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HLSL - Variable texture operation index

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I'm sort of a hack when it comes to vertex shader code, so any corrective input is appreciated. In this vertex shader function..
float vParamTex[16];

void vsMethod_Param_TE4(vNormInput v_in,
	out float4 oPos : POSITION,		// Position
	out float4 oDif : COLOR0,		// Color + Alpha
	out float2 oTex1 : TEXCOORD0,	// Primary Texture Coordinate
	out float2 oTex2 : TEXCOORD1,	// Secondary Texture Coordinate
	out float2 oTex3 : TEXCOORD2,	// Third Texture Coordinate
	out float2 oTex4 : TEXCOORD3 )	// Fourth Texture Coordinate
	// Transform Vertex
	float3 vp = mul( float4(v_in.Pos.xyz, 1.0f), WorldMatrix);
	oPos = mul( float4(vp, 1.0f), ViewProj);

	// Set Diffuse to show light
	float3 normal = normalize( mul( v_in.normal, WorldMatrix ) );
	oDif = GetLightDiffuse( normal );

	// Set param coordinates
	oTex1.x = ( v_in.Tex.x * vParamTex[0] ) + vParamTex[2];
	oTex1.y = ( v_in.Tex.y * vParamTex[1] ) + vParamTex[3];
	oTex2.x = ( v_in.Tex.x * vParamTex[4] ) + vParamTex[6];
	oTex2.y = ( v_in.Tex.y * vParamTex[5] ) + vParamTex[7];
	oTex3.x = ( v_in.Tex.x * vParamTex[8] ) + vParamTex[10];
	oTex3.y = ( v_in.Tex.y * vParamTex[9] ) + vParamTex[11];

	// Fake reflection effect
	float3 vz = normalize( cross( normal, float3(vParamTex[14], vParamTex[15], 0.0f) ) );
	float3 vx = normalize( cross( normal, vz ) );
	vp += normalize( vp - Camera.Position ) * 192.0;
	oTex4.x = dot( vp, vx ) * vParamTex[12];
	oTex4.y = dot( vp, vz ) * vParamTex[13];

..I'm handling oTex4 at the bottom in a specific way. Now I've found that I actually need to control which texture layer (oTex1, oTex2, oTex3, oTex4) recieves this special operation per material (not per vertex). I guess I need each object material to be able to specify a texture index. Anyone have advice about how to best go about this? Can I reference the textures by array/index (ie: oTex[2].x)? And even if I can, how can I write this function in a managable way? My first guess would be to just handle all texture layers normally, then overwrite one of them at the bottom, which would waste some processing. It's a mess. I appreciate any help.

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I'm guessing DX9 (right?).

You can use a volume texture, where each slice is a texture. Then you'd use the Z element when fetching from the texture to index into which texture to use. Alternatively, a CubeMap might work as well.

With DX10 you should be able to use Texture Arrays.

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