Cycles : Crash with interactive shader updates on subdiv meshes

[murraystevenson]

Description

Editing shader parameters on a subdiv mesh while interactively rendering can crash Cycles.

Steps to reproduce

1. Open the scene below
2. Switch to the Cycles viewport.
3. Turn on full shading.
4. Use the virtual slider to rapidly edit the shader's emission.r parameter.
5. Enable the MeshType node.
6. Use the virtual slider to again rapidly edit the shader's emission.r parameter.
7. Cycles should quickly crash.

Debug log

Click to Expand

import Gaffer
import GafferCycles
import GafferScene
import IECore
import imath

Gaffer.Metadata.registerValue( parent, "serialiser:milestoneVersion", 1, persistent=False )
Gaffer.Metadata.registerValue( parent, "serialiser:majorVersion", 2, persistent=False )
Gaffer.Metadata.registerValue( parent, "serialiser:minorVersion", 0, persistent=False )
Gaffer.Metadata.registerValue( parent, "serialiser:patchVersion", 0, persistent=False )

__children = {}

__children["Sphere"] = GafferScene.Sphere( "Sphere" )
parent.addChild( __children["Sphere"] )
__children["Sphere"].addChild( Gaffer.V2fPlug( "__uiPosition", defaultValue = imath.V2f( 0, 0 ), flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic, ) )
__children["MeshType"] = GafferScene.MeshType( "MeshType" )
parent.addChild( __children["MeshType"] )
__children["MeshType"].addChild( Gaffer.V2fPlug( "__uiPosition", defaultValue = imath.V2f( 0, 0 ), flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic, ) )
__children["PathFilter"] = GafferScene.PathFilter( "PathFilter" )
parent.addChild( __children["PathFilter"] )
__children["PathFilter"].addChild( Gaffer.V2fPlug( "__uiPosition", defaultValue = imath.V2f( 0, 0 ), flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic, ) )
__children["principled_bsdf"] = GafferCycles.CyclesShader( "principled_bsdf" )
parent.addChild( __children["principled_bsdf"] )
__children["principled_bsdf"].loadShader( "principled_bsdf" )
__children["principled_bsdf"].addChild( Gaffer.V2fPlug( "__uiPosition", defaultValue = imath.V2f( 0, 0 ), flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic, ) )
__children["ShaderAssignment"] = GafferScene.ShaderAssignment( "ShaderAssignment" )
parent.addChild( __children["ShaderAssignment"] )
__children["ShaderAssignment"].addChild( Gaffer.V2fPlug( "__uiPosition", defaultValue = imath.V2f( 0, 0 ), flags = Gaffer.Plug.Flags.Default | Gaffer.Plug.Flags.Dynamic, ) )
__children["Sphere"]["__uiPosition"].setValue( imath.V2f( -9.03114128, 27.5871067 ) )
__children["MeshType"]["enabled"].setValue( False )
__children["MeshType"]["in"].setInput( __children["Sphere"]["out"] )
__children["MeshType"]["filter"].setInput( __children["PathFilter"]["out"] )
__children["MeshType"]["meshType"].setValue( 'catmullClark' )
__children["MeshType"]["__uiPosition"].setValue( imath.V2f( -9.03028107, 19.4230461 ) )
__children["PathFilter"]["paths"].setValue( IECore.StringVectorData( [ '/sphere' ] ) )
__children["PathFilter"]["__uiPosition"].setValue( imath.V2f( 3.96971893, 25.5050774 ) )
__children["principled_bsdf"]["__uiPosition"].setValue( imath.V2f( -30.9004307, 11.2589846 ) )
__children["ShaderAssignment"]["in"].setInput( __children["MeshType"]["out"] )
__children["ShaderAssignment"]["filter"].setInput( __children["PathFilter"]["out"] )
__children["ShaderAssignment"]["shader"].setInput( __children["principled_bsdf"]["out"]["BSDF"] )
__children["ShaderAssignment"]["__uiPosition"].setValue( imath.V2f( -9.03028107, 11.2589836 ) )


del __children

[boberfly]

Hey @murraystevenson

I've not been too happy with Cycles' subdivision code in-general and found that it isn't very reliable at all and need to revert to using MeshType - linear to get stability back.

One thing I'm going to propose back to the Cycles devs is to just use Embree's subdivision code. They probably won't accept it as it'll be CPU-only initially but by the reports of the Intel devs, the next Embree release will allow GPU support and not just for their Arc line of GPUs but other vendors as well. No idea how this plays with OptiX though but this should be something that Blender should solve.

The other option is we just use OpenSubdiv under the hood and call it a day?

[johnhaddon]

I've not been too happy with Cycles' subdivision code in-general and found that it isn't very reliable at all and need to revert to using MeshType - linear to get stability back.

Is this true in general for Cycles users outside of Gaffer? Do they all avoid Cycles' adaptive subdivision and use Blender's pre-render uniform subdivision instead?

[boberfly]

I've not been too happy with Cycles' subdivision code in-general and found that it isn't very reliable at all and need to revert to using MeshType - linear to get stability back.

Is this true in general for Cycles users outside of Gaffer? Do they all avoid Cycles' adaptive subdivision and use Blender's pre-render uniform subdivision instead?

Good question - I am only going by what @skwerner was telling me that is notorious for being buggy in Blender when using it live from the viewport and I think motion blur but my memory is fuzzy on the details.

[boberfly]

Here's the tracker for adaptive subdivision, seems like it hasn't got much attention:
https://developer.blender.org/T53901

[johnhaddon]

Some more details :

• The crash appears to be in Cycles, somewhere in the BVH build. We don't really want to be doing a BVH build just for a parameter change.
• If instead of changing emission above, you change the sphere radius, then it doesn't crash. Even though changing the radius is presumably also triggering a BVH build.

Not sure what this means, but to my mind it makes it more likely that the problem is on our side.

[johnhaddon]

Well, the crash is sometimes in the BVH build. Sometimes it is in OSL.