I've somehow gone and changed my settings to stop the displacement from the COP Preview Material node from showing in the scene view. When the material is tested with Karma, it's fine. Checking the display options, I do have displacement enabled.


When switching to OpenGL, it does correctly show the displacement.

Is there something I can do, or might have missed, to get Vulkan to work with displacement again?

I feel ya I also get this problem every so often and its quite annoying.
It isnt consistent for me so hard to trace down. It will be working away fine and I save v01, v02, v03 and then shut off for the day come back the next day no displacement in v03, if I get lucky it is still working in v01.

I also found that switching to openGL will force the displacement to recook but not an ideal solution as I much prefer the vulkan viewport when it is working correctly.

I have played with every setting in the display options refreshing the scene with the labs tools but nothing makes the displacement kick back in.

Sometimes I will create a very basic displacement setup and then copy paste the old broken scene into it. This works sometimes.

I thought perhaps it was just my card so tried new drivers, new houdini versions but same probelm.
If anyone has a magic solution for this, it would be great.

You can search through previous posts; this issue has already been discussed here. The tricky part is that the viewport needs tangents (and maybe normals too?) to display height, and OpenGL calculates them automatically under the hood, while Vulkan doesn't calculate them by default because it's faster and they're usually not needed. There are two options: 1) enable tangents in the display settings under Geometry. There's a setting in the bottom right called "Auto generate tangents." It's disabled by default. You need to enable Mikkt because you need Mikkt tangents. Will Fast work? Honestly, I don't remember. I remember the developers specifically mentioning Mikkt.
2) The second option to avoid constant and unnecessary autocalculation of tangents is to manually generate them on the geometry where they're needed. And that's the right solution. In theory, they're created by the Polyframe node, with the default names tangentu and tangentv. The main thing is to select the style—MikkT—in the menu. And according to the developers, the viewport should pick them up and work.

But! Back then, no matter what I tried, I couldn't get the viewport to show the height with the Polyframe tangents. So I settled on autocalculation—set it to default. And that's how it stayed. But since this conversation and testing were a long time ago, it's possible it was a bug, and now everything works with manual tangents as well—try it and let us know. Frankly, I haven't had time for more testing.