# Single-pass stereo rendering for Android

> Explains how to use single-pass stereo rendering in your Android application.

Unity supports [single-pass stereo rendering](/engine/6000.7/manual/xr/graphics/stereo-rendering/single-pass-instancing.md) for Android devices that support multiview. Multiview consists of the [GL\_OVR\_multiview2](https://www.khronos.org/registry/OpenGL/extensions/OVR/OVR_multiview2.txt) and [GL\_OVR\_multiview\_multisampled\_render\_to\_texture](https://www.khronos.org/registry/OpenGL/extensions/OVR/OVR_multiview_multisampled_render_to_texture.txt) OpenGL ES extensions. These extensions require shaders to use a 2D texture array that consists of two slices, one slice per eye.

## Shader code requirements

To use single-pass stereo rendering with custom shaders, you may need to include additional shader code. You don't need to include additional code if your custom shaders are:

* Surface Shaders that don't have custom vertex processing.
* Fixed-function pipeline shaders.

> **Note:**
>
> These shader changes are compatible with multi-pass stereo rendering.

### Modify your shaders

If you want to use the `unity_StereoEyeIndex` built-in shader variable to know which eye the GPU is rendering to, you must declare `UNITY_VERTEX_OUTPUT_STEREO` in any shader stage output structs that you have. For example:

```lang-hlsl
struct v2f {
    float2 uv : TEXCOOR0;
    float4 vertex : SV_POSITION;
    UNITY_VERTEX_OUTPUT_STEREO
};
```

To initialize the output data, use `UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO()` in the vertex shader function. For example:

```lang-hlsl
v2f vert (appdata v)
{
	v2f o;
	UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
    o.vertex = UnityObjectToClipPos(v.vertex);
    o.uv = TRANSFORM_TEX(v.uv, _MainTex);
    return o;
}
```

To initialize `unity_StereoEyeIndex` in subsequent stages, add `UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX()` at the beginning. For example:

```lang-hlsl
half4 frag (v2f i) : SV_Target
{
    UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
	// sample the texture
	half4 col = tex2D(_MainTex, i.uv);
	// apply fog
	UNITY_APPLY_FOG(i.fogCoord, col);
	return col;
}
```

If your shaders use other shader stages, use the `UNITY_TRANSFER_VERTEX_OUTPUT_STEREO()` macro to transfer the eye index to the subsequent stages.

> **Tip:**
>
> To calculate the final position of the object, it's best practice to use `UnityObjectToClipPos(IN.vertex)` instead of `mul(UNITY_MATRIX_MVP, IN.vertex)`.

## Post-Processing Shaders

You must update post-processing shaders to deal with the eye textures being a 2D texture array. To help with this, Unity includes the `UNITY_DECLARE_SCREENSPACE_TEXTURE()` macro. To make textures work in both multi-pass and single-pass modes, wrap each textures in this macro. Also, when you sample the texture, use the `UNITY_SAMPLE_SCREENSPACE_TEXTURE()` macro.

This macro requires that you call `UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX()` beforehand when in single-pass mode. Unity also includes similar macros for depth textures and screen space shadow maps. You can see the full list at the bottom of `HLSLSupport.cginc`.