I'm making a windows forms application with opengl view and I'm using c#, openTK . I need to get the mouse coords converted to the opengl world coords. Well, my Y coord gets converted wrong. It's hard to explain, so here is the video: http://tinypic.com/r/23sal8k/6. I'm constantly pressing left mouse button and the red dot should be where the mouse is(but it isn't). How to fix this? Here is the code:

       private void glview_MouseDown(object sender, System.Windows.Forms.MouseEventArgs e)
        {
            if (e.Button == MouseButtons.Left)
            {
                Point worldCoords = convertScreenToWorldCoords(e.X, e.Y);
                shitx = worldCoords.X;
                shity = worldCoords.Y;
            }
        }




        // functions:
        public static Point convertScreenToWorldCoords(int x, int y)
        {
            int[] viewport = new int[4];
            Matrix4 modelViewMatrix, projectionMatrix;
            GL.GetFloat(GetPName.ModelviewMatrix, out modelViewMatrix);
            GL.GetFloat(GetPName.ProjectionMatrix, out projectionMatrix);
            GL.GetInteger(GetPName.Viewport, viewport);
            Vector2 mouse;
            mouse.X = x;
            mouse.Y = viewport[3] - y;
            Vector4 vector = UnProject(ref projectionMatrix, modelViewMatrix, new Size(viewport[2], viewport[3]), mouse);
            Point coords = new Point((int)vector.X, (int)vector.Y);
            return coords;
        }
        public static Vector4 UnProject(ref Matrix4 projection, Matrix4 view, Size viewport, Vector2 mouse)
        {
            Vector4 vec;


            vec.X = 2.0f * mouse.X / (float)viewport.Width - 1;
            vec.Y = -(2.0f * mouse.Y / (float)viewport.Height - 1);
            vec.Z = 0;
            vec.W = 1.0f;


            Matrix4 viewInv = Matrix4.Invert(view);
            Matrix4 projInv = Matrix4.Invert(projection);


            Vector4.Transform(ref vec, ref projInv, out vec);
            Vector4.Transform(ref vec, ref viewInv, out vec);


            if (vec.W > float.Epsilon || vec.W < float.Epsilon)
            {
                vec.X /= vec.W;
                vec.Y /= vec.W;
                vec.Z /= vec.W;
            }


            return vec;
        }
 

Thanks for reply. Strange but it doesn't work. Using both methods the dot just shows at a bigger distance of mouse:

and I can't do modelview[13] = -modelview[13] because modelview variable is Matrix4, not array of integers or something else...

I just noticed one thing, it's hard to explain in words, so here it is :

I actually understood what you meant by inverting one of the component of matrix but I don't know how to do that in opentk because modelViewMatrix variable is a Matrix4 struct. This doesn't change anything:

modelViewMatrix.M13 = -modelViewMatrix.M13;

I removed that minus in UnProject function, so now:

and if I change

mouse.Y = viewport[3] - y;

to

mouse.Y = viewport[3] - y - 130;

then it gets converted almost fine. I've implemented zooming, so when I zoom in the view, I can see that X coord isn't very accurate too.

maybe there is another way to convert that?

Also, I forgot to mention that my application is 2D if that makes any sense.

EDIT: it's also possible that unproject function is wrong, I took it from opentk forum written by someone... I see learning opengl is really difficult .

Finally, I got this fixed. I just changed

mouse.Y = viewport[3] - y;

to

mouse.Y = y + (ClientRectangle.Height - glview.Size.Height);

still not very accurate but not very bad.

I have implemented zooming, so when I zoom in I see that it isn't very accurate(sometimes dot gets in a right place, sometimes not):

When I zoom out, I see it's accurate enough. This is how I implemented zooming:

        // zoom in: zoom *= 2;
        // zoom out: zoom /= 2;
        public void UpdateCameraZoom()
        {
            Matrix4 projection = Matrix4.CreateOrthographic(ClientRectangle.Width * 5 / zoom, ClientRectangle.Height * 5 / zoom, 0.0f, 64.0f);
            GL.MatrixMode(MatrixMode.Projection);
            GL.LoadMatrix(ref projection);
        }

What I did to fix this, I just increased the width and height of projection volume(as you can see I multiply by 5) and increased other objects size(so they won't be so small) :).