float4 ps_main( PS_IN In ) : COLOR
{
    //  Convert to Pixel, use proper rounding
    int DestinationPixelY = (In.UV0.y*ViewPortSize.y) + 0.5;
   
    // Detect odd pixel row
    int Odd=(DestinationPixelY%2);
   
    // Calculate source pixel
    // Odd pixels are from 2nd viewport at half of viewportsize
    int SourcePixel = ((DestinationPixelY-Odd) + Odd*ViewPortSize.y)*0.5;
   
    // offset 0.5 to center of pixel
    // Scale to 0..1
    float2 UV  =float2(In.UV0.x,(SourcePixel+0.5)/ViewPortSize.y);
   
    float4 tc = tex2D(BaseTexture, UV);
    return tc;
}

 while ( spApp->Run() )
  {
     // Needs to be reset on resize..
     g_spContextP2[0]->SetViewport ( 0, 0, iSizeX, iSizeY/2 );
     g_spContextP2[1]->SetViewport ( 0, iSizeY/2, iSizeX, iSizeY/2 );

     // Check keys
     fFOVX -= ::Key.IsPressed(VGLK_1) * 1.0f;
     fFOVX += ::Key.IsPressed(VGLK_2) * 1.0f;

     fYaw -= ::Key.IsPressed(VGLK_3)*0.1f;
     fYaw += ::Key.IsPressed(VGLK_4)*0.1f;

     fDistanceEyeToCenter -= ::Key.IsPressed(VGLK_5)*0.1f;
     fDistanceEyeToCenter += ::Key.IsPressed(VGLK_6)*0.1f;

               // Switch eye's
     if ( ::Key.IsPressed(VGLK_7) )
     {
        fYaw *= -1.0f;
        fDistanceEyeToCenter *= -1.0f;
     }
               // Toggle Pitch correction, between, -1, 0 and 1
               // To tilt/pitch the camera 0.5 scanline down or up
     if ( ::Key.IsPressed(VGLK_8) )
     {
        iUsePitchCorrection = (((iUsePitchCorrection+1)+1)%3)-1;
     }
               // set some nice default
     if ( ::Key.IsPressed(VGLK_9) )
     {
        fFOVX = fOrgX;
        fFOVY = fOrgY;
        fYaw = -2.0f;
        fDistanceEyeToCenter = -2.0f;
     }
               // disable stereoscopic view
     if ( ::Key.IsPressed(VGLK_0) )
     {
        fFOVX = fOrgX; fFOVY = fOrgY; fYaw = 0.0f; fDistanceEyeToCenter = 0.0f;
     }

     // Update Field of view
     // Since the context ignores our viewportsize, they now look all the same.
     g_spContextP1->SetFOV ( fFOVX, fFOVY);
     g_spContextP2[0]->SetFOV ( fFOVX, fFOVY);
     g_spContextP2[1]->SetFOV ( fFOVX, fFOVY);

               // Left side of the screen shows the normal camera position
     spCameraP1->AttachToEntity(pMouseCamera, VisVector_cl(0, 0,0) );

               // left eye slightly translated and rotated
     ::Math.EulerToMatrix(-fYaw,0,0, rotMat);
     spCameraP2[0]->AttachToEntity(pMouseCamera, VisVector_cl(0, fDistanceEyeToCenter,0), &rotMat );

               // right eye slightly translated and rotated, optionally pitch correction, too improve resolution
     ::Math.EulerToMatrix( fYaw,iUsePitchCorrection*fFOVY/iSizeY,0, rotMat);
     spCameraP2[1]->AttachToEntity(pMouseCamera, VisVector_cl(0,-fDistanceEyeToCenter,0), &rotMat );
  }

float4  cHalfScreenSize : register(c2) = { 640 / 2, 480 / 2, 0.5 / 640, 0.5 / 480 };

struct PS_INPUT
{
   float2 t : TEXCOORD0;
};

float4 shaderHorizontalInterlacedOptimized(PS_INPUT input) : COLOR
{
    if (frac(input.t.y * cHalfScreenSize.y) < 0.5f) // y odd check
      return tex2D(sL, input.t + float2(0, cHalfScreenSize.w));
   else
      return tex2D(sR, input.t - float2(0, cHalfScreenSize.w));
}

If ( x < 1. ) {..}
else if ( x < 2. ) {..}

    ps_2_0
    def c0, -0.5, 0, 0, 0
    dcl t0.xy
    dcl_2d s0
    dcl_2d s1
    texld r0, t0, s0
    texld r1, t0, s1
    mul r2.w, t0.y, c2.y
    frc r2.x, r2.w
    add r2.x, r2.x, c0.x
    cmp r0, r2.x, r1, r0
    mov oC0, r0