# view_state.rs - source
Source of the Rust file `maplibre/src/render/view_state.rs`.

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    use std::{
        f64,
        ops::{Deref, DerefMut},
    };

    use cgmath::{prelude::*, *};

    use crate::{
        coords::{ViewRegion, WorldCoords, Zoom, ZoomLevel},
        render::camera::{
            Camera, EdgeInsets, InvertedViewProjection, Perspective, ViewProjection, FLIP_Y,
            OPENGL_TO_WGPU_MATRIX,
        },
        util::{
            math::{bounds_from_points, Aabb2, Aabb3, Plane},
            ChangeObserver,
        },
        window::{LogicalSize, PhysicalSize},
    };

    const VIEW_REGION_PADDING: i32 = 1;
    const MAX_N_TILES: usize = 512;

    pub struct ViewState {
        zoom: ChangeObserver<Zoom>,
        camera: ChangeObserver<Camera>,
        perspective: Perspective,

        width: f64,
        height: f64,
        edge_insets: EdgeInsets,
    }

    impl ViewState {
        pub fn new<F: Into<Rad<f64>>, P: Into<Deg<f64>>>(
            window_size: PhysicalSize,
            position: WorldCoords,
            zoom: Zoom,
            pitch: P,
            fovy: F,
        ) -> Self {
            let camera = Camera::new((position.x, position.y), Deg(0.0), pitch.into());

            let perspective = Perspective::new(fovy);

            Self {
                zoom: ChangeObserver::new(zoom),
                camera: ChangeObserver::new(camera),
                perspective,
                width: window_size.width() as f64,
                height: window_size.height() as f64,
                edge_insets: EdgeInsets {
                    top: 0.0,
                    bottom: 0.0,
                    left: 0.0,
                    right: 0.0,
                },
            }
        }
        pub fn set_edge_insets(&mut self, edge_insets: EdgeInsets) {
            self.edge_insets = edge_insets;
        }

        pub fn edge_insets(&self) -> &EdgeInsets {
            &self.edge_insets
        }

        pub fn resize(&mut self, size: LogicalSize) {
            self.width = size.width() as f64;
            self.height = size.height() as f64;
        }

        pub fn create_view_region(&self, visible_level: ZoomLevel) -> Option<ViewRegion> {
            self.view_region_bounding_box(&self.view_projection().invert())
                .map(|bounding_box| {
                    ViewRegion::new(
                        bounding_box,
                        VIEW_REGION_PADDING,
                        MAX_N_TILES,
                        *self.zoom,
                        visible_level,
                    )
                })
        }

        pub fn get_intersection_time(
            ray_origin: Vector3<f64>,
            ray_direction: Vector3<f64>,
            plane_origin: Vector3<f64>,
            plane_normal: Vector3<f64>,
        ) -> f64 {
            let m = plane_origin - ray_origin;
            let distance = (m).dot(plane_normal);

            let approach_speed = ray_direction.dot(plane_normal);

            // Returns an infinity if the ray is
            // parallel to the plane and never intersects,
            // or NaN if the ray is in the plane
            // and intersects everywhere.
            return distance / approach_speed;

            // Otherwise returns t such that
            // ray_origin + t * rayDirection
            // is in the plane, to within rounding error.
        }

        pub fn furthest_distance(&self, camera_height: f64, center_offset: Point2<f64>) -> f64 {
            let perspective = &self.perspective;
            let width = self.width;
            let height = self.height;
            let camera = self.camera.position();

            let y = perspective.y_tan();
            let x = perspective.x_tan(width, height);
            let offset_x = perspective.offset_x(center_offset, width);
            let offset_y = perspective.offset_y(center_offset, height);

            let rotation = Matrix4::from_angle_x(self.camera.get_pitch())
                * Matrix4::from_angle_y(self.camera.get_yaw())
                * Matrix4::from_angle_z(self.camera.get_roll());

            let rays = [
                Vector3::new(x * (1.0 - offset_x), y * (1.0 - offset_y), 1.0),
                Vector3::new(x * (-1.0 - offset_x), y * (1.0 - offset_y), 1.0),
                Vector3::new(x * (1.0 - offset_x), y * (-1.0 - offset_y), 1.0),
                Vector3::new(x * (-1.0 - offset_x), y * (-1.0 - offset_y), 1.0),
            ];
            let ray_origin = Vector3::new(-camera.x, -camera.y, -camera_height);

            let plane_origin = Vector3::new(-camera.x, -camera.y, 0.0);
            let plane_normal = (rotation * Vector4::new(0.0, 0.0, 1.0, 1.0)).truncate();

            rays.iter()
                .map(|ray| Self::get_intersection_time(ray_origin, *ray, plane_origin, plane_normal))
                .fold(0. / 0., f64::max)
        }

        pub fn camera_to_center_distance(&self) -> f64 {
            let height = self.height;

            let fovy = self.perspective.fovy();
            let half_fovy = fovy / 2.0;

            // Camera height, such that given a certain field-of-view, exactly height/2 are visible on ground.
            let camera_to_center_distance = (height / 2.0) / (half_fovy.tan()); // TODO: Not sure why it is height here and not width
            camera_to_center_distance
        }

        /// This function matches how maplibre-gl-js implements perspective and cameras at the time
        /// of the mapbox -> maplibre fork: [src/geo/transform.ts#L680](https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L680)
        #[tracing::instrument(skip_all)]
        pub fn view_projection(&self) -> ViewProjection {
            let width = self.width;
            let height = self.height;

            let center = self.edge_insets.center(width, height);
            // Offset between wanted center and usual/normal center
            let center_offset = center - Vector2::new(width, height) / 2.0;

            let camera_to_center_distance = self.camera_to_center_distance();

            let camera_matrix = self.camera.calc_matrix(camera_to_center_distance);

            // Add a bit extra to avoid precision problems when a fragment's distance is exactly `furthest_distance`
            let far_z = self.furthest_distance(camera_to_center_distance, center_offset) * 1.01;

            // The larger the value of near_z is
            // - the more depth precision is available for features (good)
            // - clipping starts appearing sooner when the camera is close to 3d features (bad)
            //
            // Smaller values worked well for mapbox-gl-js but deckgl was encountering precision issues
            // when rendering it's layers using custom layers. This value was experimentally chosen and
            // seems to solve z-fighting issues in deckgl while not clipping buildings too close to the camera.
            //
            // TODO remove: In tile.vertex.wgsl we are setting each layer's final `z` in ndc space to `z_index`.
            // This means that regardless of the `znear` value all layers will be rendered as part
            // of the near plane.
            // These values have been selected experimentally:
            // https://www.sjbaker.org/steve/omniv/love_your_z_buffer.html
            let near_z = height / 50.0;

            let mut perspective =
                self.perspective
                    .calc_matrix_with_center(width, height, near_z, far_z, center_offset);

            //let mut perspective = self.perspective.calc_matrix(width / height, near_z, far_z);
            // Apply center of perspective offset, in order to move the vanishing point
            //perspective.z[0] = -center_offset.x * 2.0 / width;
            //perspective.z[1] = center_offset.y * 2.0 / height;

            // Apply camera and move camera away from ground
            let view_projection = perspective * camera_matrix;

            // TODO for the below TODOs, check GitHub blame to get an idea of what these matrices are used for!
            // TODO mercatorMatrix https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L725-L727
            // TODO scale vertically to meters per pixel (inverse of ground resolution): https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L729-L730
            // TODO alignedProjMatrix https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L735-L747
            // TODO labelPlaneMatrix https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L749-L752C14
            // TODO glCoordMatrix https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L754-L758
            // TODO pixelMatrix, pixelMatrixInverse https://github.com/maplibre/maplibre-gl-js/blob/e78ad7944ef768e67416daa4af86b0464bd0f617/src/geo/transform.ts#L760-L761

            ViewProjection(FLIP_Y * OPENGL_TO_WGPU_MATRIX * view_projection)
        }

        pub fn zoom(&self) -> Zoom {
            *self.zoom
        }

        pub fn did_zoom_change(&self) -> bool {
            self.zoom.did_change(0.05)
        }

        pub fn update_zoom(&mut self, new_zoom: Zoom) {
            *self.zoom = new_zoom;
            log::info!("zoom: {new_zoom}");
        }

        pub fn camera(&self) -> &Camera {
            self.camera.deref()
        }

        pub fn camera_mut(&mut self) -> &mut Camera {
            self.camera.deref_mut()
        }

        pub fn did_camera_change(&self) -> bool {
            self.camera.did_change(0.05)
        }

        pub fn update_references(&mut self) {
            self.camera.update_reference();
            self.zoom.update_reference();
        }

        /// A transform which can be used to transform between clip and window space.
        /// Adopted from [here](https://docs.microsoft.com/en-us/windows/win32/direct3d9/viewports-and-clipping#viewport-rectangle) (Direct3D).
        fn clip_to_window_transform(&self) -> Matrix4<f64> {
            let min_depth = 0.0;
            let max_depth = 1.0;
            let x = 0.0;
            let y = 0.0;
            let ox = x + self.width / 2.0;
            let oy = y + self.height / 2.0;
            let oz = min_depth;
            let pz = max_depth - min_depth;
            Matrix4::from_cols(
                Vector4::new(self.width / 2.0, 0.0, 0.0, 0.0),
                Vector4::new(0.0, -self.height / 2.0, 0.0, 0.0),
                Vector4::new(0.0, 0.0, pz, 0.0),
                Vector4::new(ox, oy, oz, 1.0),
            )
        }

        /// Transforms coordinates in clip space to window coordinates.
        ///
        /// Adopted from [here](https://docs.microsoft.com/en-us/windows/win32/dxtecharts/the-direct3d-transformation-pipeline) (Direct3D).
        fn clip_to_window(&self, clip: &Vector4<f64>) -> Vector4<f64> {
            #[rustfmt::skip]
            let ndc = Vector4::new(
                clip.x / clip.w,
                clip.y / clip.w,
                clip.z / clip.w,
                1.0
            );

            self.clip_to_window_transform() * ndc
        }
        /// Alternative implementation to `clip_to_window`. Transforms coordinates in clip space to
        /// window coordinates.
        ///
        /// Adopted from [here](https://www.khronos.org/registry/vulkan/specs/1.2-extensions/man/html/VkViewport.html)
        /// and [here](https://matthewwellings.com/blog/the-new-vulkan-coordinate-system/) (Vulkan).
        fn clip_to_window_vulkan(&self, clip: &Vector4<f64>) -> Vector3<f64> {
            #[rustfmt::skip]
                let ndc = Vector4::new(
                clip.x / clip.w,
                clip.y / clip.w,
                clip.z / clip.w,
                1.0
            );

            let min_depth = 0.0;
            let max_depth = 1.0;

            let x = 0.0;
            let y = 0.0;
            let ox = x + self.width / 2.0;
            let oy = y + self.height / 2.0;
            let oz = min_depth;
            let px = self.width;
            let py = self.height;
            let pz = max_depth - min_depth;
            let xd = ndc.x;
            let yd = ndc.y;
            let zd = ndc.z;
            Vector3::new(px / 2.0 * xd + ox, py / 2.0 * yd + oy, pz * zd + oz)
        }

        /// Order of transformations reversed: https://computergraphics.stackexchange.com/questions/6087/screen-space-coordinates-to-eye-space-conversion/6093
        /// `w` is lost.
        ///
        /// OpenGL explanation: https://www.khronos.org/opengl/wiki/Compute_eye_space_from_window_space#From_window_to_ndc
        fn window_to_world(
            &self,
            window: &Vector3<f64>,
            inverted_view_proj: &InvertedViewProjection,
        ) -> Vector3<f64> {
            #[rustfmt::skip]
                let fixed_window = Vector4::new(
                window.x,
                window.y,
                window.z,
                1.0
            );

            let ndc = self.clip_to_window_transform().invert().unwrap() * fixed_window;
            let unprojected = inverted_view_proj.project(ndc);

            Vector3::new(
                unprojected.x / unprojected.w,
                unprojected.y / unprojected.w,
                unprojected.z / unprojected.w,
            )
        }

        /// Alternative implementation to `window_to_world`
        ///
        /// Adopted from [here](https://docs.rs/nalgebra-glm/latest/src/nalgebra_glm/ext/matrix_projection.rs.html#164-181).
        fn window_to_world_nalgebra(
            window: &Vector3<f64>,
            inverted_view_proj: &InvertedViewProjection,
            width: f64,
            height: f64,
        ) -> Vector3<f64> {
            let pt = Vector4::new(
                2.0 * (window.x - 0.0) / width - 1.0,
                2.0 * (height - window.y - 0.0) / height - 1.0,
                window.z,
                1.0,
            );
            let unprojected = inverted_view_proj.project(pt);

            Vector3::new(
                unprojected.x / unprojected.w,
                unprojected.y / unprojected.w,
                unprojected.z / unprojected.w,
            )
        }

        /// Gets the world coordinates for the specified `window` coordinates on the `z=0` plane.
        pub fn window_to_world_at_ground(
            &self,
            window: &Vector2<f64>,
            inverted_view_proj: &InvertedViewProjection,
            bound: bool,
        ) -> Option<Vector2<f64>> {
            let near_world =
                self.window_to_world(&Vector3::new(window.x, window.y, 0.0), inverted_view_proj);

            let far_world =
                self.window_to_world(&Vector3::new(window.x, window.y, 1.0), inverted_view_proj);

            // for z = 0 in world coordinates
            // Idea comes from: https://dondi.lmu.build/share/cg/unproject-explained.pdf
            let u = -near_world.z / (far_world.z - near_world.z);
            if !bound || (0.0..=1.01).contains(&u) {
                let result = near_world + u * (far_world - near_world);
                Some(Vector2::new(result.x, result.y))
            } else {
                None
            }
        }

        /// Calculates an [`Aabb2`] bounding box which contains at least the visible area on the `z=0`
        /// plane. One can think of it as being the bounding box of the geometry which forms the
        /// intersection between the viewing frustum and the `z=0` plane.
        ///
        /// This implementation works in the world 3D space. It casts rays from the corners of the
        /// window to calculate intersections points with the `z=0` plane. Then a bounding box is
        /// calculated.
        ///
        /// *Note:* It is possible that no such bounding box exists. This is the case if the `z=0` plane
        /// is not in view.
        pub fn view_region_bounding_box(
            &self,
            inverted_view_proj: &InvertedViewProjection,
        ) -> Option<Aabb2<f64>> {
            let screen_bounding_box = [
                Vector2::new(0.0, 0.0),
                Vector2::new(self.width, 0.0),
                Vector2::new(self.width, self.height),
                Vector2::new(0.0, self.height),
            ]
            .map(|point| self.window_to_world_at_ground(&point, inverted_view_proj, false));

            let (min, max) = bounds_from_points(
                screen_bounding_box
                    .into_iter()
                    .flatten()
                    .map(|point| [point.x, point.y]),
            )?;

            Some(Aabb2::new(Point2::from(min), Point2::from(max)))
        }
        /// An alternative implementation for `view_bounding_box`.
        ///
        /// This implementation works in the NDC space. We are creating a plane in the world 3D space.
        /// Then we are transforming it to the NDC space. In NDC space it is easy to calculate
        /// the intersection points between an Aabb3 and a plane. The resulting Aabb2 is returned.
        pub fn view_region_bounding_box_ndc(&self) -> Option<Aabb2<f64>> {
            let view_proj = self.view_projection();
            let a = view_proj.project(Vector4::new(0.0, 0.0, 0.0, 1.0));
            let b = view_proj.project(Vector4::new(1.0, 0.0, 0.0, 1.0));
            let c = view_proj.project(Vector4::new(1.0, 1.0, 0.0, 1.0));

            let a_ndc = self.clip_to_window(&a).truncate();
            let b_ndc = self.clip_to_window(&b).truncate();
            let c_ndc = self.clip_to_window(&c).truncate();
            let to_ndc = Vector3::new(1.0 / self.width, 1.0 / self.height, 1.0);
            let plane: Plane<f64> = Plane::from_points(
                Point3::from_vec(a_ndc.mul_element_wise(to_ndc)),
                Point3::from_vec(b_ndc.mul_element_wise(to_ndc)),
                Point3::from_vec(c_ndc.mul_element_wise(to_ndc)),
            )?;

            let points = plane.intersection_points_aabb3(&Aabb3::new(
                Point3::new(0.0, 0.0, 0.0),
                Point3::new(1.0, 1.0, 1.0),
            ));

            let inverted_view_proj = view_proj.invert();

            let from_ndc = Vector3::new(self.width, self.height, 1.0);
            let vec = points
                .iter()
                .map(|point| {
                    self.window_to_world(&point.mul_element_wise(from_ndc), &inverted_view_proj)
                })
                .collect::<Vec<_>>();

            let min_x = vec
                .iter()
                .map(|point| point.x)
                .min_by(|a, b| a.partial_cmp(b).unwrap())?;
            let min_y = vec
                .iter()
                .map(|point| point.y)
                .min_by(|a, b| a.partial_cmp(b).unwrap())?;
            let max_x = vec
                .iter()
                .map(|point| point.x)
                .max_by(|a, b| a.partial_cmp(b).unwrap())?;
            let max_y = vec
                .iter()
                .map(|point| point.y)
                .max_by(|a, b| a.partial_cmp(b).unwrap())?;
            Some(Aabb2::new(
                Point2::new(min_x, min_y),
                Point2::new(max_x, max_y),
            ))
        }
    }

    #[cfg(test)]
    mod tests {
        use cgmath::{Deg, Matrix4, Vector2, Vector4};

        use crate::{
            coords::{WorldCoords, Zoom},
            render::view_state::ViewState,
            window::PhysicalSize,
        };

        #[test]
        fn conform_transformation() {
            let fov = Deg(60.0);
            let mut state = ViewState::new(
                PhysicalSize::new(800, 600).unwrap(),
                WorldCoords::at_ground(0.0, 0.0),
                Zoom::new(10.0),
                Deg(0.0),
                fov,
            );

            //state.furthest_distance(state.camera_to_center_distance(), Point2::new(0.0, 0.0));

            let projection = state.view_projection().invert();

            let bottom_left = state
                .window_to_world_at_ground(&Vector2::new(0.0, 0.0), &projection, true)
                .unwrap();
            println!("bottom left on ground {:?}", bottom_left);
            let top_right = state
                .window_to_world_at_ground(&Vector2::new(state.width, state.height), &projection, true)
                .unwrap();
            println!("top right on ground {:?}", top_right);

            let mut rotated = Matrix4::from_angle_x(Deg(-30.0))
                * Vector4::new(bottom_left.x, bottom_left.y, 0.0, 0.0);

            println!("bottom left rotated around x axis {:?}", rotated);

            rotated = Matrix4::from_angle_y(Deg(-30.0)) * rotated;

            println!("bottom left rotated around x and y axis {:?}", rotated);

            state.camera.set_pitch(Deg(30.0));
            //state.camera.set_yaw(Deg(-30.0));

            // TODO: verify far distance plane calculation
        }
    }