解决VR中UGUI world space UI会被其他物体遮挡的问题

• 问题：
  在制作VR内容时，通常使用的都是UGUI，一般会将Canvas的render mode设置为world space，但是这样设置过后，因为UI是直接放在了场景里面，很容易被其他物体挡住。
• 解决方法：
  Unity官方的一个VR例子中给出了一个shader的解决方案。

1. 将如下的shader文件挂在材质上
2. 将这个材质赋予需要总是显示在所有物体前面的UI控件上。

创建UIShader.shader文件

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "UI/Overlay"
{
	Properties
	{
		[PerRendererData] _MainTex("Font Texture", 2D) = "white" {}

	_Color("Tint", Color) = (1,1,1,1)

		_StencilComp("Stencil Comparison", Float) = 8
		_Stencil("Stencil ID", Float) = 0
		_StencilOp("Stencil Operation", Float) = 0
		_StencilWriteMask("Stencil Write Mask", Float) = 255
		_StencilReadMask("Stencil Read Mask", Float) = 255

		_ColorMask("Color Mask", Float) = 15
	}

		SubShader
	{
		LOD 100

		Tags
	{
		"Queue" = "Transparent"
		"IgnoreProjector" = "True"
		"RenderType" = "Transparent"
		"PreviewType" = "Plane"
		"CanUseSpriteAtlas" = "True"
	}

		Stencil
	{
		Ref[_Stencil]
		Comp[_StencilComp]
		Pass[_StencilOp]
		ReadMask[_StencilReadMask]
		WriteMask[_StencilWriteMask]
	}

		Cull Off
		Lighting Off
		ZWrite Off
		ZTest Always
		Offset -1, -1
		Blend SrcAlpha OneMinusSrcAlpha
		ColorMask[_ColorMask]

		Pass
	{
		CGPROGRAM
#pragma vertex vert
#pragma fragment frag
#include "UnityCG.cginc"
#include "UnityUI.cginc"

		struct appdata_t
	{
		float4 vertex : POSITION;
		float2 texcoord : TEXCOORD0;
		float4 color : COLOR;
	};

	struct v2f
	{
		float4 vertex : SV_POSITION;
		half2 texcoord : TEXCOORD0;
		fixed4 color : COLOR;
	};

	sampler2D _MainTex;
	float4 _MainTex_ST;
	fixed4 _Color;
	fixed4 _TextureSampleAdd;

	v2f vert(appdata_t v)
	{
		v2f o;
		o.vertex = UnityObjectToClipPos(v.vertex);
		o.texcoord = TRANSFORM_TEX(v.texcoord, _MainTex);
		o.color = v.color * _Color;
#ifdef UNITY_HALF_TEXEL_OFFSET
		o.vertex.xy += (_ScreenParams.zw - 1.0)*float2(-1,1);
#endif

		return o;
	}

	fixed4 frag(v2f i) : SV_Target
	{
		fixed4 col = (tex2D(_MainTex, i.texcoord) + _TextureSampleAdd) * i.color;
	clip(col.a - 0.01);
	return col;
	}
		ENDCG
	}
	}
}