glm::vec3 precision C++

Question

I'm creating a collision detection system in C++ using the glm library.

I have an array of vertices defined as std::vector<glm::vec3> vertices and a function to calculate the maxX, y, z defined as

GLfloat AssetInstance::maxX()
{
    GLfloat max = vertices.at(0).x;

    for(glm::vec3 vertex : vertices)
        if(vertex.x > max) max = vertex.x;

    return max;
}

but if I run the following code:

std::vector<glm::vec3> testVector;
testVector.push_back(glm::vec3(3.0500346, 1.0, 1.0));
testVector.push_back(glm::vec3(3.0500344, 2.0, 2.0));
testVector.push_back(glm::vec3(3.0500343, 3.0, 3.0));

std::cout << maxX(testVector) << std::endl;

the output is 3.05003

I thought that glm::vec3 was double and that double was more precise than that? Is there a better way to do this? My maxX isn't returning precise enough results.

Borgleader Borgleader · Accepted Answer · 2014-10-31T20:43:03

Try setprecision. By default its 6 which is what youre getting.

#include <iostream>
#include <iomanip>
#include <cmath>
#include <limits>
int main()
{
    std::cout << "default precision (6): " << 3.0500346 << '\n';
    std::cout << "std::precision(10):    " << std::setprecision(10) << 3.0500346 << '\n';

    return 0;
}

Output (Coliru link):

clang++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp && ./a.out
default precision (6): 3.05003
std::precision(10):    3.0500346

On another note, you're returning a GLFloat which is well a float, not a double, so no matter what glm uses, you're converting it to a float. So ultimately, calling maxX gives you float precision, not double.

P.S: Looking at the docs it looks like there is a dvec type which makes me doubt that vec uses double by default.

glm::vec3 precision C++

2 Answers