Section Planes

Tutorial 5 in the Intermediate Twilight Render V2 Tutorial Series


One of the more unique features of SketchUp is Section Planes.  Since the release of V2, Twilight Render has had the ability to render Section Planes as they appear in SketchUp.  With a little work you can even animate Section Planes to create dynamic construction layouts or product deconstructions.


Having a little familiarity with how Section Planes and Components interact in SketchUp will help you create the most effective scenes.  Knowing how Twilight Render treats Section Planes differently will help you set up your scenes for the correct render.


SketchUp Components and Groups are used to create a hierarchy.  At the top (or bottom, depending on how you look at it) is the world or root.  Adding a Component adds another level to the hierarchy.  Placing a child Component within the first (nested components) adds another level, and so on.  The world root, as well as any given component, may contain any number of sibling components.


So what does this have to do with rendering Section Planes?


Section Planes only effect geometry, components, and groups that are at the same level of hierarchy and below.


In our example scene here, we have three components added to a world (with some room geometry scattered about): a table and two chairs.  The chair on the right is a simple single component.  The table contains 2 nested components, the top and bottom.  The chair on the left contains 7 nested components.  We will look at how Section Planes and components interact in our scene.


First, root-level, or world-level, Section Planes cut everything.


You can easily see a demonstration of this by adding a Section Plane to the world root. 


Because all geometry is beneath the world root level, all geometry gets cut by the Section Plane.  No matter how deeply nested, all components and groups are cut by a root-level section plane.


Twilight Render produces images that reproduce this exactly the same for root-level section planes. 


Notice that all the components are universally cut, even when nested.  Notice too, that the room backdrop is also cut, revealing the background environment light.


It's important to remember that all geometry is cut by a root Section Plane, no matter what.



Don't forget,  you must turn Section Plane rendering on!  By default, Section Plane rendering is off and must be enabled under

Settings => Section Plane Rendering



 Now let's try something different.  After removing the Section Plane from the root, lets add section planes to our two nested components.  Remember, both the table and the left chair are components with sub components.  If we add a section plane just inside the compnent, SketchUp will cut all the interior (nested) components.


But when we render... we don't get the same result!  If you notice in the image on the left, no Section Plane is rendered, yet the SketchUp view shows differently (you can click on the image to view it larger).  Don't panic.  We will show you how to work around this issue and even use it to your advantage.


The reason this occurs has to do entirely with the nature of Components.  In SketchUp, all component instances are identical copies of the same definition.  This is true of components in Twilight Render too.  But in SketchUp, Section Planes are essentially a visual trick and are exempt from the 'identical copy' rule.  Not so in Twilight Render where Section Planes are geometry / material properties.


Knowing that, consider two instances of a component table.  Table A is cut vertically by a section plane but Table B is cut horizontally.  How can the two be considered identical copies any longer?  They can't and that is why nested components are never cut by the Section Planes of their component parents.  (Yes, root-level Section Planes can cut Components; there is a reason why it's different but it has to do with multiple coordinate systems that you don't really want to know about).


So if we can't cut nested components, how do we cut geometry without cutting the entire scene? 


Even if Twilight Render won't cut nested components, it will render Section Planes that cut through a component's local geometry.  That's any face in the component not in a nested Component.


In a lot of cases, this will be all you need.  In many other cases, this will allow you to create complex renders that you actually can't create in SketchUp.  Consider how you could cut into a house without affecting the furniture components within it?




Taking what we just learned, we can go one step further.  In SketchUp, Groups are hierarchical just like Components.  Unlike Components, however, Groups can not be used to create multiple identical instances.  They are inherently unique.  Because of that, Twilight treats them as if they were simple geometry all at the same level in the hierarchy.


What that means for us is that, by converting our nested components to groups, we can ensure that a Section Plane included in the component cuts through all the nested geometry.


Perhaps you can begin to see how, by working with Groups and Components, we can accomplish a whole variety of effects in controlling where the Section Plane is rendered.


Just remember:

  1. Section Planes only affect geometry at the same level or lower.
  2. Root-Level or Global Section Planes cut everything.
  3. Rendering Section Planes never crosses the Component boundary (except root-level section planes).
  4. Rendering Section Planes treat Groups exactly the same as ungrouped geometry.


 One of the coolest uses of rendered Section Planes is using them in an animation.


If you set up a Section Plane in SketchUp with multiple scenes, you can animate the position and rotation of the Section Plane as SketchUp animated from scene to scene.  It would be great to be able to reproduce this effect in a render!


Unfortunately, if we try to do that, we quickly run into problems.  It seems that, unless the scene animation is controlled by SketchUp's animation 'Play' button, Section Planes quickly fall apart and even loose their position completely.  Twilight Render needs to be able to precisely control the position of the animation to render the right scene view.  So what do we do?


 Twilight Render has something called a Twilight Render Section.  This is a special component created by Twilight Render to be a stand-in for section planes.  It won't cut your scene in SketchUp, but as an actual Component, you can do anything to it that can be done to any component.

  1. Add a Section Plane where you want it (don't forget everything we learned above).
  2. Right-click on the Section Plane
  3. Choose Twilight Render V2 -> Convert to Twilight Section


By adding one of the many third-party plugins for animation, you can control the position and rotation of the TWR Section as your animation progresses.  Set up a normal TWR Animation render, get some coffee while it renders, and at the end you will have fully animated section cuts!




 Twilight Render Pro users have some additional features.  We decided that, since a Section Plane is just a mathematical description of a Plane, why could't we use other shapes?  Why not a Section Sphere, Cube, or Cylinder?


So that's exactly what we did.  Operating exactly the same as a Twilight Render Section, Pro users can insert spheres, cubes, and cylinders that cut the geometry in a scene (following exactly the same rules as Section Planes).

  1. Click on Extensions in the main menu.
  2. Choose the Twilight Render V2 sub-menu.
  3. Choose Objects -> Section Sphere (or Plane, Cube, Cylinder).
  4. A component will be added to your scene and will follow your mouse until you click to place it.
  5. By default, each Section Object (or Shape) is 1 meter in size.  After inserting, you can scale it to your desired size.

You should immediately see something significant.  A Section Plane is unbounded; it cuts everything along the plane (following the above rules).  But Section Object don't act that way!  They have specific boundaries that, with some careful arrangement, let you achieve a whole variety of cutout effects!  And when you scale them into ellipses and rectangles, the resulting cutout is an exact match!


Just like Twilight Render Section Planes, the Section Shapes can be animated.  And not just in position but in size as well.  And as mentioned above, that scale doesn't have to be uniform, allowing you to create rectangles and ellipses of any size and shape.


Section Shapes have another great feature.  Just like a Section Plane, you can reverse the direction.  For a shape like a cube or a sphere, that turns the cut area inside out!  So instead of cutting a hole in an object, you  can render just the part that falls within the cube, sphere, or cylinder.

  1. Add a Section Shape.
  2. Position and scale the shape to your needs.
  3. Right-click on the shape component and choose Twilight Render V2 -> Invert.
  4. Render!
Friday, April 19, 2024

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