Matt thank you so much. I'll give this a try soon!
On Tue, Mar 24, 2015 at 2:50 PM, Matt Lind <[email protected]> wrote: > You just need to pull the right vectors from mental ray and do the > computation yourself, which isn't hard. > > You need 3 pieces of data to perform the computation: > - intersection point (state->pnt) > - light position > - camera position (state->org) > > The intersection point is the point on the surface being shaded. Mental ray > refers to this as state->pnt (or state->point). You can access this value > using a state shader. It'll have to be used in a rendertree applied as a > material or texture on a surface as state->point doesn't exist for some > other types of shaders. > > The viewing angle is the vector formed by subtracting the intersection point > from the camera position. Mental ray calls this state->dir (viewing > direction), which you might be able to obtain using one of the state > shaders, but if you do, you'll need to negate it as state->dir points from > camera to surface. You need the opposite direction so the vector is aligned > from surface to camera to match the orientation of the other vector you'll > be computing. > > The light to surface vector is formed by subtracting the intersection point > from the light position. In code this is trivial, but in the rendertree > it's a bit cumbersome for lack of nodes to isolate information from a single > light. Therefore, you'll have to plug your light positions into the tree > explicitly. One node to represent the position of each light you want to > consider for the effect. > > When you have both vectors, normalize them to be unit vectors. Next, do a > dot product. The result is the cosine of the angle between the two vectors. > This is ratio you seek to drive the retro-reflective intensity. However, > you will likely need to clamp or rescale this value to fit a range you > choose for artistic reasons. For example, the cosine will cover the full > [0...180] degree range between the two vectors meaning you'll get a linear > increase/decrease of intensity as you step through the entire range. > However, as Peter pointed out, you don't want that. You want the intense > response to be isolated to a very narrow angular range when the two vectors > are very closely aligned to indicate the viewing angle is very similar to > the reflection angle. So take the cosine and pump it into a rescale node > and redefine your output range to be narrow and you'll get the effect you're > looking for (eg; from [0...1] to [0...0.05]). if you don't use a rescale > node, you can do the simple arithmetic yourself with a node or two. > > one speed bump you may possibly run into is getting all the positions in the > same coordinate space before you do the computations. Mental ray defines > values in 'internal space' which changes depending on the context which the > shader is evaluated. Usually it means world space, but in the case of > state->pnt it might mean object space. If you run into this problem, insert > a coordinate conversion node between the position vector and it's output > targets. > > Matt > > > > > > Date: Tue, 24 Mar 2015 11:16:59 -0500 > From: Patrick Neese <[email protected]> > Subject: Retro reflective Materials? > To: [email protected] > > > I'm looking for something like cat eyes or road signs. Where the > reflection of the light source is affected by angle between the light > to surface to camera. It should also take into account the light > color. If a cat turns it's eyes away the intensity/reflection changes > and/or if the camera shifts off axis from the surface or the light > shifts off axis the intensity/reflection also changes. > > So, I've experimented with some Incidence nodes with some crazy > mixing...but it isn't exactly what I would like. > > Does anyone know of a way to get a proper retro-reflective material? > Did I miss something in some of the shaders or a shader itself? One of > the BRDF settings? > > Thank you for your time. > > Patrick >
