Chapter 9: Cannot implement plane

krotus
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Posts: 15

Chapter 9: Cannot implement plane Apr 5, 2020 23:10:23 GMT

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Post by krotus on Apr 5, 2020 23:10:23 GMT

When I started this project, I knew that there would be more than spheres so I created a Primitive class that other shapes would inherit. One of the things that happens when a new shape is created is that it's given a numerical code to identify what kind of shape it is, and a unique integer ID, starting with 0 (for each kind of shape) and ascending from there. This code is working for any kind of shape.

But I'm having a problem implementing a plane.

All tests are passing up to "The normal of a plane is constant everywhere". (Page 122 of the book)

Here is the original NormalAt() functon from Chapter 6: (Page 82 of the book)

Public Shared Function NormalAt(MyShape As Primitive, MyWorldPoint As Tuple(Of Double, Double, Double, Double)) As Tuple(Of Double, Double, Double, Double)

        Dim result As Tuple(Of Double, Double, Double, Double)

        Dim ObjectPoint As Tuple(Of Double, Double, Double, Double) = MyMath.MultiplyMatrixByTuple(MyMath.GetInvertedMatrix(MyShape.Transformation), MyWorldPoint)
		
	'From page 86 of the book
        Dim Position As Tuple(Of Double, Double, Double, Double) = New Tuple(Of Double, Double, Double, Double)(0, 0, 0, 1)
		
        Dim ObjectNormal As Tuple(Of Double, Double, Double, Double) = MyMath.SubtractPointsAndVectors(ObjectPoint, Position)
        Dim WorldNormal As Tuple(Of Double, Double, Double, Double) = MyMath.MultiplyMatrixByTuple(MyMath.TransposeMatrix(MyMath.GetInvertedMatrix(MyShape.Transformation)), ObjectNormal)

        'Set the W of the WorldNormal vector to 0
        WorldNormal = New Tuple(Of Double, Double, Double, Double)(WorldNormal.Item1, WorldNormal.Item2, WorldNormal.Item3, 0)

        'Normalize the WorldNormal
        result = MyMath.NormalizeVector(WorldNormal)

        Return result

End Function

Now in Chapter 9, we're changing it: (Page 120 of the book)

Public Shared Function NormalAt(MyShape As Primitive, MyWorldPoint As Tuple(Of Double, Double, Double, Double)) As Tuple(Of Double, Double, Double, Double)

	Dim result As Tuple(Of Double, Double, Double, Double)

	Dim LocalPoint As Tuple(Of Double, Double, Double, Double) = MyMath.MultiplyMatrixByTuple(MyMath.GetInvertedMatrix(MyShape.Transformation), MyWorldPoint)
	Dim LocalNormal As Tuple(Of Double, Double, Double, Double)

	'Determine the primtive type
	Select Case MyShape.Code
		Case 0
			LocalNormal = Sphere.GetLocalNormalOfPointAt(MyShape, LocalPoint)
		Case 2
		LocalNormal = Plane.GetLocalNormalOfPointAt(MyShape, LocalPoint)
	End Select

	Dim WorldNormal As Tuple(Of Double, Double, Double, Double) = MyMath.MultiplyMatrixByTuple(MyMath.TransposeMatrix(MyMath.GetInvertedMatrix(MyShape.Transformation)), LocalNormal)

	'Reset the W of the WorldNormal vector to 0
	WorldNormal = New Tuple(Of Double, Double, Double, Double)(WorldNormal.Item1, WorldNormal.Item2, WorldNormal.Item3, 0)

	'Normalize the WorldNormal
	result = MyMath.NormalizeVector(WorldNormal)

	Return result

End Function

So the real difference is that the local normal is now created by the shape's LocalNormalAt() function. So what is this function supposed to do? According to the text, it "accepts a point in local (object) space, and returns the normal in the same space." How does it do that? We're passing it the shape and the local point. In the Chapter 6 version of NormalAt(), Position was subtracted from the ObjectPoint. In Chapter 9, ObjectPoint becomes LocalPoint. So if I pass the shape and LocalPoint to LocalNormalAt(), then I would be subtracting Position from LocalPoint. That works fine for a sphere since Position is point(0, 0, 0, 1) and the shape's position is also (0, 0, 0, 1):

Public Shared Function GetLocalNormalOfPointAt(MyShape As Sphere, MyLocalPoint As Tuple(Of Double, Double, Double, Double)) As Tuple(Of Double, Double, Double, Double)

        Dim result As Tuple(Of Double, Double, Double, Double)

        result = MyMath.SubtractPointsAndVectors(MyLocalPoint, MyShape.Position)

        'Set W to 0
        result = New Tuple(Of Double, Double, Double, Double)(result.Item1, result.Item2, result.Item3, 0)

        Return result

End Function

but with a plane, if I use Position as point(0, 0, 0, 1), the vector comes out vector(0, 0, 0).

The first test for Plane shows that the local normal of a local point (point(0, 0, 0, 1)) should be vector(0, 1, 0) but how am I supposed to get that if all that LocalNormalAt() does is subtract Position from LocalPoint? What do you use for Position?

Since the plane is infinite in X and Z and passes through the origin, then Y should always be 0. But if every point on a plane has a vector of vector(0, 1, 0), am I supposed to calculate a Position so that any local point on the plane has that vector of vector(0, 1, 0)?

Last Edit: Apr 6, 2020 16:56:38 GMT by krotus

jebbly
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Posts: 3

Chapter 9: Cannot implement plane Apr 7, 2020 20:12:59 GMT

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Post by jebbly on Apr 7, 2020 20:12:59 GMT

You want the "localNormal" function to be a virtual function that is overridden by a concrete subclass. Then, each shape can implement their own method of calculating their local normal - currently, the way you're calculating it is specific to spheres only. You would want your "localNormal" for Sphere to subtract Position from LocalPoint, but you would want the "localNormal" for Plane to always return vector(0, 1, 0) regardless of the point of intersection.

krotus
New Member

Posts: 15

Chapter 9: Cannot implement plane Apr 7, 2020 21:09:20 GMT

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Post by krotus on Apr 7, 2020 21:09:20 GMT

Apr 7, 2020 20:12:59 GMT jebbly said:

You want the "localNormal" function to be a virtual function that is overridden by a concrete subclass. Then, each shape can implement their own method of calculating their local normal - currently, the way you're calculating it is specific to spheres only. You would want your "localNormal" for Sphere to subtract Position from LocalPoint, but you would want the "localNormal" for Plane to always return vector(0, 1, 0) regardless of the point of intersection.

Actually, if you look at the second code block, I'm determining shape by the shape's code and calling the GetLocalNormalOfPointAt() function for that particular shape.

So, your answer seems to answer my last question, that I should be forcing GetLocalNormalOfPointAt() to return vector(0, 1, 0). If that is the case, then if I implement this:

    Public Shared Function GetLocalNormalOfPointAt(MyShape As Primitive, MyLocalPoint As Tuple(Of Double, Double, Double, Double)) As Tuple(Of Double, Double, Double, Double)

        Dim result As Tuple(Of Double, Double, Double, Double)
        Dim MyPoint As Tuple(Of Double, Double, Double, Double) = New Tuple(Of Double, Double, Double, Double)(MyLocalPoint.Item1, -1, MyLocalPoint.Item3, 1)

        result = MyMath.SubtractPointsAndVectors(MyLocalPoint, MyPoint)

        'Set W to 0
        result = New Tuple(Of Double, Double, Double, Double)(result.Item1, result.Item2, result.Item3, 0)

        Return result

    End Function

I get a return of vector(0, 1, 0). However, the plane still does not render. That is with the use of the scene settings from Chapters 7 & 8 minus the spheres used as a floor and walls and the addition of a plane.

Last Edit: Apr 7, 2020 21:09:58 GMT by krotus

jebbly New Member Posts: 3	Chapter 9: Cannot implement plane Apr 8, 2020 4:49:36 GMT Quote Select Post Deselect Post Link to Post Member Give Gift Back to Top Post by jebbly on Apr 8, 2020 4:49:36 GMT Ah sorry, I think I missed that part. If the plane is not rendering at all, you may want to look into the intersection logic. Are those tests still passing?

Post by krotus on Apr 5, 2020 23:10:23 GMT

Post by jebbly on Apr 7, 2020 20:12:59 GMT

Post by krotus on Apr 7, 2020 21:09:20 GMT

Post by jebbly on Apr 8, 2020 4:49:36 GMT