Rendering with Hidden Geometry in Rhino & VRay


Section Perspective.jpg

This workflow shows a simple method for rendering 3d models to show cut sections or plans while maintaining true lighting. We will utilize specific tools in Rhino to split our model, than create transparent materials in VRay to "hide" our cut-away model while leaving its ability to block light.
Uses Tool(s) Rhinoceros , VRay

Rendering Cut Sections

A rendered section or plan can be a great way to show how light is present in your design, but they can easily show a completely different understanding without proper execution. It's important to remember that when cutting away at your model for the purpose of rendering sections that the pieces you hide to show the interior of your design still block light - turning them off or deleting them will allow light to enter your section on a way that is untrue to the actual design. This workflow will show you had to create your rendered sections correctly.

Before You Begin

Make sure your lighting is set up as you'd like it, so that you know what to expect when your rendering is complete. You can alter your lighting at any time throughout this process, but it would be better to understand how light is supposed to enter your model so that you can place your section cuts properly.

Clean up your model as needed. Any surfaces that are unnecessarily complex or bad surfaces in general will cause the sectioning of the model to take longer than it should or to fail entirely, and may cause any renderings to come out with undesired effects.


Step 1: Prepare the Section Cut

Whether you're creating a rendered plan or a rendered section, you'll need to create a cut plane to split your model. You can begin by orienting your c-Plane to a face of design that is parallel with the cut you wish to make - for a section cut, this would be any of the elevations; for a plan, it would be the top or bottom planes. Type the Plane command and draw your plane on the newly oriented c-Plane, then move it or rotate it to the desired location for the cut. Make sure the plan extends through every part of the design that needs to be cut - extend beyond the boundary of the design just to be sure. If the plane doesn't cut through any selected object entirely, you will be unable to properly produce your section.

If you have a non-orthogonal design or a cut that is at a non-orthogonal angle, another option would be to draw the plane in whichever direction you need it and projecting the plane. Orienting the c-plane is a step to save time for simpler models.

Step 2: Cut the Model

Select all the objects that wil be cut by this section and type in the ' Split command, and choose the newly created plane as your cutting plane. Depending on the complexity of your model this operation could take some time, but if you cleaned up your model before beginning and strategically placed your cutting plane, it should take no more than a few seconds to a few minutes. Once the operation is completed, you can delete the plane you used to create the cut or place it into a new layer and turn it off.

You should now have your building split entirely into two masses. Next, you should cap any objects that were cut by the cutting plane that you want to see in the rendering, such as the floor or walls, so that they render as solid objects in the final image. Select the objects that were cut and type in the Cap command - but do not cap the objects that you wish to hide later. You may need to move the objects away from each other in order to complete this step; if so, make sure to move them back to their original positions once you've capped the section.

Step 3: "Hide" the Unseen

Now we'll need make the sectioned mass of our building disappear but still block light as if it were there. To do that, we need to create a new VRay material that is both transparent to the camera but opaque to any lighting.

In the VRay Materials dialog, create a new material and title it in a way that you'll know it's unique from all other layers - such as "hidden section" or "section material" - and add a Refraction layer to the already default Diffuse layer by expanding the VRay material and right clicking on the Refraction layer and clicking Add new layer . In the Diffuse layer, make the diffuse color black (value of 0) and the transparency color white (value of 255). In the Refraction layer, the refraction color should maintain its default of white and the transparency its default of black, but change the IOR to 1.

Apply the material to the desired objects by selecting the objects then right clicking the new material and clicking Apply material to object(s) .

Step 4: Render

Now you're ready to render. Your final render may take more manipulation in Photoshop, but all of your lighting in the interior of the model will be accurate.

Common Problems

  • My section is still letting light through as if the rest of the model weren't there - what happened?
    • You may have set the wrong settings on the VRay material. Go back to Step 3 and ensure that you did add a refraction layer and have the correct settings there. If the settings were correct, you may not have placed the sectioned objects back to their original positions after capping the section cut. If neither of these answers solve your issue, you may have bad geometry in the model that needs to be cleaned up.
  • The place where the section actually cuts through my model - at the floors, walls, etc. - is showing a weird texture when I render.
    • This happens in VRay when two surfaces are touching. You may have capped the sectioned mass that you hid with the VRay material. If so, go back and delete that capped face by exploding the object, deleting the face, and joining the remaining surfaces back together.
  • I want to make my section cut look like a poched area, but I don't want to take it into Photoshop or Illustrator to do it. Can I do it here?
    • To create a section poche look, you can take the planes of the walls, floors, etc. where the section cut through and explode those masses. Then, take the planes and place them on a new layer with a new solid diffuse VRay material (with a strategically chosen color), and re-join any of the surfaces you exploded. Now when you render, the cut planes will render with a solid color.