Using the Sketchup Inference System

Creating accurate models in Sketchup is so easy with the built in Sketchup Inference System. You might not even notice it, but Sketchup will lock references to different points and lines in your model to help you align your geometry. In this post, you’ll not only learn the basics of the Sketchup inference system, but you’ll learn some advanced techniques for using the inference system as well.

What is the Inference System?

It’s not something you can configure, or turn on and off. It’s a core part of the functionality of Sketchup that makes the program so easy to use. Learning how to use and interact with it is really easy. Learn a few of these tricks and take full advantage of the robust inference system built into Sketchup.

The inference system is basically a system that locks your cursor in reference to any point, edge, axis, face, guide or imaginary line.

When you are drawing in Sketchup, and you want to add a line from an existing point in your model, you hover your mouse over that point. When you get close enough to that point, Sketchup will guess that you are trying to reference that point and it will lock to it and you’ll see a little green circle pop up over that point to let you know you’ve referenced it.

Without the inference system, it would be impossible for you to connect exactly to that point just by trying to land on that point with your mouse. You’ll always be just a little bit off, and you wouldn’t make a connection.

The Inference System can be used while doing pretty much anything in Sketchup, not just when you’re trying to start a line. It can be used while moving an object, creating construction guides, finishing a line, rectangle, circle, etc., altering your axes, creating dimension, and more.

There are many different types of Sketchup inferences, but they can be broken down into three main categories. Point Inferences, Linear Inferences, and Shape Inferences. Watch the video below, then scroll down for a step by step explanation of the Sketchup Inference System.


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Point Inferences

Point Inferences are based off of the exact location of your cursor. You will find them when you hover over endpoints, midpoints, edges, faces, and intersections. You would typically snap to a Point Inference whenever you are starting or finishing an action, (such as drawing or moving). You also would use a Point Inference to define the starting point of a Linear Inference.

Sketchup Point Inference

Endpoint – Green circle at any endpoint of an edge.

Midpoint – Light blue circle at the midpoint of an edge. If you need to reference the midpoint a lot, create a construction guide off of a perpendicular line, and reference the midpoint of the edge. This will create an infinite guide along the midpoint.

On Edge – Red Square along an edge. Keep in mind, that this doesn’t snap you to a finite point. It will only lock you along the edge you are on. Endpoint and Midpoint inferencing, on the other hand, lock you to a specific point in space.

On Face – Dark Blue/Purple (Am I colorblind?) diamond on a face. This is a critical indicator for drawing on the surface of a face. If you don’t see this, you are not drawing on the face of an object.

Intersection – Red X on two intersecting faces or lines. This will happen if you don’t have a common edge at locations where geometry is intersecting.

Point in Group – Magenta colors. When you are referencing points from geometry that is inside a group or component, all of the above shapes still apply. The only difference is they will all be colored magenta.

Origin - UPDATE 1/15/13 Oops. I forgot this one. If you hover your mouse over the intersection of the axes, you’ll find the origin. It is a type of End Point Inference that is shown by a yellow circle.

Linear Inferences

Linear Inferences give you the ability to EXTEND an imaginary line originating from a Point Inference. This is incredibly awesome! I used to create tons of construction guides because I didn’t realize how to use Linear Inferences or didn’t know they existed. But now, by using them I save a ton of time whenever I’m modeling in Sketchup.
Sketchup Linear Inferencing
To find a Linear Inference, first you must hover over a Point Inference. When you first hover over a point that Sketchup recognizes as a Point Inference, the tool tip will change color, as you know. But if you hang there for a second, a little text box will pop up that tells you what type of inference it has locked to. This also means that Sketchup is temporarily “tracking” that Point Inference. You can then drag your mouse away from that point and you’ll notice a line being pulled away from that point, along an axis.

You can have linear inferences along the green, red, and blue axis, and sometimes it will recognize a perpendicular or parallel axis as well. (Sketchup won’t always lock to a perpendicular line, especially when there’s a lot of geometry.)

To really enjoy the power of Linear Inferences, follow these three tips:

  1. Lock a Linear Inference – Once you’ve established a Linear Inference, hold down the SHIFT key on your keyboard. This will lock the cursor focus along that axis, allowing you to reference yet another Point Inference in your model!
  2. Lock axis with Arrow Keys – After you’ve started to draw a line or make a move, press one of the arrow keys on your keyboard. It’s a quick way to lock axis without having to first find the axis you want to lock to. Up is blue axis, Left is green axis, and right is red axis.
  3. Point Inference from TWO PointsSketchup 2 point InferenceHover over your first Point Inference until the text box pops up at your cursor. Then, carefully drag your mouse (without clicking) to another Point Inference. Once you’ve locked onto that one, drag your mouse (still without clicking) along a Linear Inference until you’ve reached the point at which the two Point Inferences would have intersected. Sketchup will then draw a Linear Inference from both of the points you just focused on, giving you an intersection point to reference from.

Other Inferences

Besides point and linear inferences, there are other ones called “Shape Inferences” that you can use while drawing to help you create specific shapes. These special inferences are available to snap to while using the Rectangle Tool and the Arc Tool. There are a few more oddball inferences I haven’t talked about yet too, but I rarely attempt to use them.

  • Square – While using the Rectangle Tool, you can snap to a perfect square shape.
  • Golden Section – While using the Rectangle Tool, you can snap to a golden section
  • Tangent – While using the Arc Tool, Sketchup can detect the tangent arc of two lines.
  • Half Circle – While using the Arc Tool, you can snap to a half circle shape.
  • Parallel – While using the Line Tool, you can (sometimes) create an inference that is parallel to an edge that is not aligned to an axis.
  • Perpendicular – While using the Line Tool, you can (sometimes) create an inference that is perpendicular to an edge that is not aligned to an axis.

Use the Inference System to your advantage

Knowing how to use the inference system is critical to your success as a Sketchup modeler. It will help you understand how to keep everything in line and just the way you want it. Many frustrating problems can be prevented by paying attention to what you are snapping to. Many times I’ve been drawing something that I think is on the same plane, but then later realized that I was just a little bit off! If I had just paid attention to the inference system I would have been all set.

Good Luck! (and can somebody tell me the difference between the word “reference” and “inference”?, lol) EDIT – Thanks Kevin for explaining the difference below!

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About Matt Donley

Sketchup user since 2007, I launched MasterSketchup as a way to help other people learn this fun program. My goal is to create easy to follow tutorials that help make 3D modeling more accessible to anyone interested in learning. You'll find many of my videos and tutorials featured on the official Sketchup Facebook, Twitter, and Google+ pages.

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9 Responses to Using the Sketchup Inference System

  1. Miguel Angel Bautista says:

    Great Matt!! Keep on giving all of us your knowledge to improve our technique.
    Thanks a lot.

  2. Bernard Katz says:

    Good clearly written post about “inference” with Sketchup!

  3. Kevin McGarrahan says:

    The difference between reference and inference is activity. A reference is a static element (i.e. the origin, a point, a line, or a face) to which you refer for the beginning of any other element. An inference is a dynamic action by which SketchUp determines (infer: derive by reasoning) what you want to do based upon how you move from the reference.

    • Matt Donley says:

      Kevin, Thank you! looks like I’ve got some revisions to make….

    • Martha says:

      I’m forever looking for the best translation of the word ‘inference’ to Dutch. The best word, mathematically, would be ‘afleiding’, but in every-day-Dutch, this word will mainly mean ‘distraction’. Therefore I usually still use ‘referentie’, which is much more specific in spacial terms, even though it’s not a precise translation…

  4. Thomas says:

    Hi Matt,

    Thanks for your clear and to the point video about inference. That’s a lot of information in five minutes! Seems to me there’s one more that would deserve to be on your video: that’s the linear inference (magenta) that allows you to extend (with the Line tool) an existing line or edge that is not parallel to one of the axes. I find it very useful.

    Best.

  5. Austin says:

    This is great! How would this work with, say, a sphere component for example? Sketchup doesn’t seem to snap to point inferences of more complex 3-D objects. My goal is to be able to align the centers of two components, but they aren’t just simple shapes with straight edges so I’m unable to grab the midpoint to align the two by their midpoints. Hope that makes sense.

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