Weird member of float4 structure in the HLSL

Question

I have encountered a weird x.wwww component of the float4 x vector in a HLSL code. I haven't seen such a member or member function of float4 structures in HLSL, so please tell me what is the meaning of that. It is in the code of this pixel shader:

float4 PSConstantColor( PS_PARTICLE_INPUT input ) : SV_TARGET
{
   // Sample particle texture
   float4 vColor = g_baseTexture.Sample( g_samLinear, input.Tex ).wwww;

   // Clip fully transparent pixels
   clip( vColor.a - 1.0/255.0 );

   // Return color
   return vColor;
}

At first I thought it should be the w component replicated to all the members of the float4 vColor vector, but it does not make sense for the color value, does it?

Answer 1

From my understanding they are called swizzle operators. In this case they make a float4 of the 4th element in the given float4:

vColor = (w, w, w, w)

Where w is g_baseTexture.Sample( g_samLinear, input.Tex ).w

Here is an example of a swizzle operator in practice:

float3 cross( float3 a, float3 b ) {
    return float3( a.y*b.z - a.z*b.y, a.z*b.x - a.x*b.z, a.z*b.y - a.y*b.z );
}

and its equivalent swizzle implementation

float3 cross( float3 a, float3 b ) {
    return a.yzx*b.zxy - a.zxy*b.yzx;
}