Has anyone encounter this this stripping artifact during "RayTracing in one Weekend"?

Question

I am trying to port the "RayTracing in One Weekend" into metal compute shader. I encounter this strip artifacts in my project:

Is it because my random generator does not work well?

Does anyone have a clue?

// 参照 https://www.pcg-random.org/ 实现
typedef struct { uint64_t state;  uint64_t inc; } pcg32_random_t;

uint32_t pcg32_random_r(thread pcg32_random_t* rng)
{
    uint64_t oldstate = rng->state;
    rng->state = oldstate * 6364136223846793005ULL + rng->inc;
    uint32_t xorshifted = ((oldstate >> 18u) ^ oldstate) >> 27u;
    uint32_t rot = oldstate >> 59u;
    return (xorshifted >> rot) | (xorshifted << ((-rot) & 31));
}

void pcg32_srandom_r(thread pcg32_random_t* rng, uint64_t initstate, uint64_t initseq)
{
    rng->state = 0U;
    rng->inc = (initseq << 1u) | 1u;
    pcg32_random_r(rng);
    rng->state += initstate;
    pcg32_random_r(rng);
}

// 生成0～1之间的浮点数
float randomF(thread pcg32_random_t* rng)
{
    //return pcg32_random_r(rng)/float(UINT_MAX);
    return ldexp(float(pcg32_random_r(rng)), -32);
}

// 生成x_min ~ x_max之间的浮点数
float randomRange(thread pcg32_random_t* rng, float x_min, float x_max){
    return randomF(rng) * (x_max - x_min) + x_min;
}

Answer 1

Seeing these "circles aroune the image center" artifact is almost always a dead give-away for z-fighting (rays along any such "circle" always have the same distance to any flat object you're looking at, so they either all round up or all round down, giving you this artifact).

This z-fighting then translates to this artifact because sometimes all the rays on such a circle are inside the sphere (meaning their shadow rays get self-occluded by the sphere), or all outside (they do what they should). What you want to do is offset the ray origin of the shadow rays a tiny bit (say, 1e-3f) along the normal direction at the hitpoint.

You may also want to read up on Carsten Waechter's article in Ray Tracing Gems 1 - it's for triangles, but explains the problem and potential solution very well.

Answer 2

I found this link.It says that the primary ray hit point is either above or below the sphere's surface a litter bit due to the float precision error. It is a z-fighting problem