From e06b096994704f1e4e428135bc43b2487f77e6ff Mon Sep 17 00:00:00 2001
From: Morris Tabor <80684659+mortarroad@users.noreply.github.com>
Date: Thu, 25 Mar 2021 17:26:23 +0100
Subject: [PATCH] Fix ParticlesMaterial spread

---
 doc/classes/ParticlesMaterial.xml      |  4 ++--
 scene/resources/particles_material.cpp | 17 ++++++++++++-----
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/doc/classes/ParticlesMaterial.xml b/doc/classes/ParticlesMaterial.xml
index 0223379458f..63bd4ca3c6c 100644
--- a/doc/classes/ParticlesMaterial.xml
+++ b/doc/classes/ParticlesMaterial.xml
@@ -180,7 +180,7 @@
 			If [code]true[/code], particles rotate around Y axis by [member angle].
 		</member>
 		<member name="flatness" type="float" setter="set_flatness" getter="get_flatness" default="0.0">
-			Amount of [member spread] in Y/Z plane. A value of [code]1[/code] restricts particles to X/Z plane.
+			Amount of [member spread] along the Y axis.
 		</member>
 		<member name="gravity" type="Vector3" setter="set_gravity" getter="get_gravity" default="Vector3( 0, -9.8, 0 )">
 			Gravity applied to every particle.
@@ -241,7 +241,7 @@
 			Scale randomness ratio.
 		</member>
 		<member name="spread" type="float" setter="set_spread" getter="get_spread" default="45.0">
-			Each particle's initial direction range from [code]+spread[/code] to [code]-spread[/code] degrees. Applied to X/Z plane and Y/Z planes.
+			Each particle's initial direction range from [code]+spread[/code] to [code]-spread[/code] degrees.
 		</member>
 		<member name="tangential_accel" type="float" setter="set_param" getter="get_param" default="0.0">
 			Tangential acceleration applied to each particle. Tangential acceleration is perpendicular to the particle's velocity giving the particles a swirling motion.
diff --git a/scene/resources/particles_material.cpp b/scene/resources/particles_material.cpp
index 18260d6c39d..f872f57a0ab 100644
--- a/scene/resources/particles_material.cpp
+++ b/scene/resources/particles_material.cpp
@@ -319,14 +319,21 @@ void ParticlesMaterial::_update_shader() {
 		//initiate velocity spread in 3D
 		code += "		float angle1_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad;\n";
 		code += "		float angle2_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad * (1.0 - flatness);\n";
-		code += "		angle1_rad += direction.z != 0.0 ? atan(direction.x, direction.z) : sign(direction.x) * (pi / 2.0);\n";
-		code += "		angle2_rad += direction.z != 0.0 ? atan(direction.y, abs(direction.z)) : (direction.x != 0.0 ? atan(direction.y, abs(direction.x)) : sign(direction.y) * (pi / 2.0));\n";
 		code += "		vec3 direction_xz = vec3(sin(angle1_rad), 0.0, cos(angle1_rad));\n";
 		code += "		vec3 direction_yz = vec3(0.0, sin(angle2_rad), cos(angle2_rad));\n";
 		code += "		direction_yz.z = direction_yz.z / max(0.0001,sqrt(abs(direction_yz.z))); // better uniform distribution\n";
-		code += "		vec3 vec_direction = vec3(direction_xz.x * direction_yz.z, direction_yz.y, direction_xz.z * direction_yz.z);\n";
-		code += "		vec_direction = normalize(vec_direction);\n";
-		code += "		VELOCITY = vec_direction * initial_linear_velocity * mix(1.0, rand_from_seed(alt_seed), initial_linear_velocity_random);\n";
+		code += "		vec3 spread_direction = vec3(direction_xz.x * direction_yz.z, direction_yz.y, direction_xz.z * direction_yz.z);\n";
+		code += "		vec3 direction_nrm = normalize(direction);\n";
+		code += "		// rotate spread to direction\n";
+		code += "		vec3 binormal = cross(vec3(0.0, 1.0, 0.0), direction_nrm);\n";
+		code += "		if (length(binormal) < 0.0001) {\n";
+		code += "			// direction is parallel to Y. Choose Z as the binormal.\n";
+		code += "			binormal = vec3(0.0, 0.0, 1.0);\n";
+		code += "		}\n";
+		code += "		binormal = normalize(binormal);\n";
+		code += "		vec3 normal = cross(binormal, direction_nrm);\n";
+		code += "		spread_direction = binormal * spread_direction.x + normal * spread_direction.y + direction_nrm * spread_direction.z;\n";
+		code += "		VELOCITY = spread_direction * initial_linear_velocity * mix(1.0, rand_from_seed(alt_seed), initial_linear_velocity_random);\n";
 	}
 
 	code += "		float base_angle = (initial_angle + tex_angle) * mix(1.0, angle_rand, initial_angle_random);\n";