GPUParticles2D additive blending artifacts

[dcarlile42]

Godot version

v4.0.stable.official [92bee43]

System information

Windows 11, Vulkan, NVIDIA GeForce RTX 2060 with Max-Q Design, Driver Version 531.18

Issue description

CPU is AMD Ryzen 9 4900HS with Radeon Graphics, running with set DISABLE_LAYER_AMD_SWITCHABLE_GRAPHICS_1=1 to force GeForce GPU.

When trying to use additive blending with 2D GPU Particles in 4.0 I'm getting strange artifacts that don't appear in 3.5. I actually get the same artifacts using CPU particles as well. Using Compatibility the artifacts are better, but still not as smooth as 3.5 and very noisy.

Godot 3.5

Godot 4.0

Godot 4.0 Compatibility

Steps to reproduce

• Set up a GPUParticles2D node using a 32x32 "smoke" texture designed for additive blending
• Texture should use linear filtering
• add CanvasItemMaterial, set Blend Mode to Add
• ~1200 particles moving to the left at speed 128
• life 7 seconds
• color ramp down to transparent at end of life
• see example projects for all the particle settings

Minimal reproduction project

ParticleBlendingIssue_20230311.zip

[clayjohn]

This looks like it was caused by our move to using RGB8 render targets in 2D. I tested my theory by modifying the OpenGL renderer to use an RGBA16 render target (as it does in 3.x) and it restored the same smooth blending that the 3.x version had.

We discussed adding support for a "high precision" mode (in #61667), but in the end decided against it as most users didn't need high precision 2D rendering, they mostly wanted it for data processing or for 3D effects, so we went with another direction. But this case shows that there are situations were an HDR 2D render target may be needed.

[limbonaut]

We have encountered a similar issue. Particle effects that use hue and blending are not producing result as good as in 3.5. Here is a video comparison of the same particle effect in both Godot 4.0 and 3.5:

effect-repro-6000.mp4

[Calinou]

We have encountered a similar issue. Particle effects that use hue and blending are not producing result as good as in 3.5. Here is a video comparison of the same particle effect in both Godot 4.0 and 3.5:

This is due to a separate issue: #75153

[dcarlile42]

I can confirm this change resolves my use case. A big thanks to everyone involved.