Hi,
I'm in the process of learning my way around shaders and thought I'd start with something simple. What I'm trying to achieve is a simple pixel shader that will color each pixel in a model a random gray tone. The pixels should be colored independently of eachother.

So basically, I'm trying to draw a random monochrome noise onto the screen, in the shape of a 3D model, if that makes more sense.

I was able to make the whole model a solid gray color and have it flicker as a whole, but I have not been able to color individual pixels.
I've done a bit of research on this topic and came up with this:

Code:
//Flicker/Noise Shader

float4 vecTime;

float4 ps_flicker(in float2 screenSpace : VPOS): COLOR {
	return noise(float3(screenSpace.x,screenSpace.y,vecTime.w));
}

technique flicker
{
	pass p0
	{
		PixelShader = compile ps_1_1 ps_flicker();
	}
}


I'm basically trying to use the screen position of the pixel as a sort of random seed for my random number generator.

The problem is that the engine doesn't seem to recognize the : VPOS semantic. The DirectX9 documentation listed it under Pixel Shader Semantics and the examples along with it said that this is how it should be used. However, when I first look at the model, I get an error X4502: invalid ps_2_0 input semantic 'VPOS'. I've also tried the DirectX10 equivalent, 'SV_Position', but to no avail. I receive a similar error when I do that.

Can anyone help me with this?