Particle based trail

A continuous trail effect for creating sword slashes, light sabers, etc. It works by packing trail data into the TRANSFORM, making it accessible by the draw pass shader.

This effect should work in Godot 3, and Godot 4 with minor changes.

SET UP:

1. Add a new GPU Particles node into the scene
2. Set Fixed FPS greater than 0, and set Speed Scale to the same value as Fixed FPS. This number determines the length of each step in the trail
3. Set Amount to a power of 2 (e.g. 8, 16, 32, 64), and set Lifetime to the same value as Amount. This number determines the number of steps in the trail
4. Set Local Coords to false
5. Assign the particle shader to the Process Material slot
6. Add a new QuadMesh to the draw pass, and assign the following spatial shader. This is required to render the trail

   shader_type spatial;
   
   render_mode unshaded,blend_add,world_vertex_coords,cull_disabled;
   
   uniform sampler2D tex;
   
   void vertex(){
   	vec3 a = mix(WORLD_MATRIX[3].xyz,WORLD_MATRIX[2].xyz,UV.y);
   	vec3 b = mix(WORLD_MATRIX[0].xyz,WORLD_MATRIX[1].xyz,UV.y);
   	VERTEX = mix(a,b,UV.x);
   	
   	UV.x = (INSTANCE_CUSTOM.w-1.0 - UV.x)/INSTANCE_CUSTOM.z;
   }
   
   void fragment(){
   	vec4 T = texture(tex,UV);
   	ALBEDO = T.xyz;
   	ALPHA = T.w;
   }

USAGE:

You need to restart emission after changing settings

Pass a texture or color ramp to the draw pass shader

The trail emission line segment is defined between -1 to 1 of the local Y axis, scale the GPU Particals node as required

Shader code

shader_type particles;

render_mode keep_data,disable_force,disable_velocity;

void vertex() {
	vec3 a = (EMISSION_TRANSFORM*vec4(0,-1,0,1)).xyz;
	vec3 b = (EMISSION_TRANSFORM*vec4(0,1,0,1)).xyz;
	
	if(CUSTOM.w == LIFETIME){
		CUSTOM.z = LIFETIME;
		CUSTOM.w = 0.0;
	}
	
	if(CUSTOM.w == 0.0){
		TRANSFORM[0].xyz = b;
		TRANSFORM[1].xyz = a;
		TRANSFORM[2].xyz = a;
		TRANSFORM[3].xyz = b;
	}
	if(CUSTOM.w == 1.0){
		TRANSFORM[0].xyz = b;
		TRANSFORM[1].xyz = a;
	}
	
	CUSTOM.w++;
}

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