LWJGL - 3D Mouse picking

Newbie & Debugging Questions
#1

I’ve been trying to figure out how to unproject top-left screen coords to a ray (origin and direction vector) in world coords.

But I can’t crack it. Does anyone have any good references?

Preferably using modern OpenGL

#2

Hi,

what you need to do is apply the inverse of the world-to-NDC transformation to your normalized-device-coordinates (NDC) space.
You therefore have to have the projection and view matrices of your scene, which OpenGL uses to transform a vertex from world-to-NDC space.
Once you have these, you need to invert the combined view-projection matrix to obtain the reverse transformation.
After that, you apply that transformation to the corner coordinates of your NDC space, which are -1 and +1 in both X and Y. Z can be 0.0 and W must be 1.

You might also have a look at the ray tracing tutorial in LWJGL: http://blog.lwjgl.org/ray-tracing-with-opengl-compute-shaders-part-i/

This tutorial uses modern OpenGL (OpenGL 4.3 Compute Shaders actually, also with a backport to OpenGL 3.0 GPGPU with fragment shaders) and uses NDC-to-world transformations to generate the rays for each framebuffer pixel, based on a pinhole camera model.

Cheers,
Kai

#3

Thanks this looks like a great resource.

#4

For your concrete problem, you do not have to read it all, though.
Especially the first 2/3 of the article can be a bit long-winded, if you are not so much into ray tracing, history and problems of rendering in general… :slight_smile:

You can jump right into the section saying “Setting up the camera”.
Or better, have a look at the source code whose references are found in hyperlinks beginning with “accompanying …”.

#5

Oh wow Display got replaced by GLFW. Interesting.

Time to update!

#6

Okay after browsing and stuff I was able to put this together from snippets people left around:


public Ray getPickRay(float mouseX, float mouseY) {
        float screenSpaceX = ((2f * mouseX) / GameConstants.width - 1f) * aspectRatio;
        float screenSpaceY = ((-2f * mouseY) / GameConstants.height + 1);

        float viewRatio = Mathf.tan(Mathf.toRadians(fov) / 2f);

        screenSpaceX *= viewRatio;
        screenSpaceY *= viewRatio;

        Vector3f cameraSpaceNear = new Vector3f(screenSpaceX * zNear, screenSpaceY * zNear, -zNear);
        Vector3f cameraSpaceFar = new Vector3f(screenSpaceX * zFar, screenSpaceY * zFar, -zFar);

        Matrix4f tmpView = new Matrix4f(viewMatrix);
        Matrix4f invView = tmpView.invert();

        Vector3f worldSpaceNear = new Vector3f();
        Matrix4f.transform(invView, cameraSpaceNear, worldSpaceNear);

        Vector3f worldSpaceFar = new Vector3f();
        Matrix4f.transform(invView, cameraSpaceFar, worldSpaceFar);

        return ray.set(
                new Vector3f(worldSpaceNear.x, worldSpaceNear.y, worldSpaceNear.z),
                new Vector3f(
                        worldSpaceFar.x - worldSpaceNear.x,
                        worldSpaceFar.y - worldSpaceNear.y,
                        worldSpaceFar.z - worldSpaceNear.z
                ).normalize()
        );
    }

This works fine but I’m trying to make it work using my inverse projViewMatrix. For learning purposes.

I know it’s possible I just can’t find the right way.

Any ideas?