Skip to content

Instantly share code, notes, and snippets.

@apples
Created March 24, 2021 19:58
Show Gist options
  • Save apples/ec0f0a76d3b2270582f158a2b879c61a to your computer and use it in GitHub Desktop.
Save apples/ec0f0a76d3b2270582f158a2b879c61a to your computer and use it in GitHub Desktop.
Quaternion (un)packing function, smallest-three representation.
#pragma once
#include <glm/gtc/quaternion.hpp>
#include <cassert>
#include <cmath>
#include <cstdint>
/** Creates smallest-three packing of quaternion, 10 bits per component */
inline auto pack_quaternion(glm::quat q) -> std::uint32_t {
constexpr auto quantize_max = 0b11'1111'1111u;
// [0..1]: index of largest
// [2..11]: first smallest
// [12..21]: second smallest
// [22..31]: third smallest
auto pack = std::uint32_t{};
auto is_biggest = [&](int i) -> bool {
auto x = std::abs(q[i]);
for (int j = 0; j < 4; ++j) {
if (j == i) {
continue;
}
auto y = std::abs(q[j]);
if (x < y) {
return false;
}
}
return true;
};
auto biggest = ([&] {
for (int i = 0; i < 4; ++i) {
if (is_biggest(i)) {
return i;
}
}
return 0;
})();
if (q[biggest] < 0) {
q = -q;
}
pack |= biggest;
for (auto i = 0; i < 3; ++i) {
auto e = q[i >= biggest ? i + 1 : i];
assert(e <= q[biggest]);
auto quantized = static_cast<std::uint32_t>((e + 1.f) / 2.f * quantize_max);
assert((quantized & quantize_max) == quantized);
pack |= quantized << (2 + 10 * i);
assert((pack >> (2 + 10 * i)) == quantized);
}
return pack;
}
/** Unpacks quaternion from smallest-three packing, 10 bits per component */
inline auto unpack_quaternion(std::uint32_t p) -> glm::quat {
constexpr auto quantize_max = 0b11'1111'1111u;
auto quat = glm::quat{};
auto biggest = p & 0b11;
auto acc = 0.f;
for (auto i = 0; i < 3; ++i) {
auto quantized = (p >> (2 + 10 * i)) & quantize_max;
auto v = float(quantized) / float(quantize_max) * 2.f - 1.f;
assert(v >= -1.f && v <= 1.f);
acc += v * v;
quat[i >= biggest ? i + 1 : i] = v;
}
quat[biggest] = std::sqrt(1.f - acc);
assert(quat[biggest] >= -1.f && quat[biggest] <= 1.f);
return glm::normalize(quat);
}
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment