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I have a plane in 3D but would like to add a GRID on top somehow.

What I would like to do is to somehow divide my plane into squares so that if I click on a certain location on my plane I will know on what area of the GRID I have clicked.

You would need to create a world-space ray by unprojecting the normalized device coordinates of the mouse click position and then do a ray-plane intersection. Assuming an intersection occurs, all you would need to do is then transform this world space intersection point into the area encompassed by your ground plane grid and then do what you want from there.

I know how to do a ray intersept from the mouse click coordinates to world coordinates.

The problem I don't get is how can I check that it intersept a certain area on my plane?

Imagine a chess board. if I click on the top left corner of the board I would want the top left square to either change color or at least that I will be notified that, the square has been clicked on.

Any example or more detailed explanation of how this is done??

How could I get the Model object size in the world, or location so that I can know how big my grid should be.

he problem I don't get is how can I check that it intersept a certain area on my plane?

You need 2 extra direction vectors + a position to define a coordinate frame. As an example, consider the grid centred at position (0,1,0) - let's call that 'W', with the plane normal also pointing up in 'Y' (0,1,0). We need to define the 2 directions for your plane (i.e. the +X, and +Z directions).

Compute your intersection point on the plane.

IP = getIntersectPointOnPlane(plane, selection_ray);

Make the intersect point relative to the grid centre, i.e.

IP_relative = IP - W

Now compute the X and Z coordinates using a dot product.

Another option, since you're going to want to color/texture the chosen area, is to break the plane into many individual triangles/quads, and then you wouldn't need to know where on a triangle(or the plane as a whole) the ray intersects; just which triangle, and color/texture it (and it's neighbor) accordingly.

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