Merge pull request godotengine#91223 from Arnklit/particle-cylinder-emission-distribution-fix

Fix particle cylinder emission shape random distribution

Author: akien-mga

scene/3d/cpu_particles_3d.cpp

@@ -880,7 +880,7 @@ void CPUParticles3D::_particles_process(double p_delta) {
 				} break;
 				case EMISSION_SHAPE_RING: {
 					real_t ring_random_angle = Math::randf() * Math_TAU;
-					real_t ring_random_radius = Math::randf() * (emission_ring_radius - emission_ring_inner_radius) + emission_ring_inner_radius;
+					real_t ring_random_radius = Math::sqrt(Math::randf() * (emission_ring_radius - emission_ring_inner_radius * emission_ring_inner_radius) + emission_ring_inner_radius * emission_ring_inner_radius);
 					Vector3 axis = emission_ring_axis == Vector3(0.0, 0.0, 0.0) ? Vector3(0.0, 0.0, 1.0) : emission_ring_axis.normalized();
 					Vector3 ortho_axis;
 					if (axis.abs() == Vector3(1.0, 0.0, 0.0)) {

scene/resources/particle_process_material.cpp

@@ -634,7 +634,7 @@ void ParticleProcessMaterial::_update_shader() {
 	if (emission_shape == EMISSION_SHAPE_RING) {
 		code += "		\n";
 		code += "		float ring_spawn_angle = rand_from_seed(alt_seed) * 2.0 * pi;\n";
-		code += "		float ring_random_radius = rand_from_seed(alt_seed) * (emission_ring_radius - emission_ring_inner_radius) + emission_ring_inner_radius;\n";
+		code += "		float ring_random_radius = sqrt(rand_from_seed(alt_seed) * (emission_ring_radius - emission_ring_inner_radius * emission_ring_inner_radius) + emission_ring_inner_radius * emission_ring_inner_radius);\n";
 		code += "		vec3 axis = emission_ring_axis == vec3(0.0) ? vec3(0.0, 0.0, 1.0) : normalize(emission_ring_axis);\n";
 		code += "		vec3 ortho_axis = vec3(0.0);\n";
 		code += "		if (abs(axis) == vec3(1.0, 0.0, 0.0)) {\n";