Unexpected `compMin` behavior.
MaxSac opened this issue · comments
I tried to calculate the compMin
from a vector which includes a NaN
.
The result seems to be dependent on the position of the NaN
in the vector.
It may be a different behavior than the c++ standard suggests [IEEE floating-point arithmetic (IEC 60559)] (and my naive intuition 😆 ).
#include <glm/glm.hpp>
#include <glm/gtx/component_wise.hpp>
#include <iostream>
#include <limits>
int main(int argc, char* argv[])
{
// Option I.
std::cout << glm::compMin(glm::vec2(std::numeric_limits<float>::quiet_NaN(), 1.f)) << std::endl;
// >>> nan
std::cout << glm::compMin(glm::vec2(1.f, std::numeric_limits<float>::quiet_NaN())) << std::endl;
// >>> 1
// Option II.
std::cout << std::fmin(std::numeric_limits<float>::quiet_NaN(), 1.f) << std::endl;
// >>> 1
std::cout << std::fmin(1.f, std::numeric_limits<float>::quiet_NaN()) << std::endl;
// >>> 1
return 0;
}
Is this kind of functionality intentional?
GLSL defines min as:
Returns y if y < x; otherwise it returns x
which should match std::min
's behaviour (not std::fmin
). Unfortunately, the inconsistency in special-value behaviour between IEEE-operators vs ternary-logic will emerge elsewhere too.
Erm, the comment, the example don't match. Without clarification, I will just close this issue. Maybe that's a feature request for a fcompMin
function. unclear.
Sorry for the incorrect minimal working example. I wrote it yesterday in a hurry and have now corrected it.
To give a little bit of background.
I tried to calculate the interaction distance from a ray to a box, as sketched in the picture.
I wrote the following code and was quite surprised that a) and b) gave different results.
#include <array>
#include <glm/glm.hpp>
#include <glm/gtx/component_wise.hpp>
#include <iostream>
#include <limits>
std::array<float, 2> intersection(glm::vec2 corner1, glm::vec2 corner2,
glm::vec2 origin, glm::vec2 invDirection)
{
auto tMin = (corner1 - origin) * invDirection;
auto tMax = (corner2 - origin) * invDirection;
auto tEnter = glm::min(tMin, tMax);
auto tExit = glm::max(tMin, tMax);
auto enterMax = glm::compMax(tEnter);
auto exitMin = glm::compMin(tExit);
return std::array<float, 2> { enterMax, exitMin };
}
int main(int argc, char* argv[])
{
auto c1 = glm::vec2(1, 1);
auto c2 = glm::vec2(2, 2);
// a)
auto origin1 = glm::vec2(1, 0);
auto invDirection1 = glm::vec2(std::numeric_limits<float>::quiet_NaN(), 1.f);
auto intersection_points1 = intersection(c1, c2, origin1, invDirection1);
std::cout << "entry: " << intersection_points1[0] << ", exit: " << intersection_points1[1] << std::endl;
// >>> entry: nan, exit: nan
// b)
auto origin2 = glm::vec2(0, 1);
auto invDirection2 = glm::vec2(1.f, std::numeric_limits<float>::quiet_NaN());
auto intersection_points2 = intersection(c1, c2, origin2, invDirection2);
std::cout << "entry: " << intersection_points2[0] << ", exit: " << intersection_points2[1] << std::endl;
// >>> entry: 1, exit: 2
return 0;
}
Technically I can understand what happens, but it was against my naive expectation.
I guess using something like fcompMax
would help in this case and would be a nice feature.