Use LWJGL to render a scende into a PNG without showing windows

RenderToPng

import static org.lwjgl.glfw.GLFW.GLFW_CONTEXT_VERSION_MAJOR;
import static org.lwjgl.glfw.GLFW.GLFW_CONTEXT_VERSION_MINOR;
import static org.lwjgl.glfw.GLFW.GLFW_OPENGL_CORE_PROFILE;
import static org.lwjgl.glfw.GLFW.GLFW_OPENGL_FORWARD_COMPAT;
import static org.lwjgl.glfw.GLFW.GLFW_OPENGL_PROFILE;
import static org.lwjgl.glfw.GLFW.GLFW_SAMPLES;
import static org.lwjgl.glfw.GLFW.GLFW_VISIBLE;
import static org.lwjgl.glfw.GLFW.glfwCreateWindow;
import static org.lwjgl.glfw.GLFW.glfwDestroyWindow;
import static org.lwjgl.glfw.GLFW.glfwInit;
import static org.lwjgl.glfw.GLFW.glfwMakeContextCurrent;
import static org.lwjgl.glfw.GLFW.glfwTerminate;
import static org.lwjgl.glfw.GLFW.glfwWindowHint;
import static org.lwjgl.opengl.EXTFramebufferObject.GL_COLOR_ATTACHMENT0_EXT;
import static org.lwjgl.opengl.EXTFramebufferObject.GL_FRAMEBUFFER_EXT;
import static org.lwjgl.opengl.EXTFramebufferObject.glBindFramebufferEXT;
import static org.lwjgl.opengl.EXTFramebufferObject.glFramebufferTexture2DEXT;
import static org.lwjgl.opengl.EXTFramebufferObject.glGenFramebuffersEXT;
import static org.lwjgl.opengl.GL11.GL_COLOR_BUFFER_BIT;
import static org.lwjgl.opengl.GL11.GL_FALSE;
import static org.lwjgl.opengl.GL11.GL_FLOAT;
import static org.lwjgl.opengl.GL11.GL_INT;
import static org.lwjgl.opengl.GL11.GL_LINEAR;
import static org.lwjgl.opengl.GL11.GL_RGBA;
import static org.lwjgl.opengl.GL11.GL_RGBA8;
import static org.lwjgl.opengl.GL11.GL_TEXTURE_2D;
import static org.lwjgl.opengl.GL11.GL_TEXTURE_MIN_FILTER;
import static org.lwjgl.opengl.GL11.GL_TRIANGLES;
import static org.lwjgl.opengl.GL11.GL_TRUE;
import static org.lwjgl.opengl.GL11.GL_UNSIGNED_BYTE;
import static org.lwjgl.opengl.GL11.glBindTexture;
import static org.lwjgl.opengl.GL11.glClear;
import static org.lwjgl.opengl.GL11.glDrawArrays;
import static org.lwjgl.opengl.GL11.glGenTextures;
import static org.lwjgl.opengl.GL11.glGetTexImage;
import static org.lwjgl.opengl.GL11.glTexImage2D;
import static org.lwjgl.opengl.GL11.glTexParameterf;
import static org.lwjgl.opengl.GL11.glViewport;
import static org.lwjgl.opengl.GL15.GL_ARRAY_BUFFER;
import static org.lwjgl.opengl.GL15.GL_STATIC_DRAW;
import static org.lwjgl.opengl.GL15.glBindBuffer;
import static org.lwjgl.opengl.GL15.glBufferData;
import static org.lwjgl.opengl.GL15.glDeleteBuffers;
import static org.lwjgl.opengl.GL15.glGenBuffers;
import static org.lwjgl.opengl.GL20.glDisableVertexAttribArray;
import static org.lwjgl.opengl.GL20.glEnableVertexAttribArray;
import static org.lwjgl.opengl.GL20.glVertexAttribPointer;
import static org.lwjgl.opengl.GL30.glBindVertexArray;
import static org.lwjgl.opengl.GL30.glDeleteVertexArrays;
import static org.lwjgl.opengl.GL30.glGenVertexArrays;
import static org.lwjgl.system.MemoryUtil.NULL;

import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.FloatBuffer;

import javax.imageio.ImageIO;

import org.lwjgl.BufferUtils;
import org.lwjgl.opengl.GLContext;

public class RenderToPng
{
    private int vaoID;
    private int vboID;
    
    int screenTextureID;
    int framebufferID;
    
	private long windowID;
    
  

	private void initFramebuffer() {
		framebufferID = glGenFramebuffersEXT();                                         // create a new framebuffer
		screenTextureID = glGenTextures();                                               // and a new texture used as a color buffer
    
		glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID);                        // switch to the new framebuffer
    
		// initialize color texture
		glBindTexture(GL_TEXTURE_2D, screenTextureID);                                   // Bind the colorbuffer texture
		glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);               // make it linear filterd
		glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, 512, 512, 0,GL_RGBA, GL_INT, (java.nio.ByteBuffer) null);  // Create the texture data
		glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT,GL_COLOR_ATTACHMENT0_EXT,GL_TEXTURE_2D, screenTextureID, 0); // attach it to the framebuffer
    
		glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);                                    // Swithch back to normal framebuffer rendering
	}

	private void initWindow() {
		// Window Hints for OpenGL context
         glfwWindowHint(GLFW_SAMPLES, 4);
         glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
         glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 2);
         glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
         glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
         
         glfwWindowHint(GLFW_VISIBLE, GL_FALSE);

         
         windowID = glfwCreateWindow(512, 512, "Invisible", NULL, NULL);

         if (windowID == NULL)
         {
             System.err.println("Error creating a window");
             System.exit(1);
         }

         glfwMakeContextCurrent(windowID);
         GLContext.createFromCurrent();
	}

	private void initGeometry() 
	{
	    // Generate and bind a Vertex Array
        vaoID = glGenVertexArrays();
        glBindVertexArray(vaoID);

        // The vertices of our Triangle
        float[] vertices = new float[]
        {
            +0.0f, +0.8f,    // Top coordinate
            -0.8f, -0.8f,    // Bottom-left coordinate
            +0.8f, -0.8f     // Bottom-right coordinate
        };

        // Create a FloatBuffer of vertices
        FloatBuffer verticesBuffer = BufferUtils.createFloatBuffer(vertices.length);
        verticesBuffer.put(vertices).flip();

        // Create a Buffer Object and upload the vertices buffer
        vboID = glGenBuffers();
        glBindBuffer(GL_ARRAY_BUFFER, vboID);
        glBufferData(GL_ARRAY_BUFFER, verticesBuffer, GL_STATIC_DRAW);

        glVertexAttribPointer(0, 2, GL_FLOAT, false, 0, 0);
        glBindVertexArray(0);
	}
	
    public void renderToTexture() {
        
        glViewport (0, 0, 512, 512);                                    // set The Current Viewport to the fbo size
 
        glBindTexture(GL_TEXTURE_2D, 0);                                // unlink textures because if we dont it all is gonna fail
        glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, framebufferID);        // switch to rendering on our FBO
 
        renderAsNormal();
        
        ByteBuffer buffer = extractRawTextureData();
        encodePng(buffer);
         
    }

	private void encodePng(ByteBuffer buffer) 
	{
		
        BufferedImage image = new BufferedImage(512, 512, BufferedImage.TYPE_INT_RGB);
           
        // possibly there's a faster method?
        for(int x = 0; x < 512; x++)
        {
            for(int y = 0; y < 512; y++)
            {
                int i = (x + (512 * y)) * 4;
                int r = buffer.get(i) & 0xFF;
                int g = buffer.get(i + 1) & 0xFF;
                int b = buffer.get(i + 2) & 0xFF;
                image.setRGB(x, 512 - (y + 1), (0xFF << 24) | (r << 16) | (g << 8) | b);
            }
        }
           
        try {
            ImageIO.write(image, "PNG", new File("out.png"));
        } catch (IOException e) { e.printStackTrace(); }
	}

	private ByteBuffer extractRawTextureData() {
		ByteBuffer buffer = ByteBuffer.allocateDirect(512 * 512 * 4).order(ByteOrder.nativeOrder());
        glBindTexture(GL_TEXTURE_2D, screenTextureID);
        glGetTexImage(GL_TEXTURE_2D, 0, GL_RGBA, GL_UNSIGNED_BYTE, buffer);
		return buffer;
	}
	

    public void renderAsNormal()
    {
        // Clear the screen
        glClear(GL_COLOR_BUFFER_BIT);

        // Bind the vertex array and enable our location
        glBindVertexArray(vaoID);
        glEnableVertexAttribArray(0);

        // Draw a triangle of 3 vertices
        glDrawArrays(GL_TRIANGLES, 0, 3);

        // Disable our location
        glDisableVertexAttribArray(0);
        glBindVertexArray(0);

    }

    public void disposeAll()
    {

        // Dispose the vertex array
        glBindVertexArray(0);
        glDeleteVertexArrays(vaoID);

        // Dispose the buffer object
        glBindBuffer(GL_ARRAY_BUFFER, 0);
        glDeleteBuffers(vboID);
        
        
        // Destroy the window
        glfwDestroyWindow(windowID);
        glfwTerminate();

    }
   
    public void run()
    {
    	if (glfwInit() != GL_TRUE)
        {
            System.err.println("Error initializing GLFW");
            System.exit(1);
        }

    	initWindow();
        initGeometry();	
        initFramebuffer();
        
        renderToTexture();

        disposeAll();
    }

    public static void main(String[] args)
    {
        new RenderToPng().run();
    }
}