Neon Box Border Glow

shader_type canvas_item;

// === Neon Box Border Glow Shader ===
// A clean, rewritten rectangular glow effect.
// Features an animated two-color gradient (default white & purple)
// travelling along the borders of the bounding box.

// Colors for the animated gradient
uniform vec4 color1 : source_color = vec4(1.0, 1.0, 1.0, 1.0); // White
uniform vec4 color2 : source_color = vec4(0.6, 0.2, 1.0, 1.0); // Purple

// Glow settings
uniform float thickness : hint_range(1.0, 50.0) = 8.0;
uniform float corner_radius : hint_range(0.0, 100.0) = 12.0;
uniform float speed : hint_range(0.0, 10.0) = 3.0;
uniform float intensity : hint_range(1.0, 5.0) = 2.0;

// Size of the node in pixels. 
// For Control nodes, pass `size` from GDScript. 
// For Sprites, pass `texture.get_size()`.
// Example: material.set_shader_parameter("rect_size", size)
uniform vec2 rect_size = vec2(64.0, 64.0);

void fragment() {
	// Current pixel coordinate mapped from UV
	vec2 px = UV * rect_size;
	
	// Center coordinate
	vec2 center_px = px - rect_size * 0.5;
	vec2 half_size = rect_size * 0.5;
	
	// Rounded Box SDF (Signed Distance Field)
	// Calculates distance to the nearest edge of a rounded rectangle.
	// Negative values are inside the box, positive are outside.
	vec2 q = abs(center_px) - half_size + vec2(corner_radius);
	float dist_to_box = min(max(q.x, q.y), 0.0) + length(max(q, 0.0)) - corner_radius;
	
	// We want the distance FROM the edge inwards. 
	// Values outside the box (positive dist) will have negative min_d.
	float min_d = -dist_to_box;
	
	if (min_d < 0.0 || min_d > thickness) {
		// Outside the rounded box (corners) OR deep inside the border
		COLOR = vec4(0.0);
	} else {
		// Create an animated pulsing gradient
		// Using a combination of X and Y for a diagonal sweep effect
		float t = TIME * speed;
		float pos_factor = (px.x + px.y) / max(rect_size.x + rect_size.y, 1.0);
		
		// Sine wave mapped to 0..1 for blending
		float blend = (sin(t - pos_factor * 6.28318) + 1.0) * 0.5;
		vec3 color_mix = mix(color1.rgb, color2.rgb, blend);
		
		// Soft gradient alpha: 1.0 at the extreme edge, fading to 0.0 at the inner boundary
		float edge_alpha = 1.0 - smoothstep(0.0, thickness, min_d);
		edge_alpha = sqrt(edge_alpha); // Non-linear curve to soften the glow falloff
		
		// Final output
		COLOR = vec4(color_mix * intensity, edge_alpha);
	}
}