Ray Tracing

[cherm]

Hi there,

The following may sound studpid to you guys, so please bear with me. I am quited new to Houdini. I am now doing a render test with my simple scene. The problem is I just couldn't get the ray tracing to work. In my scene, I have a sphere and a grid, and i want them to reflect each other. Both of them have a phong shader on (VEX Layered). And I did turned on Ray trace in Mantra, but they didn't reflect at all. Can any of you lead me the way? Thank you very much.

CHERM

[Jason]

Mantra will ray-trace reflections with you having to turn on raytracing, by the way. The "mantra -a" option to ray trace just means that it'll also raytrace the primary ray.

Try using a VEX Metal Shader; does that work for you? You can build your own reflective materials in VOPs too if you use a Lighting Model VOP and add the result to a Reflected Light VOP and wire the result into the Cf Output Variable. This should get you out of the gate:)

Good luck and write back with your experiences.

Jason

[cherm]

Mantra will ray-trace reflections with you having to turn on raytracing, by the way. The "mantra -a" option to ray trace just means that it'll also raytrace the primary ray.

Try using a VEX Metal Shader; does that work for you? You can build your own reflective materials in VOPs too if you use a Lighting Model VOP and add the result to a Reflected Light VOP and wire the result into the Cf Output Variable. This should get you out of the gate:)

Good luck and write back with your experiences.

Jason

Thanks Jason for bearing with me. I am so new to Houdini.

I did tried the vex metal, and it gave me the reflection. What if I have my sphere with a glass shader on and have my grid with my texture on, how can I get the reflection on the grid? I guess it is quite different from Maya. In Maya, when I apply just a phong shader to my objects and turn on Ray Trace, it gives me reflection right away. I thought this would work the same in Houdini. Or maybe it does, but I just don't know how. By the way, in Mantra, I increased the detail of Ray tracing, but nothing happened. Do you know why?

Thanks again

Cherm

[stevenong]

Hi Cherm,

Turning on Raytracing does not necessarily mean you'll get reflections. To have reflections, you have to use a shader with the Reflection function built-in. The few shaders that have it are VEX Layered Surface, VEX Super Material, VEX Metal and VEX Glass. I noticed the default settings of the VEX Layered Surface does not give you an obvious reflection so please look at my settings for your own tests.

I've attached a file which gives the attached image. Hope it will help you to discover more about shading & rendering in Houdini. It might feel a bit slow rendering because I made the light an area light. Please feel free to turn it off.

Have fun!

Cheers!

steven

reflections.hip

[cherm]

Hi Cherm,

Turning on Raytracing does not necessarily mean you'll get reflections. To have reflections, you have to use a shader with the Reflection function built-in. The few shaders that have it are VEX Layered Surface, VEX Super Material, VEX Metal and VEX Glass. I noticed the default settings of the VEX Layered Surface does not give you an obvious reflection so please look at my settings for your own tests.

I've attached a file which gives the attached image. Hope it will help you to discover more about shading & rendering in Houdini. It might feel a bit slow rendering because I made the light an area light. Please feel free to turn it off.

Have fun!

Cheers!

steven

thanks Steven

you saved my life. Correct me if I'm wrong....One thing I noticed is that in the VEX Layered Surface, if I unticked Apply Fresnel Reflections off the reflection will appear. May I ask why? And I have two other stupid questions to ask. One is what is ^ for? I saw you put it in the reflection mask, and the second is what is Ray Bias in the glass shader for?

cheers again

Cherm

[symek]

Starting from the beggining:

What you are facing here is that mantra is a programmable rendering engine,. This means that it does precisely what you are asking it for. Your tube directing mantra is a shader. Shader tell mantra how to treat a surface. If you start creating your own shaders (in VOPs its really simply) you will understand why mantra sometimes render reflection and sometimes not. Fresnel Reflection for example is, as I suppose, just these few rays which are reflected back to light source when rays hit a surface on some angle. In other words this shader behaves like a writer wanted to but not like Houdini treats reflections in general. You got what I'm saying? Shader can be written in arbitrary way what can be confusing for someone (like me some time ago) who get used to identifying in a wrong fashion shaders and surface qualities. They are not equal. Just in some app. there are general shaders applied to any surface.

As to you second question "^" is one of a few signs belonging to filtering expressions and means "except this"

* ^light1 = all except light1

cheers,

sy.

Edited May 16, 2007 by SYmek

[sibarrick]

Fresnel reflections make the reflection stronger at more glancing angles.

Take a close look at a glass and you should see that there are stronger reflections where the glass curves away from you and weaker reflections where you can look straight through it.

[cherm]

billion thanks to you all

One last question....I heard that the VEX Glass shader in Houdini is busted. Is this true? I have problems myself. I just couldn't get any reflection from my environment sphere which has a texture map on.

cheers

Cherm

[symek]

billion thanks to you all

One last question....I heard that the VEX Glass shader in Houdini is busted. Is this true? I have problems myself. I just couldn't get any reflection from my environment sphere which has a texture map on.

cheers

Cherm

well, you know, it is not so bad... . Houdni's default glass shader is just not so phisicly accurate and works on an simpliest model of glass. But for sure it supports enviromental reflection. Did my self many times. If you'd like to use evnmap, just use it in shader (Map Tab). One of many choices. Using normal geometry with texture should work also. Do you have any lights there (i.e. inside a sphere)? If not you have no ray to trace ...

cheers,

sy.