Partial Post-Process Effect Application in UE4

After a long long time (yay summer?) I'm back with a post on what I found to be a rather useful technique. Imagine you have a post-processing effect that you apply to your viewport. But what if you want to apply it only to a certain portion of the screen? What if you want to use say edge detection only on a certain object?

Unsurprisingly, this is very doable in Unreal Engine using Custom Depth buffer that allows you to select only some objects and write there scene depth into it, while ignoring other objects.

Side note, this blog was inspired by one by Tom Looman and I most definitely suggest you go through it too, as it contains more information on issues and limitations of Custom Depth buffer.

Idea

Idea is actually very simple. Since Custom Depth buffers allows you to track only the objects that interest you, you can use it to decide whether to apply the effect on it or not. If the value in Custom Depth buffer is smaller than that in the of the Scene Depth buffer, you should apply the post-processing effect. Otherwise keep the image the same.

Setup

This approach also doesn't require writing any more code. First off, we have to decide which object will get to have post-processing applied. You can do this by clicking on the desired object and checking Render CustomDepth Pass as seen bellow. 

If you have trouble finding it just write "custom" into search field.

This will add the object to Custom Depth buffer. You can visualize how your Custom Depth buffer currently looks via Buffer Visualization option.

In my case I added two objects into the buffer and here's what it looks like. The Depth buffer is shown on left, while Custom Depth buffer on right.

So far so good. Now let's change our post-processing effect so it takes Custom Depth buffer into consideration. In our case, I'll be using Sobel edge detection effect. Current material setup looks like this.

So first off we need to add Custom Depth and Scene Depth nodes. Both of these are SceneTexture nodes, only thing you need to do is to change type like this.

Now only plug these two values into If node. Since If node doesn't allow to work with vectors, we need to somehow get a single float out of the color. This can be easily done by using Mask node and taking R channel (since depth maps are written as greyscale images, you can actually take B or G channel if you want to).

Once this is done we plug masked Custom Depth as value B and masked Scene Depth as value A. If Scene Depth value is higher or equal to Custom Depth value you want to apply the effect (this means that nothing is blocking the object in Custom Depth buffer). If the value of Scene Depth is smaller, leave the original scene color (this means something is standing in front of the object where you want to apply the effect).

The setup looks like this.

Note that PostProcessInput0 color pin returns float4 so we use Mask node in order to only get RGB values. 

Final result can for example look something like this.

Application

Partial application of post-processing effect can have many uses (so go wild :) ) ranging from outline detection, object highlighting, and so on. Before I wrap up this post I'd like to show a simple use with Sobel edge detection and First Person camera that can (sorta) mimic say a use of a special lens. The effect we want to achieve looks like this.

In this case we will have a plane in front of player's camera that will be marked for Custom Depth buffer. Everything that is "occluded" by the plane will have the effect applied.

All you need to do is add the plane to your hierarchy in your character blueprint (I parented it with Mesh so that it will move with camera, I also assume that you already have the PostProcess component applied to your character and that it has correct material set up). Make sure to place plane in eye-height of the player.

Next mark plane for Custom Depth pass as shown in Setup section of the post. Yay should work!

Not quiet unfortunately. Since our plane is still a solid object it will "block" everything beyond it so the effect will only be applied to the plane. As mentioned in Tom Looman's post, making plane transparent won't quite work. We need a way to not physically render the plane and yet to keep it in our Custom Depth buffer. Fortunately, this is pretty easy too! All you need to do is uncheck a checkbox.

Select the plane and in Render section deselect "Render in Main Pass" as shown below.

And that's it! You should now have your own partially applied post-processing effect. 

Results

To sum up the tutorial, here are once again results of the effect.

Notes

- Custom Depth buffer can't be applied to everything, but it is a very useful technique. So to save up some headache be sure to read some of its limitation and issue

Resources

Tom Looman - more information on Custom Depth buffer and effects it can produce

Official documentation - contains information on post-processing materials, custom depth, material blending and more

Sobel edge detection - tutorial for Sobel edge detector used in this post