IncludeInstanced.hlsl

// https://twitter.com/Cyanilux/status/1396848736022802435?s=20

#ifndef GRASS_INSTANCED_INCLUDED
#define GRASS_INSTANCED_INCLUDED

float2 random2(float2 st)
{
	return frac(sin(dot(st.xy, float2(12.9898, 78.233))) * 43758.5453);
}

// ----------------------------------------------------------------------------------

// Graph should contain Boolean Keyword, "PROCEDURAL_INSTANCING_ON", Global, Multi-Compile.
// Must have two Custom Functions in vertex stage. One is used to attach this file (see Instancing_float below),
// and another to set #pragma instancing_options :

// It must use the String mode as this cannot be defined in includes.
// Without this, you will get "UNITY_INSTANCING_PROCEDURAL_FUNC must be defined" Shader Error.
/*
Out = In;
#pragma instancing_options procedural:vertInstancingSetup
*/
// I've found this works fine, but it might make sense for the pragma to be defined outside of a function,
// so could also use this slightly hacky method too
/*
Out = In;
}
#pragma instancing_options procedural:vertInstancingSetup
void dummy(){
*/

// ----------------------------------------------------------------------------------

struct InstanceData { int index; float4 color; };

StructuredBuffer<InstanceData> _PerInstanceData;

void GetInstancedIndex_float(out int Out)
{
	Out = 1;
#ifndef SHADERGRAPH_PREVIEW
#if UNITY_ANY_INSTANCING_ENABLED
	Out = _PerInstanceData[unity_InstanceID].index;
#endif
#endif
}

void GetInstancedIndexRandom_float(out float Out)
{
	int idx;
	GetInstancedIndex_float( idx );
	Out = random2( float2( idx , 0 ) );
}

void GetInstancedColor_float(out float4 Out) 
{
	Out = float4( 1,1,1,1 );
#ifndef SHADERGRAPH_PREVIEW
#if UNITY_ANY_INSTANCING_ENABLED
	Out = _PerInstanceData[unity_InstanceID].color;
#endif
#endif
}

struct InstanceMatrix { float4x4 m; };

StructuredBuffer<InstanceMatrix> _PerInstanceMatrix;

#if UNITY_ANY_INSTANCING_ENABLED

	// Updates the unity_ObjectToWorld / unity_WorldToObject matrices so our matrix is taken into account

	// Based on : 
	// https://github.com/Unity-Technologies/Graphics/blob/master/com.unity.shadergraph/Editor/Generation/Targets/BuiltIn/ShaderLibrary/ParticlesInstancing.hlsl
	// and/or
	// https://github.com/TwoTailsGames/Unity-Built-in-Shaders/blob/master/CGIncludes/UnityStandardParticleInstancing.cginc

	void vertInstancingMatrices(inout float4x4 objectToWorld, out float4x4 worldToObject) 
	{	
		InstanceMatrix data = _PerInstanceMatrix[ unity_InstanceID ];

		if ( data.m._44 == 0 )
		{
			float4 z = float4( 0, 0, 0, 0 );
			worldToObject = objectToWorld = float4x4( z, z, z, z );
			return;
		}

		objectToWorld = mul( objectToWorld, data.m );

		// Transform matrix (override current)
		// I prefer keeping positions relative to the bounds passed into DrawMeshInstancedIndirect so use the above instead
		//objectToWorld._11_21_31_41 = float4(data.m._11_21_31, 0.0f);
		//objectToWorld._12_22_32_42 = float4(data.m._12_22_32, 0.0f);
		//objectToWorld._13_23_33_43 = float4(data.m._13_23_33, 0.0f);
		//objectToWorld._14_24_34_44 = float4(data.m._14_24_34, 1.0f);

		// Inverse transform matrix
		float3x3 w2oRotation;
		w2oRotation[0] = objectToWorld[1].yzx * objectToWorld[2].zxy - objectToWorld[1].zxy * objectToWorld[2].yzx;
		w2oRotation[1] = objectToWorld[0].zxy * objectToWorld[2].yzx - objectToWorld[0].yzx * objectToWorld[2].zxy;
		w2oRotation[2] = objectToWorld[0].yzx * objectToWorld[1].zxy - objectToWorld[0].zxy * objectToWorld[1].yzx;

		float det = dot(objectToWorld[0].xyz, w2oRotation[0]);
		w2oRotation = transpose(w2oRotation);
		w2oRotation *= rcp(det);
		float3 w2oPosition = mul(w2oRotation, -objectToWorld._14_24_34);

		worldToObject._11_21_31_41 = float4(w2oRotation._11_21_31, 0.0f);
		worldToObject._12_22_32_42 = float4(w2oRotation._12_22_32, 0.0f);
		worldToObject._13_23_33_43 = float4(w2oRotation._13_23_33, 0.0f);
		worldToObject._14_24_34_44 = float4(w2oPosition, 1.0f);
	}

	void vertInstancingSetup() 
	{
		vertInstancingMatrices( unity_ObjectToWorld , unity_WorldToObject );
	}

#endif

// Shader Graph Functions

// Obtain InstanceID. e.g. Can be used as a Seed into Random Range node to generate random data per instance
void GetInstanceID_float(out float Out){
	Out = 0;
	#ifndef SHADERGRAPH_PREVIEW
	#if UNITY_ANY_INSTANCING_ENABLED
	Out = unity_InstanceID;
	#endif
	#endif
}

// Just passes the position through, allows us to actually attach this file to the graph.
// Should be placed somewhere in the vertex stage, e.g. right before connecting the object space position.
void Instancing_float(float3 Position, out float3 Out)
{
	Out = Position;
}

#endif