Rendering Soft outlines in Unreal Engine 4

Ever since I first wrote about creating object outlines in Unreal Engine 4 I have wondered if it was possible to render them as soft outlines instead of harsh binary lines. A good example of soft outlines can be found in Valve’s games like Left 4 Dead or CS:GO.

There wasn’t an easy way to blur the pixels at the time nor a decent way to down-sample the render target containing the outlined objects in order to make this blur operation cheaper. Overall it came to be too costly to come with a decent solution and so I stuck with the binary outlines you may have been in several of my earlier posts. Recently I decided to revisit this issue by doing a quick experiment using a modified version of SpiralBlur, a material node that’s available in Unreal Engine 4, that is using some custom (HLSL) shader code to blur the pixels using a spiral blur method.

Looks pretty decent, right? The effect is more expensive than the binary outlines since we perform several blur steps in the SpiralBlur-node to get to look decently smooth. Without going the custom engine route there is no way to downsample the post-process step where we sample and blur the information from the Custom Depth buffer. Later in this post I will talk about performance of the effect itself.

The Material Graph

Unfortunately I couldn’t find any official UE4 documentation on the Spiral Blur node, there is a wiki page on how to use the material node. The built-in implementation takes the scene textures and over several iterations creates….a spiral blur. The default settings are at about 128 iterations, which is pretty heavy! I’ve used this node as a reference to create my own, which samples the Custom Depth buffer instead of the scene color.

The node graph for it is reasonably simple and most of the logic happens inside the custom node which I added as a code sample below.

Performance

Performance was recorded on a 850M mobile GPU at 1280×720 with ~1.5ms measured and on my GTX 980 Ti it runs at 1920×1080 with 0.8ms for the post process material. It’s heavily depending on the amount of iterations in the spiral blur. You will want to keep the DistanceSteps and RadialSteps as low as possible while maintaining a smooth edge. In the demo I settled on 4 DistanceSteps and 8 RadialSteps which is 32 iterations.

Conclusion

To conclude, the answer is yes! It’s entirely possible to make soft outlines in Unreal Engine 4. I imagine there may be more efficient ways of blurring the custom depth buffer to get similar results, I simply leveraged the available shader code to quickly get to a proof of concept. It’s important to note that being far away from the object with an outline can make it look slightly less smooth, a thinner outline helps and is a matter of tweaking.

References

18 Comments

  1. This looks like it could be incredibly beneficial to my project!

    I unfortunately get errors when implementing it though – “redefinition of ‘CurColor”‘ is the initial error. If I remove that duplicate line from the code that goes in the custom node I get a bunch of other errors about missing declarations and not found Intrinsic functions. I tried this out in both 4.16 and 4.17 to no avail.

    Any way you could either send me a copy of the function or help me get it working?
    Much appreciated!

    -DSP

  2. @Noodlespagoodle I’m using 4.17, I removed the duplicate line and had no other errors. Did you copy/paste the Scalars from the engine SpiralBlur, maybe you made a tiny mistake there? <3

    • That’s 10000 units, I don’t even quite remember why I went for that – in practise defines the available range of custom depth (if you lower this value you’ll see it breaks beyond a certain range, since everything will be clamped to 1’s. If I’d clean this up I would probably map the custom depth range between 0 and 1 properly (This really was just a quick proof of concept on the method)

  3. Hi, I am trying to use this example but I have a couple of questions.

    Does the output of the PP_BlurredCustomDepth_OccludedOnly blueprint is connected to the Emissive Color property of the material node or the Base Color property?

    Also, I am getting a compiler error when trying to compile the material, it shows “ERROR” just below the custom function (LODZERO_SpiralBlur – Texture), and in the logs it goes:

    “Error [SM5] SceneTexture expressions cannot use post process inputs or scene color in non post process domain materials”

    Seems that its complaining for the UV property is not connected, although you do not have it in the image either, so, there must be something I am doing wrong.

    Any help is appreciated!

    Regards,
    Lermy

    • Hi Lermy,

      You did not set the “Material Domain” to be a post-process and it’s still considered a surface material. Click on the main node in your material to access the properties and change it to be a Post Process. This will expose ONLY the emissive channel.

      Hope that helps,

      Tom

  4. Hi! When “saving” it shows a pop up with this message:

    The current material has compilation errors, so it will not render correctly in feature level SM5.
    Are you sure you wish to continue?

    After continuing, the call to LODZERO shows an error:

    Error [SM5] Function LODZERO_SpiralBlur-Texture: Missing function input ‘UV’

    I would appreciate any help!

    Thanks

  5. Hi, thanks for your guide, but I found a problem: I can’t change the outline color using “custom stencil value” (I am able to do this with normal stencil material in your previous tutorial)

  6. Hi Tom,

    Great article, I have this working and it’s great! I’m trying to set this up where by it occludes behind walls. I can achieve this using your other outline setup, by just adding a oneminus after the Determine Occlusion set of nodes, and then multiplying that just before the lerp input, that worked great for that.

    I see this is already occluding against the mesh that has Custom depth switched on. but I can’t get how to have walls etc do that same? It’s driving me a bit nuts.

    An info on that would be awesome Tom

    Aaron

  7. That’s great, thank you for this. Finally I’ve got it running. But I couldn’t figure out how you do the scalar parameter in the material function. I don’t see an input if I use the regular scalar p.
    Speed: I just tested it on Samsung galaxy S2: With steps=4 It’s running but not fluent, with steps=8 it’s waaaay to slow.

    • Hey Olaf, do you mean you couldn’t create the scalar input for the material function? Creating a scalar parameter input in a material function requires a function input node, if you just type ‘input’ into the search bar on the left of the shader window, a function input node will appear, and you can then set whether its scalar input in it’s properties. Hope that helps

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