shader_type spatial;
render_mode blend_mix, depth_draw_never, cull_disabled, specular_disabled;

uniform sampler2D albedo_tex : source_color;
uniform sampler2D depth_tex : hint_depth_texture, filter_linear_mipmap;
uniform vec3 core_bias_color : source_color = vec3(1.0, 0.6, 0.85);
uniform float core_bias_amount : hint_range(0.0, 1.0) = 0.35;
uniform float emission_strength : hint_range(0.0, 5.0) = 2.5;
uniform float soft_fade_distance : hint_range(0.0, 5.0) = 1.0;

varying float v_flicker;

void vertex() {
	// true billboard: strip rotation from the per-instance modelview, keep translation + scale
	mat4 mv = MODELVIEW_MATRIX;
	mv[0].xyz = vec3(length(mv[0].xyz), 0.0, 0.0);
	mv[1].xyz = vec3(0.0, length(mv[1].xyz), 0.0);
	mv[2].xyz = vec3(0.0, 0.0, length(mv[2].xyz));
	VERTEX = (mv * vec4(VERTEX, 1.0)).xyz;

	// INSTANCE_CUSTOM.x is GPUParticles3D's per-particle random seed, baked at spawn
	float seed = INSTANCE_CUSTOM.x;
	float flicker_hash = fract(sin((seed + floor(TIME * 6.0)) * 127.1) * 43758.5453);
	v_flicker = 0.85 + 0.15 * flicker_hash;
}

void fragment() {
	// fake a puff/sphere normal from local UV so a camera-facing billboard can still catch directional light;
	// the quad is billboarded to face the camera in vertex(), so this normal is already view-space aligned
	vec2 centered = UV * 2.0 - 1.0;
	float r2 = dot(centered, centered);
	float mask = clamp(1.0 - r2, 0.0, 1.0);
	NORMAL = normalize(vec3(centered, sqrt(max(mask, 0.001))));

	vec4 tex = texture(albedo_tex, UV);
	ALBEDO = tex.rgb * mix(vec3(1.0), core_bias_color, core_bias_amount);
	EMISSION = core_bias_color * tex.rgb * emission_strength * 0.15;
	ALPHA = tex.a * COLOR.a * mask;

	float raw_depth = texture(depth_tex, SCREEN_UV).r;
	vec3 ndc = vec3(SCREEN_UV * 2.0 - 1.0, raw_depth);
	vec4 view_pos = INV_PROJECTION_MATRIX * vec4(ndc, 1.0);
	view_pos.xyz /= view_pos.w;
	float scene_depth = -view_pos.z;
	float particle_depth = -VERTEX.z;
	ALPHA *= clamp((scene_depth - particle_depth) / soft_fade_distance, 0.0, 1.0);
}

void light() {
	float ndotl = clamp(dot(NORMAL, LIGHT), 0.0, 1.0);
	DIFFUSE_LIGHT += LIGHT_COLOR * ndotl * ALBEDO * emission_strength * v_flicker * ATTENUATION;
}