Bubblin

There are no spheres stored anywhere. map() twists a copy of the marching point, samples a single octave of simplex noise scrolling along the tunnel, and offsets it by the density bias. The result is read as a rough signed distance — negative inside a blob, positive in the gaps.

nfp.xy *= Rot(sin(TT+p.z));               // twist the field along the tunnel
float nf = simplex3d(nfp + vec3(0.,TT,0.)); // scrolling 3d simplex noise
nf += density - abs(nfp.y)*0.2;           // bias the threshold

Because it’s noise, not a real distance, the march has to creep — each step advances only 0.3× the reported distance so it never overshoots a thin membrane.

Raymarching fires a ray per pixel and steps a point along it, asking the field “how far to the nearest surface?” at each step. The trick that makes this shader glass is the distanceFactor: while the ray is travelling inside a bubble it flips the sign of the field, so the same map() now measures distance to the far wall from within.

ds = mapr.x;
ds *= distanceFactor;   // flip sdf when inside the glass
d += max(ds, DELTA);    // advance the march
if(ds<0.) { hit=true; break; }

When a ray crosses a skin it doesn’t stop — the outer loop runs it up to four times, each pass refracting it onward to the next surface.

This is the heart of it. At each hit the ray is refracted by the index ratio refraction 1.15. GLSL’s built-in refract returns a zero vector when the angle is too steep for light to escape — total internal reflection — and the shader falls back to a mirror bounce.

vec3 refraction = refract(rd, norm, refractionRatio);
if (dot(refraction, refraction) < DELTA) {  // total internal reflection
    rd = reflection;
} else {
    rd = refraction;
    refractionRatio = 1.0 / refractionRatio; // invert crossing the boundary
    distanceFactor = -distanceFactor;        // now marching inside
}

Each successful crossing inverts the index ratio (you’re now going glass→air instead of air→glass) and flips distanceFactor, so the loop seamlessly continues marching on the inside.

Surface colour is improvised from the hit normal and the procedural detail noise, tinted by three slightly-detuned sines that cycle along the tunnel depth. A single moving light adds a diffuse-plus-specular highlight on the outermost faces. The camera itself bobs camera bob 0.10 and tilts on lazy sine curves, so the drift never feels locked to a rail. fly speed 0.40 sets how fast you fall down the tunnel.

vec3 col = texture(iChannel0,uv).rgb;
col = pow(col, vec3(.7));    // gamma correction (tweaked)