Trying to build a perlin noise generator and ran into bizarre looking results. I Boiled it down to the most basic example that showcases the issue for me.
#include <stdio.h>
#include "Halide.h"
using namespace Halide;
/* Basic 2d random function from https://thebookofshaders.com/11/ */
template <class T>
T rand(T x, T y)
{
T tx = x * 12.9898f; // 1 ok
T ty = y * 78.233f; // 2 ok
T dot = ty + tx; // 3 small error begins here
T sval = sin(dot); // 4 still small
T sval_mul = sval * 43758.5453123f; //5 small error * large number = large error
T fract = sval_mul - floor(sval_mul); // 6 yeah......
T Res = fract; /* use this to select an output , issue starts at 3
return Res;
}
#define size 10
int main(int argc, char **argv) {
Var x("x"), y("y");
Func r("r");
/* There is no issue if offsetx == 0.0f*/
float offsetx = 1.0f;
r(x, y) = rand<Expr>(x + offsetx, y);
Buffer<float> res = r.realize(size, size);
for (int ix = 0; ix < size; ix++)
{
for (int iy = 0; iy < size; iy++)
{
float fval = rand<float>(ix+offsetx, iy);
if (res(ix, iy) != fval)
{
printf("%d,%d got:%f expected:%f diff:%f\n", ix, iy, res(ix,iy), fval , fval - res(ix,iy));
}
}
}
return 0;
}
output on win64 (can't test on any other platforms sorry)
1,2 got:0.117188 expected:0.467773 diff:0.350586
2,1 got:0.449219 expected:0.257813 diff:-0.191406
2,3 got:0.813477 expected:0.068359 diff:-0.745117
2,4 got:0.268311 expected:0.934204 diff:0.665894
2,5 got:0.546875 expected:0.834961 diff:0.288086
2,6 got:0.187500 expected:0.064453 diff:-0.123047
3,1 got:0.152344 expected:0.273438 diff:0.121094
3,2 got:0.285156 expected:0.972656 diff:0.687500
3,6 got:0.858398 expected:0.500488 diff:-0.357910
4,7 got:0.631470 expected:0.301025 diff:-0.330444
4,8 got:0.103516 expected:0.583008 diff:0.479492
4,9 got:0.738281 expected:0.875000 diff:0.136719
5,6 got:0.035156 expected:0.994141 diff:0.958984
6,1 got:0.320313 expected:0.728516 diff:0.408203
6,2 got:0.867188 expected:0.335938 diff:-0.531250
6,4 got:0.238281 expected:0.460938 diff:0.222656
7,1 got:0.089600 expected:0.423096 diff:0.333496
7,3 got:0.923828 expected:0.880859 diff:-0.042969
7,5 got:0.097656 expected:0.726563 diff:0.628906
The core issue seems to be that the halide output for Rand(1 + 1.0f,1) and rand(2,1) are vastly different, sadly for perlin noise these two values are interpolated and you get very odd looking results like this:
While the reference implementation is fine.
Trying to build a perlin noise generator and ran into bizarre looking results. I Boiled it down to the most basic example that showcases the issue for me.
output on win64 (can't test on any other platforms sorry)
The core issue seems to be that the halide output for Rand(1 + 1.0f,1) and rand(2,1) are vastly different, sadly for perlin noise these two values are interpolated and you get very odd looking results like this:
While the reference implementation is fine.