给Particles加点外力

Godot Engine 3.1.2 这篇文章中提到 Godot Engine的未来版本中粒子系统将支持外力 ，看了一下当前版本中自动生成的ParticlesShader ，其实Gravity就已经算是外力了，把它简单改一下实现了下面的效果，其实就是实时修改这个外力的方向。

Shader

这段Shader代码基本上都是自动生成的

shader_type particles;
uniform float spread;
uniform float flatness;
uniform float initial_linear_velocity;
uniform float initial_angle;
uniform float angular_velocity;
uniform float orbit_velocity;
uniform float linear_accel;
uniform float radial_accel;
uniform float tangent_accel;
uniform float damping;
uniform float scale;
uniform float hue_variation;
uniform float anim_speed;
uniform float anim_offset;
uniform float initial_linear_velocity_random;
uniform float initial_angle_random;
uniform float angular_velocity_random;
uniform float orbit_velocity_random;
uniform float linear_accel_random;
uniform float radial_accel_random;
uniform float tangent_accel_random;
uniform float damping_random;
uniform float scale_random;
uniform float hue_variation_random;
uniform float anim_speed_random;
uniform float anim_offset_random;
uniform vec4 color_value : hint_color;
uniform int trail_divisor;
uniform vec3 external_force;


float rand_from_seed(inout uint seed) {
	int k;
	int s = int(seed);
	if (s == 0)
	s = 305420679;
	k = s / 127773;
	s = 16807 * (s - k * 127773) - 2836 * k;
	if (s > uint(16)) ^ x) * uint(73244475);
	x = ((x >> uint(16)) ^ x) * uint(73244475);
	x = (x >> uint(16)) ^ x;
	return x;
}

void vertex() {
	uint base_number = NUMBER / uint(trail_divisor);
	uint alt_seed = hash(base_number + uint(1) + RANDOM_SEED);
	float angle_rand = rand_from_seed(alt_seed);
	float scale_rand = rand_from_seed(alt_seed);
	float hue_rot_rand = rand_from_seed(alt_seed);
	float anim_offset_rand = rand_from_seed(alt_seed);
	float pi = 3.14159;
	float degree_to_rad = pi / 180.0;

	if (RESTART) {
		float tex_linear_velocity = 0.0;
		float tex_angle = 0.0;
		float tex_anim_offset = 0.0;
		float spread_rad = spread * degree_to_rad;
		float angle1_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad;
		float angle2_rad = rand_from_seed_m1_p1(alt_seed) * spread_rad * (1.0 - flatness);
		vec3 direction_xz = vec3(sin(angle1_rad), 0.0, cos(angle1_rad));
		vec3 direction_yz = vec3(0.0, sin(angle2_rad), cos(angle2_rad));
		direction_yz.z = direction_yz.z / max(0.0001,sqrt(abs(direction_yz.z))); // better uniform distribution
		vec3 direction = vec3(direction_xz.x * direction_yz.z, direction_yz.y, direction_xz.z * direction_yz.z);
		direction = normalize(direction);
		VELOCITY = direction * initial_linear_velocity * mix(1.0, rand_from_seed(alt_seed), initial_linear_velocity_random);
		float base_angle = (initial_angle + tex_angle) * mix(1.0, angle_rand, initial_angle_random);
		CUSTOM.x = base_angle * degree_to_rad;
		CUSTOM.y = 0.0;
		CUSTOM.z = (anim_offset + tex_anim_offset) * mix(1.0, anim_offset_rand, anim_offset_random);
		VELOCITY = (EMISSION_TRANSFORM * vec4(VELOCITY, 0.0)).xyz;
		TRANSFORM = EMISSION_TRANSFORM * TRANSFORM;
	} else {
		CUSTOM.y += DELTA / LIFETIME;
		float tex_linear_velocity = 0.0;
		float tex_angular_velocity = 0.0;
		float tex_linear_accel = 0.0;
		float tex_radial_accel = 0.0;
		float tex_tangent_accel = 0.0;
		float tex_damping = 0.0;
		float tex_angle = 0.0;
		float tex_anim_speed = 0.0;
		float tex_anim_offset = 0.0;
		vec3 pos = TRANSFORM[3].xyz;
		// apply linear acceleration
		vec3 force = external_force;
		force += length(VELOCITY) > 0.0 ? normalize(VELOCITY) * (linear_accel + tex_linear_accel) * mix(1.0, rand_from_seed(alt_seed), linear_accel_random) : vec3(0.0);
		// apply radial acceleration
		vec3 org = EMISSION_TRANSFORM[3].xyz;
		vec3 diff = pos - org;
		force += length(diff) > 0.0 ? normalize(diff) * (radial_accel + tex_radial_accel) * mix(1.0, rand_from_seed(alt_seed), radial_accel_random) : vec3(0.0);
		// apply tangential acceleration;
		vec3 crossDiff = cross(normalize(diff), normalize(external_force));
		force += length(crossDiff) > 0.0 ? normalize(crossDiff) * ((tangent_accel + tex_tangent_accel) * mix(1.0, rand_from_seed(alt_seed), tangent_accel_random)) : vec3(0.0);
		// apply attractor forces
		VELOCITY += force * DELTA;
		// orbit velocity
		if (damping + tex_damping > 0.0) {
			float v = length(VELOCITY);
			float damp = (damping + tex_damping) * mix(1.0, rand_from_seed(alt_seed), damping_random);
			v -= damp * DELTA;
			if (v