I've tested assign COP plane to the principled shader, UE4 can read the texture properly. But if I assign that COP plane to unreal_material_parameter, UE4 won't read the image.

The unreal_material_parameter“ is meant to be used with material instance of an existing Unreal material created by the plugin.
(via the ”unreal_material_instance" parameter)

Please have a look at:
https://www.sidefx.com/docs/unreal18.5/_materials.html#MaterialsInstances [www.sidefx.com]

for more details

Basically you need to:
- Assign the principled shader with the texture on your houdini object.
This will ensure that the houdini material is converted to material/texture in unreal.
- Set the “unreal_material_instance” attribute on your object, to indicate that in unreal, the mesh will use a new Material Instance of an existing unreal material.
- Finally, you can use the “unreal_material_parameter” attribute, to override parameter on the newly created Material Instance.

assuming the texture parameter you're trying to override with the houdini texture is called “myTexture”
add the unreal_material_parameter_myTexture attribute, with its value set to the texture's channel name/

(if the principled shader texture in H was assigned to the diffuse/basecolor channel, set the attribute value to “diffuse”)

sysyfu
I've tested assign COP plane to the principled shader, UE4 can read the texture properly. But if I assign that COP plane to unreal_material_parameter, UE4 won't read the image.

Just curious, did you manage to get UE4 to read a COP plane by using the {PlaneName} braces in the path? If yes, what syntax did you use in your principled shader to make it work? I've tried with a simple op:/ and {PlaneName} at the end but can't seem to get it to work in UE4.

Cheers