LWJGL 3 glReadPixles always reports 255 255 255 255

[Moocow9m]

I have colored pixel 1,1 with 0.0f 0.0f 1.0f 1.0f (blue) and i got the same response as white. Can someone tell me what I did wrong?
Code:
        GL11.glReadBuffer(GL11.GL_FRONT);
        ByteBuffer rgb = BufferUtils.createByteBuffer(3);
        glReadPixels(X, Y, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, rgb);
        System.out.println((rgb.get(0) & 0xFF) + ", " + (rgb.get(1) & 0xFF) + ", " + (rgb.get(2) & 0xFF));

[Kai]

Can you show some more of your code, where you render things?
Also keep in mind that when using double-buffering then the GL_BACK buffer is the one being rendered to, before you call glfwSwapBuffers().
Also, are you maybe rendering to a Framebuffer Object?

[Moocow9m]

Code: (whole loop beware)

    private void loop() {
        GL.createCapabilities();
        glClearColor(1.0f, 1.0f, 1.0f, 1.0f);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        glOrtho(0, 600, 600, 0, 1, -1);
        glMatrixMode(GL_MODELVIEW);
        ArrayList<Shape> shapes = new ArrayList<>();
        Shape shape = new Shape();
        Color color = new Color();
        color.setColor(0.0f, 0.0f, 0.0f, 1.0f);
        shape.setShape(7, 20.0f, 15.0f, 100.0f, 100.0f, color);
        shapes.add(shape);
        Points temp;
        square.setUp();
        while (glfwWindowShouldClose(window) == GLFW_FALSE) {
            sync(60);
            frameTrack++;
            glfwPollEvents();
            glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
            glClear(GL_COLOR_BUFFER_BIT);
            switch (togglePage) {
                case "login":
                    drawShape(shapes);
                    if (usernameOn) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Username.draw();
                    if (connectError != null) {
                        glColor4f(1.0f, 0.0f, 0.0f, 1.0f);
                        temp = drawString(connectError, 260, 260);
                        temp.draw();
                    }
                    glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    login.draw();
                    temp = drawString(username, 260, 300);
                    temp.draw();
                    if (passwordOn) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Password.draw();
                    glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    String passwordTemp = "";
                    for (int i = 0; i < password.length(); i++) {
                        passwordTemp = passwordTemp + "*";
                    }
                    temp = drawString(passwordTemp, 260, 340);
                    temp.draw();
                    if ((mouseX >= 251 && mouseX <= 350) && mouseY >= 348 && mouseY <= 366) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Login.draw();
                    glColor3f(0.0f, 0.0f, 1.0f);
                    GL11.glBegin(GL11.GL_POINTS);
                    GL11.glVertex2f(1, 1);
                    GL11.glEnd();
                    new boardCreate().read(1, 1);
                    break;
                case "conectionInfo":
                    if ((mouseX >= 251 && mouseX <= 350) && mouseY >= 348 && mouseY <= 366) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Login.draw();
                    glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    conectionInfo.draw();

                    if (hostOn) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Username.draw();
                    glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    temp = drawString(host, 260, 300);
                    temp.draw();
                    if (portOn) {
                        glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    } else {
                        glColor4f(0.0f, 0.0f, 0.0f, 1.0f);
                    }
                    Password.draw();
                    temp = drawString(Port + "", 260, 340);
                    temp.draw();
                    break;
                default:
                    glColor4f(0.0f, 0.0f, 1.0f, 1.0f);
                    square.draw();
                    break;
            }
            glfwSwapBuffers(window);
            new boardCreate().read(1, 1);
        }

[Kai]

And when/where do you make the glReadPixels call there? I mean it's clearly your glClearColor which you read.
Also beware that glReadPixels takes "window coordinates". They have their Y axis point downwards.
So the y co-ordinate 0 is at the top of the window. <-- wrong. See further post.

[Moocow9m]

 new boardCreate().read(1, 1);
And I know about the 0,0 being in the top corner. The whole thing is a 600x600 window.

[Kai]

Okay, that looks about right then. The only thing I can think of now is that the pixel (1, 1) (the second pixel along +X and the second along +Y) does not contain that color, but the clearcolor. Are you absolutely sure that your rendered output at pixel (1, 1) is not the clear color (white)? Can you maybe post an image of the render result?

[Moocow9m]

glColor3f(0.0f, 0.0f, 1.0f);
                    GL11.glBegin(GL11.GL_POINTS);
                    GL11.glVertex2f(1, 1);
                    GL11.glEnd();
In code and I will send a pic in ~8min.

[Kai]

Okay, I was wrong. Window coordinates in OpenGL have their origin at the bottom-left...
So you could either switch the top and bottom arguments of glOrtho or use glReadPixels(1, height-2, ...)
Also noted here: https://www.opengl.org/sdk/docs/man/html/glReadPixels.xhtml

x, y
Specify the window coordinates of the first pixel that is read from the frame buffer. This location is the lower left corner of a rectangular block of pixels.

[Moocow9m]

Here (I cropped it to make it easier to see)

[Moocow9m]

Okay, I was wrong. Window coordinates in OpenGL have their origin at the bottom-left...
So you could either switch the top and bottom arguments of glOrtho or use glReadPixels(1, height-2, ...)
Also noted here: https://www.opengl.org/sdk/docs/man/html/glReadPixels.xhtml

x, y
Specify the window coordinates of the first pixel that is read from the frame buffer. This location is the lower left corner of a rectangular block of pixels.

The project relies on 0,0 being top-Left though... And it works...

[Kai]

The project relies on 0,0 being top-Left though...

or use glReadPixels(1, height-2, ...)

This is also possible, if supported by your driver: https://www.opengl.org/registry/specs/ARB/clip_control.txt

[Moocow9m]

The project relies on 0,0 being top-Left though...

or use glReadPixels(1, height-2, ...)

This is also possible, if supported by your driver: https://www.opengl.org/registry/specs/ARB/clip_control.txt

glReadPixels(X, Y - 2, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, rgb); gave 0,0,0...

[Kai]

glReadPixels(X, Y - 2, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, rgb); gave 0,0,0...

Not Y - 2 .... height-2, where height is the height in pixels of your viewport.

Generally: your glReadPixels(X, Y, ...) becomes: glReadPixels(X, height-Y-1, ...)

[Moocow9m]

glReadPixels(X, Y - 2, 1, 1, GL_RGB, GL_UNSIGNED_BYTE, rgb); gave 0,0,0...

Not Y - 2 .... height-2, where height is the height in pixels of your viewport.

Generally: your glReadPixels(X, Y, ...) becomes: glReadPixels(X, height-Y-1, ...)

height-Y-1 == 1-1-1.... They all equal 1 in this situation and -3 is not displayed....

[Kai]

height-Y-1 == 1-1-1....

No. "height" is the height of your viewport/render area. Your window is NOT 1 pixel in height. According to your code it is 600 Pixels.
So please understand that glReadPixels(1, 1, ...) translates to glReadPixels(1, height-1-1, ...) = glReadPixels(1, height-2) = glReadPixels(1, 600-2,...) = glReadPixels(1, 598, ...)