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111 lines
4.7 KiB
Markdown
111 lines
4.7 KiB
Markdown
# `omath::opengl_engine::CameraTrait` — plug-in trait for `projection::Camera`
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> Header: `omath/engines/opengl_engine/traits/camera_trait.hpp` • Impl: `omath/engines/opengl_engine/traits/camera_trait.cpp`
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> Namespace: `omath::opengl_engine`
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> Purpose: provide OpenGL-style **look-at**, **view**, and **projection** math to the generic `omath::projection::Camera` (satisfies `CameraEngineConcept`).
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---
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## Summary
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`CameraTrait` exposes three `static` functions:
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* `calc_look_at_angle(origin, look_at)` – computes Euler angles so the camera at `origin` looks at `look_at`. Implementation normalizes the direction, computes **pitch** as `asin(dir.y)` and **yaw** as `-atan2(dir.x, -dir.z)`; **roll** is `0`. Pitch/yaw are returned using the project's strong angle types (`PitchAngle`, `YawAngle`, `RollAngle`).
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* `calc_view_matrix(angles, origin)` – delegates to OpenGL formulas `opengl_engine::calc_view_matrix`, producing a `Mat4X4` view matrix (column-major) for the given angles and origin.
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* `calc_projection_matrix(fov, viewport, near, far)` – builds a perspective projection by calling `calc_perspective_projection_matrix(fov_degrees, aspect, near, far)`, where `aspect = viewport.aspect_ratio()`. Accepts `FieldOfView` (degrees).
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The trait's types (`ViewAngles`, `Mat4X4`, angle aliases) and helpers live in the OpenGL math headers included by the trait (`formulas.hpp`) and the shared projection header (`projection/camera.hpp`).
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---
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## API
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```cpp
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namespace omath::opengl_engine {
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class CameraTrait final {
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public:
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// Compute Euler angles (pitch/yaw/roll) to look from cam_origin to look_at.
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static ViewAngles
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calc_look_at_angle(const Vector3<float>& cam_origin,
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const Vector3<float>& look_at) noexcept;
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// Build view matrix for given angles and origin (column-major).
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static Mat4X4
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calc_view_matrix(const ViewAngles& angles,
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const Vector3<float>& cam_origin) noexcept;
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// Build perspective projection from FOV (deg), viewport, near/far (column-major).
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static Mat4X4
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calc_projection_matrix(const projection::FieldOfView& fov,
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const projection::ViewPort& view_port,
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float near, float far) noexcept;
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};
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} // namespace omath::opengl_engine
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```
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Uses: `Vector3<float>`, `ViewAngles` (pitch/yaw/roll), `Mat4X4`, `projection::FieldOfView`, `projection::ViewPort`.
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---
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## Behavior & conventions
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* **Angles from look-at** (Y-up, -Z forward coordinate system):
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```
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dir = normalize(look_at - origin)
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pitch = asin(dir.y) // +Y is up
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yaw = -atan2(dir.x, -dir.z) // horizontal rotation
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roll = 0
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```
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Returned as `PitchAngle::from_radians(...)`, `YawAngle::from_radians(...)`, etc.
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* **View matrix**: built by the OpenGL helper `opengl_engine::calc_view_matrix(angles, origin)` to match OpenGL's right-handed, Y-up, -Z forward conventions. Matrix is **column-major**.
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* **Projection**: uses `calc_perspective_projection_matrix(fov.as_degrees(), viewport.aspect_ratio(), near, far)`. Pass your **vertical FOV** in degrees via `FieldOfView`; the helper computes a standard perspective matrix (column-major).
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---
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## Using with `projection::Camera`
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Create a camera whose math is driven by this trait:
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```cpp
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using Mat4 = Mat4X4; // from OpenGL math headers (column-major)
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using Angs = ViewAngles; // pitch/yaw/roll type
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using GLcam = omath::projection::Camera<Mat4, Angs, omath::opengl_engine::CameraTrait>;
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omath::projection::ViewPort vp{1920.f, 1080.f};
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auto fov = omath::projection::FieldOfView::from_degrees(45.f);
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GLcam cam(
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/*position*/ {5.f, 3.f, 5.f},
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/*angles*/ omath::opengl_engine::CameraTrait::calc_look_at_angle({5,3,5},{0,0,0}),
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/*viewport*/ vp,
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/*fov*/ fov,
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/*near*/ 0.1f,
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/*far*/ 100.f
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);
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```
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This satisfies `CameraEngineConcept` expected by `projection::Camera` (look-at, view, projection) as declared in the trait header.
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---
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## Notes & tips
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* Ensure your `ViewAngles` aliases (`PitchAngle`, `YawAngle`, `RollAngle`) match the project's angle policy (ranges/normalization). The implementation constructs them **from radians**.
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* `aspect_ratio()` is taken directly from `ViewPort` (`width / height`), so keep both positive and non-zero.
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* `near` must be > 0 and `< far` for a valid projection matrix (enforced by your math helpers).
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* OpenGL uses **Y-up, -Z forward**: pitch angles control vertical look (positive = up), yaw controls horizontal rotation.
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* Matrices are **column-major** (no transpose needed for OpenGL shaders).
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---
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## See also
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* OpenGL math helpers in `omath/engines/opengl_engine/formulas.hpp` (view/projection builders used above).
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* Generic camera wrapper `omath::projection::Camera` and its `CameraEngineConcept` (this trait is designed to plug straight into it).
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