GPUParticles3D flickers frozen particle at world origin (0, 0, 0) when shader material has y-billboard set

[kraybit]

Godot version

v4.0.beta17.mono.official [c400205]

System information

Windows 10

Issue description

With certain settings, GPUParticles3D intermittently shows a particle at world origin with zero velocity.

These settings in GPUParticles3D caused this behavior:

• Lifetime 1 s
• Material Override / StandardMaterial3D / Billboard / Mode : Y-Billboard
• Mesh: Quad

Quirks

• Turning Y-Billboard on and off toggles the bug
• Setting Lifetime for particles to 10 s makes the bug disappear "for ever", even if setting back Lifetime to 1 s (?)

Steps to reproduce

• Create new Godot project
• Create new scene with Node3D
• Add GPUParticles3D node
• Under Process Material, select New ParticleProcessMaterial
• Click ParticleProcessMaterial for settings, there set Emisssion Shape to Box
• Under Draw Passes / Pass 1, select New QuadMesh
• Click the QuadMesh for settings, there set size to 0.1 , 0.1
• Under Geomeetry / Material Override, select New StandardMaterial3D
• Click StandardMaterial3D for settings, there set Billboard / Mode to Y-Billboard
• Set Transform y-position to 1

At world origin (0, 0, 0) there's intermittently a particle with zero velocity.

Minimal reproduction project

gpu_particles_ybillboard_bug.zip

[QbieShay]

Could you re-test with apply_scale flag enabled?

[kraybit]

If the Keep Scale flag is enabled (In the shader material), then the bug disappears!

Enabling Keep Scale fixes bug

Does this flag have any other side-effects? Also, I couldn't find any apply_scale flag?

[Calinou]

I could reproduce a similar issue with the Particle Billboard mode here: #49767 (comment)

godotengine/godot-demo-projects#849 also has a stray rain particle in the world origin that remains visible for a long time because of this.

[QbieShay]

oop sorry @kraybit . Yes, it was keep scale not apply scale :D

@Calinou particles use scale 0 to try and make particles invisible, but if keep scale is off then a new matrix is generated based on the camera matrix and the instance_custom.x as a rotation value, so the particles that should be "dead" are instead displayed at the origin (at least this is how it worked in 3.x)

[Calinou]

@Calinou particles use scale 0 to try and make particles invisible, but if keep scale is off then a new matrix is generated based on the camera matrix and the instance_custom.x as a rotation value, so the particles that should be "dead" are instead displayed at the origin (at least this is how it worked in 3.x)

Interesting, I thought their position would be changed to -INF, -INF, -INF (effectively ensuring they're outside the view frustum). This is what the Hide On Contact collision mode does. Should we make it scale particles to 0, 0, 0 instead of moving them?

[QbieShay]

I think that's what happens already, but if you don't enable scale in billboards you still see them. The inf solution sounds better

[ecmjohnson]

The inf solution is much cleaner (2 line change vs. 12) and doesn't have a performance impact on the billboarded materials since they don't need to even check anything