Custom projection support for Unity.

Projection.cs

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class TreesCamera : MonoBehaviour {
	private void OnEnable(){
		// Optional, only enable the callbacks when in the editor.
		if(Application.isEditor){
			// These callbacks are invoked for all cameras including the scene view and camera previews.
			Camera.onPreCull += ScenePreCull;
			Camera.onPostRender += ScenePostRender;
		}
	}

	private void OnDisable(){
		if(Application.isEditor){
			Camera.onPreCull -= ScenePreCull;
			Camera.onPostRender -= ScenePostRender;
		}
	}

	private void ScenePreCull(Camera cam){
		// If the camera is the scene view camera, call our OnPreCull() event method for it.
		if(cam.cameraType == CameraType.SceneView) OnPreCull();
	}

	private void ScenePostRender(Camera cam){
		// Unity's gizmos don't like it when you change the worldToCameraMatrix.
		// The workaround is to reset it after rendering.
		if(cam.cameraType == CameraType.SceneView) cam.ResetWorldToCameraMatrix();
	}

	private Camera _cam;
	private void Start(){
		_cam = GetComponent<Camera>();

		// Set the camera this script is attached to to use orthographic sorting order.
		// Instead of using the euclidean distance from the camera, objects will be sorted based on their depth into the scene.
		_cam.transparencySortMode = TransparencySortMode.Orthographic;

		GetComponent<SFRenderer>().extents = Rect.MinMaxRect(-1, -2f, 1, 1);
	}

	// This is a Unity callback and is the ideal place to set the worldToCameraMatrix.
	private void OnPreCull(){
		var cam = Camera.current;

		// First calculate the regular worldToCameraMatrix.
		// Start with transform.worldToLocalMatrix.
		var m = cam.transform.worldToLocalMatrix;

		// Then, since Unity uses OpenGL's view matrix conventions we have to flip the output z-value.
		m.SetRow(2, -m.GetRow(2));

		// Now for the custom projection.
		// Set the world's up vector to always align with the camera's up vector.
		// Add a small amount of the original up vector to ensure the matrix will be invertible.
		m.SetColumn(2, 1e-3f*m.GetColumn(2) - new Vector4(0, 1, 0, 0));

		cam.worldToCameraMatrix = m;
	}

	public static Matrix4x4 ScreenToWorldMatrix(Camera cam){
		// Make a matrix that converts from screen coordinates to clip coordinates.
		var rect = cam.pixelRect;
		var viewportMatrix = Matrix4x4.Ortho(rect.xMin, rect.xMax, rect.yMin, rect.yMax, -1, 1);

		// The camera's view-projection matrix converts from world coordinates to clip coordinates.
		var vpMatrix = cam.projectionMatrix*cam.worldToCameraMatrix;

		// Setting column 2 (z-axis) to identity makes the matrix ignore the z-axis.
		// Instead you get the value on the xy plane.
		vpMatrix.SetColumn(2, new Vector4(0, 0, 1, 0));

		// Going from right to left:
		// convert screen coords to clip coords, then clip coords to world coords.
		return vpMatrix.inverse*viewportMatrix;
	}

	public Vector2 ScreenToWorldPoint(Vector2 point){
		return ScreenToWorldMatrix(_cam).MultiplyPoint(point);
	}
}