shutdown process worked
		error = glGetError();
		while(error != 0) {
			System.out.println("OpenGL Error: " + error);
			error = glGetError();
	public static int loadShader(File file, int type) {
		try {
			Scanner sc = new Scanner(file);
			StringBuilder data = new StringBuilder();
			if(file.exists()) {
				while(sc.hasNextLine()) {
					data.append(sc.nextLine() + "\n");
			int id = glCreateShader(type);
			glShaderSource(id, data);
			return id;
		catch (FileNotFoundException e) {
			return -1;
	public static ByteBuffer storeArrayInBuffer(double[] array) {
		//8 bytes (64-bits) in a double, multiplied by the number of values in the array
		ByteBuffer buffer = BufferUtils.createByteBuffer(array.length * 8);
		for(double i : array) {
		return buffer;
	public static ByteBuffer storeArrayInBuffer(int[] array) {
		//8 bytes (64-bits) in a double, multiplied by the number of values in the array
		ByteBuffer buffer = BufferUtils.createByteBuffer(array.length * 4);
		for(int i : array) {
		return buffer;

Vertex Shader:
#version 450 core

//position is bound to attribute index 0 and color is bound to attribute index 1
in  vec2 position;
in  vec3 color;

//this vector is updated each frame to animate the triangle
uniform vec2 positionMod = vec2(1,1);

//output the outColor variable to the next shader in the chain
out vec3 outColor;

void main(void) {

	//syntax: vec4(x, y, z, w);
    gl_Position = vec4(position.x * positionMod.x, position.y * positionMod.y, 0.0, 1.0);
    //pass the output color right to the fragment shader without changing it
    outColor = color;

Fragment Shader:
#version 450 core
// It was expressed that some drivers required this next line to function properly
precision highp float;

//use the outColor variable from the vertex shader as the input to the fragment shader
in vec3 outColor;

//this is updated every frame in the main loop
uniform vec4 colorMod;

//the built in pixel color variable
out vec4 gl_FragColor;

void main(void) {
    // Pass through our original color with full opacity.
    gl_FragColor = vec4(outColor.r * colorMod.r, outColor.g * colorMod.g, outColor.b * colorMod.b, 1.0 * colorMod.a);

Please take all of this with a grain of salt since this program probably isn't written in the best way possible  :)