omath/source/engines/source_engine/formulas.cpp

//
// Created by Vlad on 3/19/2025.
//
#include <omath/engines/source_engine/formulas.hpp>

namespace omath::source_engine
{
    Vector3<float> forward_vector(const ViewAngles& angles) noexcept
    {
        const auto vec = rotation_matrix(angles) * mat_column_from_vector(k_abs_forward);

        return {vec.at(0, 0), vec.at(1, 0), vec.at(2, 0)};
    }

    Mat4X4 rotation_matrix(const ViewAngles& angles) noexcept
    {
        return mat_rotation_axis_z(angles.yaw) * mat_rotation_axis_y(angles.pitch) * mat_rotation_axis_x(angles.roll);
    }

    Vector3<float> right_vector(const ViewAngles& angles) noexcept
    {
        const auto vec = rotation_matrix(angles) * mat_column_from_vector(k_abs_right);

        return {vec.at(0, 0), vec.at(1, 0), vec.at(2, 0)};
    }
    Vector3<float> up_vector(const ViewAngles& angles) noexcept
    {
        const auto vec = rotation_matrix(angles) * mat_column_from_vector(k_abs_up);

        return {vec.at(0, 0), vec.at(1, 0), vec.at(2, 0)};
    }

    Mat4X4 calc_view_matrix(const ViewAngles& angles, const Vector3<float>& cam_origin) noexcept
    {
        return mat_camera_view(forward_vector(angles), right_vector(angles), up_vector(angles), cam_origin);
    }

    Mat4X4 calc_perspective_projection_matrix(const float field_of_view, const float aspect_ratio, const float near,
                                              const float far, const NDCDepthRange ndc_depth_range) noexcept
    {
        // Source (inherited from Quake) stores FOV as horizontal FOV at a 4:3
        // reference aspect. Convert to vertical FOV first, then use the
        // standard vfov-based projection against the caller's actual aspect.
        //   vfov = 2 · atan( tan(hfov_4:3 / 2) / (4/3) )
        constexpr float k_source_reference_aspect = 4.f / 3.f;
        const float half_hfov_4_3 = angles::degrees_to_radians(field_of_view) / 2.f;
        const float tan_half_vfov = std::tan(half_hfov_4_3) / k_source_reference_aspect;

        const float x_axis = 1.f / (aspect_ratio * tan_half_vfov);
        const float y_axis = 1.f / tan_half_vfov;

        if (ndc_depth_range == NDCDepthRange::ZERO_TO_ONE)
            return {
                    {x_axis, 0,      0,                     0},
                    {0,      y_axis, 0,                     0},
                    {0,      0,      far / (far - near),   -(near * far) / (far - near)},
                    {0,      0,      1,                     0},
            };
        if (ndc_depth_range == NDCDepthRange::NEGATIVE_ONE_TO_ONE)
            return {
                    {x_axis, 0,      0,                             0},
                    {0,      y_axis, 0,                             0},
                    {0,      0,      (far + near) / (far - near),  -(2.f * far * near) / (far - near)},
                    {0,      0,      1,                             0},
            };
        std::unreachable();
    }
} // namespace omath::source_engine