HDRP Terrain Details Blurry / Motion Vector bug

Applies to:
All Gaia versions
HDRP only
Unity versions 2023.1.0 - 2023.1.12

Symptoms:

There is a bug in the more recent Unity versions 2023.1.0 to 2023.1.12 where terrain details appear very blurry on the terrain:

This affects both Flora vegetation as Unity's GPU instanced terrain details. This is a pure Unity bug that is reproduceable without Gaia being in the project as well. This bug has been recognized by Unity:

https://issuetracker.unity3d.com/issues/motion-blur-distorts-terrain-details-in-runtime
See also 
https://issuetracker.unity3d.com/issues/hdrp-motion-vectors-are-wrong-for-terrain-not-positioned-at-the-origin-when-built-or-in-play-mode

It is not possible for Procedural Worlds to fix this issue, but we can provide workarounds (see below).

Cause:

This issue is caused by Motion Vectors not being calculated correctly for terrain details when the terrain is not positioned at x=0,y=0,z=0 in the scene. Motion Vectors are "hidden information" in the rendered image that tell how fast & where an object on the screen is moving. This information is e.g. used to process visual effects like Motion Blur, or Temporal Anti-Aliasing.
Since the motion vectors are wrong, the rendering process "thinks" the grass is moving very quickly through the scene. Per default, there is Motion Blur enabled in HDRP which results in the above issue: There is an extreme motion blur applied to the terrain details "since they are moving so fast".

Fixes / Workarounds:

The issue is supposed to be fixed in Unity 2023.1.13 which was not available yet at the time this article was written. There are various workarounds that you can apply to avoid this issue in the meantime.
The simplest way is to turn off Motion Blur in the HDRP Global Settings. Go to Edit > Project Settings > Graphics > HDRP Global Settings and find the Motion Blur checkbox in the Volume Profiles:



Please uncheck that box and restart the scene, this should immediately resolve the issue. If it does, and this solution works for you, you can consider it resolved.  If running Gaia 2023, you can click the "Ignore" button in the Project settings now to hide the warning about this issue:




If the issue still persists, or you can't see the Motion Blur checkbox, please read on.

Alternative Workarounds / More info on HDRP Settings

Please note that the HDRP settings are very flexible and allow you to override settings in various locations. The location for that Motion Blur toggle as shown in the screenshot above is the Default Volume Profile in the HDRP Global Settings. The settings you see here can vary if you have a custom Default Volume Profile set up for your project. In this case you would need to add the "Motion Blur" section as an override to your Default Volume Profile:



If you have deactivated Motion Blur in here, but you are still getting the issue, please note that there are those other locations where the motion blur might potentially get enabled again. Please check these locations to make sure that motion blur is disabled there as well.

Global or Local HDRP Volumes:

If the camera is inside a HDRP volume, the Motion Blur setting from the global volume profile in the HDRP Global Settings might be overridden. Check if your volumes do have Motion Blur override section and turn it off there.




Custom Frame Overrides:

It is possible to add custom frame overrides on the camera that allow you to override specific HDRP settings for that camera only. Check if you have custom frame overrides and if Motion Blur is enabled here. This is a bit esoteric in so far as you would normally not be able to enable Motion Blur back on if it is disabled in the other two places already, but it cannot hurt to check here:



This also gives you an option if you need to have motion blur enabled on another camera, you can then use the frame overrides to just deactivate it on one camera.

Deactivating Motion Vectors

Motion Vectors are also exposed as a HDRP setting and can be turned off at will in various locations, e.g. in the HDRP Global Settings under "Frame Settings":



Like turning off motion blur this will get rid off the issue since there are no faulty motion vectors on the terrain details anymore, but will also stop all other motion vectors from being calculated. This essentially also disables Motion Blur as a side effect. If another effect you use or custom code relies on these Motion Vectors, it will affect that as well. This setting is also available under the Quality-specific HDRP Asset settings:



and as a custom frame override on the camera as well:



If all those possible settings cannot be used in your case e.g. because they would affect other parts of your project, you will most likely need to wait for a proper fix in version 2023.1.13 as outlined in the unity issue tracker link above. We would not recommend to downgrade in order to work around the issue, since this can result in loss of HDRP settings and / or visual issue when you migrate towards an earlier version of HDRP.