GPUParticles2D: Generated emission mask is too random

[MelPeslier]

Tested versions

4.3

System information

Godot v4.3.stable - Windows 10.0.22631 - Vulkan (Mobile) - dedicated NVIDIA GeForce RTX 3060 (NVIDIA; 32.0.15.6109) - 12th Gen Intel(R) Core(TM) i7-12700KF (20 Threads)

Issue description

When loading emission mask, particles use (diferent !) randomness for each of the components (position, color, normal).
I would expect that each pixel at a posiiton have the corect coor value and not a random one. Same for normals, that each pixel keep the normal he own, and not just a random value.

What's missing is to writte on the shader a variable 'custom_seed', that's passed to all of theses 3 parameters when loading from an emission mask.

And also, let 'generate emission mask' be a function that's available via script so we can generate from any textures !

• What it should do COLORS:

• What it does COLORS :

• What it should do NORMAL:

demo_should.mp4

• What it does NORMAL:

demo_should_NOT.mp4

I made a litle tool where I do as in the godot engine, but in GDScript to generate it from a viewport texture, and I altered the shader code to have a common 'custom_seed' shared between theses params.

Here is a part of the shader :

vec3 calculate_initial_position(inout uint alt_seed, in float custom_seed) {
	...
		int point = min(emission_texture_point_count - 1, int(custom_seed * float(emission_texture_point_count)));
		ivec2 emission_tex_size = textureSize(emission_texture_points, 0);
		ivec2 emission_tex_ofs = ivec2(point % emission_tex_size.x, point / emission_tex_size.x);
		pos = texelFetch(emission_texture_points, emission_tex_ofs, 0).xyz;
	}
...
}

void calculate_initial_display_params(inout DisplayParameters params, inout uint alt_seed, in float custom_seed) {
   ...
	int point = min(emission_texture_point_count - 1, int(custom_seed * float(emission_texture_point_count)));
	ivec2 emission_tex_size = textureSize(emission_texture_points, 0);
	ivec2 emission_tex_ofs = ivec2(point % emission_tex_size.x, point / emission_tex_size.x);
	params.color *= texelFetch(emission_texture_color, emission_tex_ofs, 0);
}

void start() {
   ...
	**float custom_seed = rand_from_seed(alt_seed);**

	calculate_initial_display_params(params, alt_seed, **custom_seed**);

	if (RESTART_POSITION) {
		TRANSFORM[3].xyz = calculate_initial_position(alt_seed, **custom_seed**);
		TRANSFORM = EMISSION_TRANSFORM * TRANSFORM;
	}

	if (RESTART_VELOCITY) {
		...
		int point = min(emission_texture_point_count - 1, int(**custom_seed** * float(emission_texture_point_count)));
		ivec2 emission_tex_size = textureSize(emission_texture_points, 0);
		ivec2 emission_tex_ofs = ivec2(point % emission_tex_size.x, point / emission_tex_size.x);
		{
			mat2 rotm;
			rotm[0] = texelFetch(emission_texture_normal, emission_tex_ofs, 0).xy;
			rotm[1] = rotm[0].yx * vec2(1.0, -1.0);
			VELOCITY.xy = rotm * VELOCITY.xy;
		}

Steps to reproduce

1. Add a gpu particle 2D to your scene
2. Choose 'Load emission mask'
3. Select pixel color
4. Select emission border oriented

Minimal reproduction project (MRP)

N/A