Fullscreen Raymarching with Depth Check

For Godot 4.3.

Modified from this fullscreen raymarching shader.

Allows raymarched objects to intersect with world objects.

Follow this guide to create a fullscreen quad for the effect.

Shader code

shader_type spatial;
render_mode unshaded;

const int MAX_STEPS = 300;
const float MAX_DISTANCE = 1000.0;
const float MIN_DISTANCE = 0.001;

uniform sampler2D DEPTH_TEXTURE : source_color, hint_depth_texture;

void vertex() {
    POSITION = vec4(VERTEX.xy, 1.0, 1.0);
}

float sdSphere (vec3 p, vec3 centrum, float radius) {
    return length(centrum-p) - radius;
}

// infinte repetitions
// adapted from https://www.iquilezles.org/www/articles/distfunctions/distfunctions.htm
vec3 opRep(vec3 p, vec3 c) {
    vec3 q = mod(p+0.5*c,c)-0.5*c;
    return q;
}

float get_distance(vec3 p) {
    p = opRep(p, vec3(3)); // uncomment for repeating spheres
    return sdSphere(p, vec3(0), 1.0);
}

vec3 get_normal(vec3 p) {
    vec2 e = vec2(1e-3, 0);
    
    vec3 n = get_distance(p) - vec3(
        get_distance(p - e.xyy),
        get_distance(p - e.yxy),
        get_distance(p - e.yyx)
    );
    
    return normalize(n);
}

vec4 raymarch(vec3 ray_origin, vec3 ray_dir, float depth) {
    float t = 0.0;
    vec3 p = ray_origin;
    for (int i = 0; i < MAX_STEPS; i++)
    {
        float d = get_distance(p);
        t += d;
        if (t > MAX_DISTANCE || t >= depth)
            break;
        
        p += d * ray_dir;
        if (abs(d)  < MIN_DISTANCE)
            return vec4(get_normal(p), 1);
     }
    //return vec4(float(i) * 0.01); // uncomment for simple glow effect
    return vec4(0.0);
}

void fragment() {
    float depth = texture(DEPTH_TEXTURE, SCREEN_UV).x;
    vec3 ndc = vec3(SCREEN_UV * 2.0 - 1.0, depth);
    vec4 view = INV_PROJECTION_MATRIX * vec4(ndc, 1.0);
    view.xyz /= view.w;
    float linear_depth = -view.z;
    
    vec3 weird_uv = vec3(SCREEN_UV * 2.0 - 1.0, 0.0);
    vec4 camera = INV_VIEW_MATRIX * INV_PROJECTION_MATRIX * vec4(weird_uv, 1);
    
    vec3 ray_origin = INV_VIEW_MATRIX[3].xyz;
    vec3 ray_dir = normalize(camera.xyz);
    
    vec4 col = raymarch(ray_origin, ray_dir, linear_depth);
    
    ALPHA = col.a;
    ALBEDO = col.rgb;    
}

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