From 4e59d730fc8c51c355fae88de110d6a6992042d7 Mon Sep 17 00:00:00 2001
From: Hartmut Seichter <hartmut@technotecture.com>
Date: Fri, 9 Dec 2022 18:35:37 +0100
Subject: [PATCH] updated code with reversed projection matrix code

---
 src/main.rs    |   2 +-
 src/offaxis.rs | 106 ++++++++++++++++++++++++++++++++++---------------
 2 files changed, 75 insertions(+), 33 deletions(-)

diff --git a/src/main.rs b/src/main.rs
index 0b907b5..1232584 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -20,7 +20,7 @@ mod viewer;
 
 fn main() {
     App::new()
-        .insert_resource(ClearColor(Color::rgb(0.9, 0.9, 0.9)))
+        .insert_resource(ClearColor(Color::rgb(0.5, 0.5, 0.5)))
         .add_plugins(DefaultPlugins.set(WindowPlugin {
             window: WindowDescriptor {
                 title: "PixSpace".to_string(),
diff --git a/src/offaxis.rs b/src/offaxis.rs
index 22b67e1..bc37354 100644
--- a/src/offaxis.rs
+++ b/src/offaxis.rs
@@ -16,38 +16,6 @@ pub struct OffAxisProjection {
     aspect: f32,
 }
 
-impl CameraProjection for OffAxisProjection {
-    fn get_projection_matrix(&self) -> Mat4 {
-    
-        println!("Here we go! {:?}",self);
-
-        Mat4::perspective_rh(45.0_f32.to_radians(), 
-            self.aspect, 
-            self.near, 
-            self.far)
-
-        //Mat4::orthographic_rh(-self.aspect, self.aspect, -1.0, 1.0, self.near, self.far)
-    }
-
-    // what to do on window resize
-    fn update(&mut self, width: f32, height: f32) {
-        self.aspect = width / height;
-    }
-
-    fn far(&self) -> f32 {
-        self.far
-    }
-}
-
-impl Default for OffAxisProjection {
-    fn default() -> Self {
-        Self {
-            near: 0.0,
-            far: 1000.0,
-            aspect: 1.0,
-        }
-    }
-}
 
 pub fn make_projection_rh_from_frustum(left: f32, right: f32, bottom: f32, top: f32, near:f32, far:f32) -> Mat4
 {
@@ -65,6 +33,80 @@ pub fn make_projection_rh_from_frustum(left: f32, right: f32, bottom: f32, top:
 }
 
 
+pub fn make_projection_rh_from_frustum_reversed(left: f32, right: f32, bottom: f32, top: f32, near:f32, far:f32) -> Mat4
+{
+
+    assert!(near > 0.0);
+
+    //
+    // reversed z 0..1 projection based on https://thxforthefish.com/posts/reverse_z/
+    //
+    let a = (right + left) / (right - left);
+    let b: f32 = (top + bottom) / (top - bottom);
+    // let c= if far.abs() > f32::MAX  { -1.0 } else { -(far + near) / (far - near)};
+    // let d = if far.abs() > f32::MAX { -2.0 * near } else { -2.0 * far * near / (far - near) };
+
+    let c =  near / (far - near);
+    let d =  far * near / (far - near);
+
+
+    Mat4::from_cols( 
+        Vec4::new(2.0 * near/(right-left),0.0,0.0,0.0), 
+        Vec4::new(0.0,2.0*near/(top-bottom), 0.0,0.0),
+        Vec4::new(a, b, c, -1.0),
+        Vec4::new(0.0, 0.0, d, 0.0))
+}
+
+
+pub fn make_projection_rh_custom(fov_y: f32, aspect_ratio: f32, z_near: f32, z_far: f32) -> Mat4
+{
+    let tan_fovy = (fov_y * 0.5).tan(); // use half angle beta
+    let right = tan_fovy * aspect_ratio * z_near;
+    let left = -right;
+    let top = tan_fovy * z_near;
+    let bottom = -top;
+
+    //make_projection_rh_from_frustum(left, right, bottom, top, z_near, z_far)
+
+    make_projection_rh_from_frustum_reversed(left, right, bottom, top, z_near, z_far)
+}
+
+
+
+impl CameraProjection for OffAxisProjection {
+    fn get_projection_matrix(&self) -> Mat4 {
+
+        make_projection_rh_custom(45.0_f32.to_radians(), self.aspect, self.near, self.far)
+
+        // Mat4::perspective_rh(45.0_f32.to_radians(), 
+        //     self.aspect, 
+        //     self.near, 
+        //     self.far)
+
+    }
+
+    // what to do on window resize
+    fn update(&mut self, width: f32, height: f32) {
+        self.aspect = width / height;
+    }
+
+    fn far(&self) -> f32 {
+        println!("Z-Value");
+        self.far
+    }
+}
+
+impl Default for OffAxisProjection {
+    fn default() -> Self {
+        Self {
+            near: 0.1,
+            far: 1000.0,
+            aspect: 1.0,
+        }
+    }
+}
+
+
 pub fn offaxis_camera_setup(mut commands: Commands) {
 
     let projection = OffAxisProjection::default();