RTS Style Unit Selection in Unity 5

  • Draw a selection box using the mouse
  • Determine which units are within the selection box
  • Highlight selected units

Drawing rectangles

There are many ways to draw rectangles in Unity. In this example, I will use GUI.DrawTexture, as it is an easy and straightforward way to achieve our goal. We can draw a simple colored Rect using GUI.DrawTexture by setting GUI.color to the desired color, and then passing a white 1×1 texture to GUI.DrawTexture. I wrote a short utility class to make this easier:

Keep in mind that GUI methods (and thus also our DrawScreenRect utility method) can only be called during OnGUI(), and make sure you create the white texture only once for performance reasons.

We can now draw screen rectangles from any component in OnGUI():

With the following utility method we can also draw borders for a rect:


Using the mouse to draw a selection box

Screen space in Unity has its origin (0,0) at the bottom left of the screen. This is inconsistent with the Rect struct, which has its origin at the top left. Input.mousePosition gives us the position of the mouse in screen space, so we have to be careful when using mouse positions to create a Rect. Knowing this, it is fairly easy to create a Rect from 2 screen space (mouse) positions:

We can then write a simple script that allows us to draw a selection box with the mouse:

Selecting Units

In order to determine which objects are within the bounds of the selection box, we need to bring both the selectable objects and the selection box into the same space.

Your fist idea might be to run these tests in world space, but I personally prefer doing it in post-projection (viewport) space because it can be a bit tricky to convert a selection box into world space if your camera uses a perspective projection. With a perspective projection, the world shape of the selection box is a frustum. You’d have to calculate the correct viewprojection matrix, extract the frustum planes, and then test against all 6 planes.

Getting the viewport bounds for a selection box is much easier:

And testing if a game object is within the bounds is trivial:

Highlighting selected units using projectors

There are many different ways to highlight units. In RTS games it seems to be pretty popular to place a small circle below selected units, so that’s what we are going to do.

In order for the circles to work well with sloped terrain we are going to use projectors (rather than just drawing a circle sprite below selected units). Projectors can project materials onto other geometry, but they need a special type of shader in order to do so. The Unity 5 standard assets contain both a Project/Light shader and a Project/Multiply shader, but unfortunately neither of those shaders are appropriate for what we want to do: Add circles to geometry below selected units. We’ll have to write our own projector shader.

It will take 2 parameters: a cookie texture (alpha mask) that defines which pixels are going to be affected, and a tint color, which will define the color of the affected pixels:

Set up shader states for projection, but use additive blending:

Combine the alpha mask and the tint color to determine the final color:

Using this shader, we can set up a projector like we initially intended. In the following example I use a simple circular alpha mask to project a circle below a unit. Note the import settings I used for the alpha mask. It needs to be a cookie texture with light type spotlight (this sets the wrap mode to clamp) and alpha from grayscale needs to be checked.
Projector Settings
While we are now capable of rendering our selection circles, there are still a few issues that we need to address.



Ignoring layers

By default projectors project onto everything. In our situation, we want projectors to ignore other units in order to avoid the behaviour seen in this image. This can be achieved using the Ignore Layers property of the projector. Personally, I like to have a ground layer which contains all of the terrain, and I simply ignore all other layers in the projector.




Projections can sometimes appear on objects outside of the projectors frustum. In this example, a projection appears on the terrain above the unit. This happens because the terrains bounding box intersects the projectors frustum (even though its geometry does not). Granted, this scenario is fairly unlikely in an RTS game, but it is easily solved by introducing an attenuation factor. This will also fade out the projection when a unit stands close to a cliff.

Example Project

I have created a Unity 5 project that implements everything discussed in this post. I also extended the unit selection component to preview unit selection and output all selected units. You can find the download link below.

Download Links

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Just wanted to say thanks. This was good stuff. I have read through a bunch of articles on the same subject and most of them way overcomplicate things.
The only thing I was missing here was the highlighting selected units. It seems the current URP doesn’t support projection. It seems like its on their roadmap though with something called Decal Rendering, or at least it sounds like it will help fill the gap. We will see how that goes. For now I will have to work a different angle for that. But again thanks


Thanks so much for this. Spent quite a few hours trying to figure out a plethora of issues with those selection circles and you saved me a lot more hours