Ray-AABB (Axis Aligned Bounding Box) intersection.

intersectAABB.glsl

// adapted from intersectCube in https://github.com/evanw/webgl-path-tracing/blob/master/webgl-path-tracing.js

// compute the near and far intersections of the cube (stored in the x and y components) using the slab method
// no intersection means vec.x > vec.y (really tNear > tFar)
vec2 intersectAABB(vec3 rayOrigin, vec3 rayDir, vec3 boxMin, vec3 boxMax) {
    vec3 tMin = (boxMin - rayOrigin) / rayDir;
    vec3 tMax = (boxMax - rayOrigin) / rayDir;
    vec3 t1 = min(tMin, tMax);
    vec3 t2 = max(tMin, tMax);
    float tNear = max(max(t1.x, t1.y), t1.z);
    float tFar = min(min(t2.x, t2.y), t2.z);
    return vec2(tNear, tFar);
};

What output is expected when rayDir has some zero components causing division by zero?

@themightyoarfish as long as the min and max functions prefer returning non-NaN's it should work as intended. This code has been written quite a while ago so you might want to check whether it behaves as expected on your target device. if not, you may need to filter out those when computing tNear and tFar.

Leaving this here if anyone needed this for C# with 3 dimensional vectors.
SIMD optimized implementation that returns true if an intersection was found.

public bool Intersects(Ray ray)
{
    Vector4 tMin = (MinPosition - ray.Origin) / ray.Direction;
    Vector4 tMax = (MaxPosition - ray.Origin) / ray.Direction;
    Vector128<float> t1 = Vector4.Min(tMin, tMax).AsVector128();
    Vector128<float> t2 = Vector4.Max(tMin, tMax).AsVector128();

    Vector128<float> tNear = Sse.Max(Sse.Max(Sse.MoveHighToLow(t1, t1),
                                             Sse.Shuffle(t1, t1, 0b00_00_11_01)),
                                     t1);

    Vector128<float> tFar = Sse.Min(Sse.Min(Sse.MoveHighToLow(t2, t2),
                                            Sse.Shuffle(t2, t2, 0b00_00_11_01)),
                                    t2);

    return tNear.GetElement(0) <= tFar.GetElement(0) && tFar.GetElement(0) >= 0;
}

Obviously if you need an optimal solution for a single ray to check many boxes then get rid of the division operators.

public bool Intersects(RayAxisAlignBoxOptimizedIntersection optimizedRay)
{
    Vector4 tMin = (MinPosition - optimizedRay.Start) * optimizedRay.InverseDirection;
    Vector4 tMax = (MaxPosition - optimizedRay.Start) * optimizedRay.InverseDirection;

That type name is absurdly long.