Box Emission Shape for GPU and CPU particles is double the size it should be.

[Arnklit]

Tested versions

4.2.2.stable

System information

Godot v4.2.2.stable - Windows 10.0.22631 - Vulkan (Forward+) - dedicated AMD Radeon RX 7800 XT (Advanced Micro Devices, Inc.; 31.0.24019.1006) - AMD Ryzen 9 7900X 12-Core Processor (24 Threads)

Issue description

The box emission shape is twice the expected area.

These three all have extends of 1,1,1.

This is easy to fix in the particle processing, but I don't know if we need to wait until a major release where we are breaking back compat since it will mess up people's existing particle setups.

Steps to reproduce

Create a particle system and set it to box emission and observe the size of the emission area.

Minimal reproduction project (MRP)

particles_box_emission_issue.zip

[and-rad]

You will notice that the property of the CollisionShape3D element is called "size", not "extents". Extents describes a shape's "half size", so what you're seeing is expected and not a bug. It's just something to get used to, you'll find the same in other engines, too.

[Arnklit]

Ah couldn't find that when I first looked at this because I was unsure if it was on purpose. Thanks @and-rad . We might wanna note it here. https://docs.godotengine.org/en/stable/classes/class_particleprocessmaterial.html#class-particleprocessmaterial-property-emission-box-extents

Found this example explaining it for another engine as well.

[clayjohn]

This is sad to see. Before 4.0 we did a pass to try to move everything to "size" instead of "extents" as the difference was confusing a lot of people #72075

But I guess we missed this one. I think a note in the documentation would be helpful

[and-rad]

Did you miss it or did you maybe keep it on purpose because it might have performance implications? I'm just thinking, since it's used in the particles material, storing the information as extents rather than size might make some computations with regards to bounds and overlaps easier?

[Calinou]

This is sad to see. Before 4.0 we did a pass to try to move everything to "size" instead of "extents" as the difference was confusing a lot of people #72075

We can likely migrate this one without breaking compatibility by adding a new emission_box_size property, deprecating the old one, hiding it from the editor and using _set() to assign it to the new property automatically.