LWJGL Can't sample framebuffer texture in the shader

Question

I'm trying to make a post processing shader using a framebuffer. But when I attempt to sample the texture in the shaders it doesn't do anything.

As soon as I comment the line(in the fragment shader)

vec4 textureColour = texture(screenTexture, textureCoords);

out everything works again

FrameBuffer.java

package postprocessing;

import java.nio.ByteBuffer;

import org.lwjgl.opengl.Display;
import org.lwjgl.opengl.GL11;
import org.lwjgl.opengl.GL14;
import org.lwjgl.opengl.GL30;
import org.lwjgl.opengl.GL32;

public class FrameBuffer {

protected static final int WIDTH = Display.getWidth();
private static final int HEIGHT = Display.getHeight();

private int framebuffer;
private int texture;
private int depthBuffer;
private int depthTexture;

public FrameBuffer()
{
    initialise();
}

public void cleanUp()
{
    GL30.glDeleteFramebuffers(framebuffer);
    GL11.glDeleteTextures(texture);
    GL30.glDeleteRenderbuffers(depthBuffer);
    GL11.glDeleteTextures(depthTexture);
}

public int getTexture()
{
    return texture;
}

public int getDepthTexture()
{
    return depthTexture;
}

private void initialise()
{
    framebuffer = createFrameBuffer();
    texture = createTextureAttachment(WIDTH, HEIGHT);
    depthBuffer = createDepthBufferAttachment(WIDTH, HEIGHT);
    depthTexture = createDepthTextureAttachment(WIDTH, HEIGHT);
    unbindCurrentFrameBuffer();
}

public void bindFrameBuffer(){
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, 0);//To make sure the texture isn't bound
    GL30.glBindFramebuffer(GL30.GL_FRAMEBUFFER, framebuffer);
    GL11.glViewport(0, 0, WIDTH, HEIGHT);
}

public void unbindCurrentFrameBuffer() {//call to switch to default frame buffer
    GL30.glBindFramebuffer(GL30.GL_FRAMEBUFFER, 0);
    GL11.glViewport(0, 0, Display.getWidth(), Display.getHeight());
}

private int createFrameBuffer() {
    int frameBuffer = GL30.glGenFramebuffers();
    //generate name for frame buffer
    GL30.glBindFramebuffer(GL30.GL_FRAMEBUFFER, frameBuffer);
    //create the framebuffer
    GL11.glDrawBuffer(GL30.GL_COLOR_ATTACHMENT0);
    //indicate that we will always render to color attachment 0
    return frameBuffer;
}

private int createTextureAttachment( int width, int height) {
    int texture = GL11.glGenTextures();
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, texture);
    GL11.glTexImage2D(GL11.GL_TEXTURE_2D, 0, GL11.GL_RGB, width, height,
            0, GL11.GL_RGB, GL11.GL_UNSIGNED_BYTE, (ByteBuffer) null);
    GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MAG_FILTER, GL11.GL_LINEAR);
    GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MIN_FILTER, GL11.GL_LINEAR);
    GL32.glFramebufferTexture(GL30.GL_FRAMEBUFFER, GL30.GL_COLOR_ATTACHMENT0,
            texture, 0);
    return texture;
}

private int createDepthTextureAttachment(int width, int height){
    int texture = GL11.glGenTextures();
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, texture);
    GL11.glTexImage2D(GL11.GL_TEXTURE_2D, 0, GL14.GL_DEPTH_COMPONENT32, width, height, 0, GL11.GL_DEPTH_COMPONENT, GL11.GL_FLOAT, (ByteBuffer) null);
    GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MAG_FILTER, GL11.GL_LINEAR);
    GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MIN_FILTER, GL11.GL_LINEAR);
    GL32.glFramebufferTexture(GL30.GL_FRAMEBUFFER, GL30.GL_DEPTH_ATTACHMENT, texture, 0);
    return texture;
}

private int createDepthBufferAttachment(int width, int height) {
    int depthBuffer = GL30.glGenRenderbuffers();
    GL30.glBindRenderbuffer(GL30.GL_RENDERBUFFER, depthBuffer);
    GL30.glRenderbufferStorage(GL30.GL_RENDERBUFFER, GL11.GL_DEPTH_COMPONENT, width,
            height);
    GL30.glFramebufferRenderbuffer(GL30.GL_FRAMEBUFFER, GL30.GL_DEPTH_ATTACHMENT,
            GL30.GL_RENDERBUFFER, depthBuffer);
    return depthBuffer;
}
}

FrameBufferShader.java

package postprocessing;

import shaders.ShaderProgram;

public class FrameBufferShader extends ShaderProgram {

private static final String VERTEX_FILE = "src/postprocessing/vertexShader.txt";
private static final String FRAGMENT_FILE = "src/postprocessing/fragmentShader.txt";

private int location_texture;
private int location_depthMap;

public FrameBufferShader()
{
    super(VERTEX_FILE, FRAGMENT_FILE);
}

@Override
protected void getAllUniformLocations()
{
    bindAttributes(0, "position");
}

@Override
protected void bindAttributes()
{
    location_texture = super.getUniformLocation("screenTexture");
    location_depthMap = super.getUniformLocation("depthMap");
}

public void connectTextureUnits()
{
    super.loadInt(location_texture, 0);
    super.loadInt(location_depthMap, 1);
}
}

FrameBufferRenderer.java

package postprocessing;

import org.lwjgl.opengl.GL11;
import org.lwjgl.opengl.GL13;
import org.lwjgl.opengl.GL20;
import org.lwjgl.opengl.GL30;

import models.RawModel;
import renderEngine.Loader;

public class FrameBufferRenderer {

private RawModel quad;
private FrameBufferShader shader;
private FrameBuffer fbo;

public FrameBufferRenderer(Loader loader, FrameBufferShader shader, FrameBuffer fbo)
{
    this.fbo = fbo;
    this.shader = shader;
    setUpVAO(loader);
    shader.start();
    shader.connectTextureUnits();
    shader.stop();
}

public void render()
{
    prepareRender();
    GL11.glDrawArrays(GL11.GL_TRIANGLES, 0, quad.getVertexCount());
    unbind();
}

private void prepareRender()
{
    shader.start();
    GL30.glBindVertexArray(quad.getVaoID());
    GL20.glEnableVertexAttribArray(0);
    GL13.glActiveTexture(GL13.GL_TEXTURE0);
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, fbo.getTexture());
    GL13.glActiveTexture(GL13.GL_TEXTURE1);
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, fbo.getDepthTexture());

    GL11.glEnable(GL11.GL_BLEND);
    GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
}

private void unbind()
{
    GL11.glDisable(GL11.GL_BLEND);
    GL20.glDisableVertexAttribArray(0);
    GL30.glBindVertexArray(0);
    shader.stop();
}

private void setUpVAO(Loader loader) {
    float[] vertices = { -1, -1, 1, -1, 1, 1, -1, 1 };
    quad = loader.loadtoVAO(vertices, 2);
}
}

Vertex Shader

#version 400

in vec2 position;

void main()
{
    gl_Position = vec4(position.x, position.y, 0.0, 1.0);
}

Fragment Shader

#version 400

in vec2 textureCoords;

out vec4 outColor;

uniform sampler2D screenTexture;
uniform sampler2D depthTexture;

void main()
{
    vec4 textureColour = texture(screenTexture, textureCoords);

    outColor = vec4(1.0, 1.0, 1.0, 1.0);
}

Answer 1

You can't sample the destination texture in the shader.

What you can do is make a new texture and make that the destination texture for the previous step and use it as a normal texture in the current step.