This tutorial demonstrates how one can do some surface analysis or form finding based on relflections. The attached file uses the resulting angle for color gradient analysis, but you could use it to cull out objects.
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The reflection issue or request comes up a lot during our workshops. How do you calculate the reflection of light or sound. Rhino has the "Bounce" command built in, which accomplishes this, however, to date , grasshopper does not have a native component to do this.
Lets start by understanding what is required in order to calculate the reflections. Firstly, it all comes down to angles and so we will be working with vectors inside of GH. Lets start by creating a surface from which we will calculate the reflections. Create Profiles and Loft them so that you have a Surface to work with inside GH.
You should have the lofted surface referenced inside GH. First we need to create a source point for the light or sound source. So create and reference a point from rhino.
Next , we need to decide how many test samples / rays we want on the surface. To do this we subdivide the surface with the "Divide Surface" component. Your reference surface should be the ofted surface we created before. You should now have something that looks similar to the images below. Green points are the subdivisions of the surface and the red is our source point.
Next , we need to create lines to intersect with the surface. This is not the conventional way to find surface vectors/normals mind you. However , it will benifit you in the long run as this definition will use any lines/rays that hit a surface to calculate the reflections.
Create lines using the "2 Point Line" component, by plugging in our source point into A and our surface points into B. Make sure that you flatting the points coming from the "Surface Divide" component. Now use the "Surface|Line Intersection" component to get the intersection points of the surface and our rays/lines.
Here is where we deal with the vector calculations between the surface and the rays. Create vectors in the same way that you created the lines using "Vector 2 Point" component and like wise make sure A and B are plugged in respectively. Now we want to reverse our vectors so that they are pointing back at us. Plug these new vectors in a "reverse vectors" component.
This next part might lose those who are not familiar with functions/expressions. We want to rotate the vectors we created by 180 degrees around the surface normal so that it mimics an angle of reflection.
In order to do this we use the "Vector Rotate" component. Plug your reversed vectors into 'V' and graft the input (right click V and choose 'Graft'). Next, for our rotation vector use the Normal from our Line/Surface Intersection component and plug N into X of the "Vector Rotate" component. Now for the angle we want 180 degrees in Radians, which is the same as PI. If you do not know how to use the "Function" component, just plug in 3.141593 which is basically PI.
In order to actually see the vectors, use a "Vector Display" component as I have done.
The rest of the definition colours the reflection angles based on the intensity of the reflection, it also creates arrows and lines that can be colored. I wont go into that for this tutorial, but it is included in the final file available for download below. The angle intensity could be used to cull objects or panels and the red and yellow gradient starts to show how one can do some surface analysis or form finding based on relflections during the design process.
The definition is set up with some controls for those of you wanting to use this as a plug'n play definition.
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