Array out of range

[oxine]

raytracing.github.io/books/RayTracingTheNextWeek.html

Line 1099 in 00d0ab0

return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];

i j k can be minus value so would leads to error

[hollasch]

Original code was:

float noise(const vec3& p) const {

    float u = p.x() - floor(p.x());
    float v = p.y() - floor(p.y());
    float w = p.z() - floor(p.z());

    int i = int(4*p.x()) & 255;
    int j = int(4*p.y()) & 255;
    int k = int(4*p.z()) & 255;

    return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];
}

Current code (master; no changes in development branch) is:

double noise(const vec3& p) const {

    auto u = p.x() - floor(p.x());
    auto v = p.y() - floor(p.y());
    auto w = p.z() - floor(p.z());

    auto i = static_cast<int>(floor(p.x()));
    auto j = static_cast<int>(floor(p.y()));
    auto k = static_cast<int>(floor(p.z()));

    return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];
}

We lost the scaling by four, plus selection of the lower 8 bits, which handled negative values. Need to track down the evolution of this code.

[hollasch]

Well … that was fun. I had to go back to the archives for the original repository for The Next Week. All changes are in the new unified repository, but the combination of multiple streams of development makes tracking down the code development for this chunk non-trivial.

Anyway, this code has been broken from the very start in the repo when I first introduced it in commit d1ecb9d, in 2019-07-28. It was basically a transcription error on my part, and I'm a bit surprised that no-one's reported it until now (thank you @oxine!).

Will fix for upcoming v3.0.0 release.

[hollasch]

For the record, my original (bad) transcription was:

float noise(const vec3& p) const {
    float u = p.x() - floor(p.x());
    float v = p.y() - floor(p.y());
    float w = p.z() - floor(p.z());

    int i = floor(p.x());
    int j = floor(p.y());
    int k = floor(p.z());

    return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];
}

[hollasch]

@oxine — can you verify the following works? Thanks.

double noise(const vec3& p) const {
    auto u = p.x() - floor(p.x());
    auto v = p.y() - floor(p.y());
    auto w = p.z() - floor(p.z());

    auto i = static_cast<int>(4*p.x()) & 255;
    auto j = static_cast<int>(4*p.y()) & 255;
    auto k = static_cast<int>(4*p.z()) & 255;

    return ranfloat[perm_x[i] ^ perm_y[j] ^ perm_z[k]];
}

[oxine]

Verified，now it works well.