initial implementation for the off-axis view frustum based on previous code

src/projection.rs

@@ -57,6 +57,61 @@ pub fn make_projection_rh_custom(fov_y: f32, aspect_ratio: f32, z_near: f32, z_f
 }
 
 
+fn create_offaxis_matrices(
+    screen_lower_left: Vec3,
+    screen_lower_right: Vec3,
+    screen_upper_left: Vec3,
+    pos_eye: Vec3,
+    z_far: f32
+    ) -> (Mat4,Mat4)
+{
+
+    // 
+    let vec_right = screen_lower_right - screen_lower_left; // vr
+    let vec_up = screen_upper_left - screen_lower_left;     // vu
+    
+    let frustum_left = screen_lower_left - pos_eye;     // va
+    let frustum_right = screen_lower_right - pos_eye;   // vb
+    let frustum_up = screen_upper_left - pos_eye;       // vc
+
+    let vec_right_normalized = vec_right.normalize();
+    let vec_up_normalized = vec_up.normalize();
+
+    let vec_normal = vec_right_normalized.cross(vec_up_normalized).normalize();
+
+    let dist = -vec_right.dot(vec_normal);
+
+    let z_near = 0.001_f32.max(dist - 0.01);
+
+    let left = vec_right.dot(frustum_left) * z_near / dist;
+    let right = vec_right.dot(frustum_right) * z_near / dist;
+    let bottom = vec_up.dot(frustum_left) * z_near / dist;
+    let top = vec_up.dot(frustum_up) * z_near / dist;
+
+
+    let projection_matrix = make_projection_rh_from_frustum_reversed(left,right,bottom,top,z_near,z_far);
+
+    let view_matrix_rotation = Mat4::from_cols(
+        Vec4::new(vec_right.x, vec_up.x, vec_normal.x  ,0.0),
+        Vec4::new(vec_right.y, vec_up.y, vec_normal.y  ,0.0),
+        Vec4::new(vec_right.z, vec_up.z, vec_normal.z  ,0.0),
+        Vec4::W
+    );
+
+    let view_matrix_eye = Mat4::from_cols(
+        Vec4::X,
+        Vec4::Y,
+        Vec4::Z,
+        (-pos_eye).extend(1.0)        
+    );
+
+    let view_matrix = view_matrix_rotation * view_matrix_eye;
+
+    (view_matrix,projection_matrix)
+}
+
+
+
 #[cfg(test)]
 mod tests {
     use bevy::prelude::Mat4;