Merge #1442

1442: Add function to transform position r=jojolepro,Moxinilian,azriel91 a=vessd

## Description

Function to transform position from screen space into world space

## Additions

- `fn screen_to_world`


Co-authored-by: Sergey Veselkov <veselkovsd@yandex.ru>

amethyst_renderer/src/cam.rs

@@ -3,12 +3,15 @@
 use amethyst_assets::PrefabData;
 use amethyst_core::{
     ecs::prelude::{Component, Entity, HashMapStorage, Write, WriteStorage},
-    math::{Matrix4, Orthographic3, Perspective3},
+    math::{Matrix4, Orthographic3, Perspective3, Point2, Point3},
+    GlobalTransform,
 };
 use amethyst_error::Error;
 
 use serde::{Deserialize, Serialize};
 
+use crate::ScreenDimensions;
+
 /// The projection mode of a `Camera`.
 ///
 /// TODO: Remove and integrate with `Camera`.
@@ -76,6 +79,27 @@ impl Camera {
             std::f32::consts::FRAC_PI_3,
         ))
     }
+
+    /// Transforms position from screen space to camera space
+    pub fn position_from_screen(
+        &self,
+        screen_position: Point2<f32>,
+        camera_transform: &GlobalTransform,
+        screen_dimensions: &ScreenDimensions,
+    ) -> Point3<f32> {
+        let screen_x = 2.0 * screen_position.x / screen_dimensions.width() - 1.0;
+        let screen_y = 1.0 - 2.0 * screen_position.y / screen_dimensions.height();
+        let screen_point = Point3::new(screen_x, screen_y, 0.0).to_homogeneous();
+
+        let vector = camera_transform.0
+            * self
+                .proj
+                .try_inverse()
+                .expect("Camera projection matrix is not invertible")
+            * screen_point;
+
+        Point3::from_homogeneous(vector).expect("Vector is not homogeneous")
+    }
 }
 
 impl Component for Camera {

docs/CHANGELOG.md

@@ -27,6 +27,7 @@ it is attached to. ([#1282])
 * Add `SpriteRenderPrefab`. ([#1435])
 * Add `ScreenSpace` component. Draws entities using the screen coordinates. ([#1424])
 * Add `add_removal_to_entity` function. ([#1445])
+* Add `position_from_screen` to `Camera`. Transforms position from screen space to camera space. ([#1442])
 
 ### Changed
 
@@ -94,6 +95,7 @@ extra bounds from `AnimatablePrefab` and `AnimationSetPrefab` ([#1435])
 [#1451]: https://github.com/amethyst/amethyst/pull/1451
 [#1452]: https://github.com/amethyst/amethyst/pull/1452
 [#1454]: https://github.com/amethyst/amethyst/pull/1454
+[#1442]: https://github.com/amethyst/amethyst/pull/1442
 
 ## [0.10.0] - 2018-12