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using bitmap for placing objects on heightmap

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Well, I Currently have my terrain engine working well and I''ve think of a way to put some objects on the terrain, such as roads, trees, ect. For this, I wanna use a bitmap in witch, for example, roads are painted in red, trees in greeen ect... Now, as I use a Pixel shader written in HLSL for my terrain blending operations, I wanna know how can I do this. I don''t know why this piece of code don''t work in HLSL

//Items on the map
float4 fItems = tex2D (SamplerItems, Tex);

//Grass texture
loat4 Grass = tex2D(SamplerGrass,Tex); 

//Rock texture
float4 Rock = tex2D(SamplerRock,Tex); 

//earth texture
float4 Mud = tex2D(SamplerMud, Tex);

//Snow texture
float4 Snow = tex2D(SamplerSnow, Tex);

float4 red;
red.r = 1.0f;
red.g = 0.0f;
red.b = 0.0f;
red.a = 0.0f;

//I wanna return the rock color where my infos bitmap is painted in red
if ( red==fItems )  return Rock;

float4 BlendingWeights = tex2D(SamplerBlending, Tex);

//Blending factors encoded in a texture
float a = BlendingWeights.a;
float b = BlendingWeights.r;
float c = BlendingWeights.g;
float d = BlendingWeights.b;
return (a * Snow) + (d*Grass) + (c * Mud) + (b*Rock);
Well it seems to be logical this way but the compiler do not accpept the line "if ( red == fItems ) return Rock; Thanx for your help!

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