Creating real-time Mirrors in Unity Free (attempt 2)

OK, a completely different approach this time (based on the “new technique for Render to Texture” that I mentioned last time). And it works a lot better – lighting, shaders, etc is all there for free (but something is wrong with the side-to-side)

Note:

1. The player model is a green cube “head” that can rotate up/down, and a blue cube body that can rotate left/right
2. ALL LIGHTING ETC IS CORRECT, without needing to do anything
3. when I’m looking at center of miror – from any angle – it appears 100% correct

What’s still wrong here…

I don’t understand why the mirror is wrong when you look away from the center. My best guess is that it’s something to do with Unity’s crappy (*) “Plane” class, that I’m using to answer the question “Where does line L intersect with plane P?”.

[csharp]
Ray rayToMirror = new Ray( objectBeforeMirror.transform.position, objectBeforeMirror.transform.forward );
planeOfMirror.Raycast( rayToMirror, out intersectionDistance );
Vector3 hitPoint = rayToMirror.GetPoint( intersectionDistance );
transform.LookAt( hitPoint );
[/csharp]

How are you doing this?

In pseudo terms, because this is the most interesting bit, here’s what I did:

1. Create a “mirror” plane in Unity
2. Duplicate your main camera
3. Remove EVERY COMPONENT from the duped camera (you only want the camera itself)
4. Attach a mini script that moves the duped camera to the opposite side of the mirror, and re-positions it based on main-camera’s position … every frame
5. Attach a tiny 1-line “DepthMask” Shader to the “mirror” so that it will “cut-out” of the game-world
6. Set your camera “depths” so that main camera is rendered last (depth = 1), dupe camera is first (depth=0)
7. Set your main camera to STOP clearing the screen every frame (set clear mode to “Depth”)
8. … et voila!

Why are you doing this?

Because I can. And because: I’m planning an exciting game project where I’ll be hacking apart the whole of Unity’s rendering system to do as many weird tricks as I can. Stuff like this is helping me to learn how Unity abstracts the GL/D3D rendering – and what I can get away with.

And … it’s an interesting exercise just to see if I can do it at all – simply and high performance – without using RenderTexture (Unity’s implementation of Render to Texture)).

Of course, you should just buy Unity Pro instead; it has MANY improvements (including R-to-T).

My scripts so far

Move the duped camera – every frame – to opposite side of mirror

From: http://en.wikibooks.org/wiki/GLSL_Programming/Unity/Mirrors

NB: I have *heavily modified* the original – it did something weird with negative scales, and it got rotations mixed-up. I had to get the rotations precisely correct to make mine work, so … the rotations bit is different here.

[csharp]
using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class MirrorMirrifyObject : MonoBehaviour
{
public GameObject objectBeforeMirror;
public GameObject mirrorPlane;

void Update ()
{
if (null != mirrorPlane) {
if (null != objectBeforeMirror) {
transform.position = objectBeforeMirror.transform.position;
transform.rotation = objectBeforeMirror.transform.rotation;

Vector3 positionInMirrorSpace = mirrorPlane.transform.InverseTransformPoint( objectBeforeMirror.transform.position );

positionInMirrorSpace.y = -positionInMirrorSpace.y;

transform.position = mirrorPlane.transform.TransformPoint( positionInMirrorSpace );

/** object is now in correct position, but looking in parallel look direction to original */
/* So, cast a ray from the original object along look-direction until you hit the plane,
* then make the cloned object "look at" that position */
Vector3 mirrorsNormal = mirrorPlane.transform.localRotation * new Vector3( 0f, 1, 0f ); // Unity planes always start with normal pointing up
Plane planeOfMirror = new Plane( mirrorsNormal, mirrorPlane.transform.position );
float intersectionDistance;
Ray rayToMirror = new Ray( objectBeforeMirror.transform.position, objectBeforeMirror.transform.forward );
planeOfMirror.Raycast( rayToMirror, out intersectionDistance );
Vector3 hitPoint = rayToMirror.GetPoint( intersectionDistance );

transform.LookAt( hitPoint );
}
}
}
}
[/csharp]

Shader that makes an object “cut-out” a hole in the camera (depth-mask)

From: http://wiki.unity3d.com/index.php?title=DepthMask

NB: I tried some alternative versions of this that didn’t work
NB: *I have modified the original*. If you look at theirs, they do NOT want it to obscure the entire scene, but I did, so see how I moved it slightly higher in the render-queue…
[c]
Shader "Custom/DepthMask" {

SubShader {
// Render the mask BEFORE regular geometry, AND ALSO before masked geometry and
// transparent things.

Tags {"Queue" = "Geometry-10" }

// Don’t draw in the RGBA channels; just the depth buffer
// ADAM: causes it to mask-out that part of the screen, and
// no further rendering will happen there

ColorMask 0
ZWrite On

// Do nothing specific in the pass:

Pass {}
}
}
[/c]