     {
         Pixel d;
         initPixel(&d);
-        Scalar sf = 0.f;
+        float sf = -1.f;
 
         // composite slices for this image line
         for (int slice=firstSlice; slice!=lastSlice; slice += sliceStep)

            if(preInt)
            {
                const float sb = volume(vidx, line-iPosY, slice, principal);
                if(slice != firstSlice)
                {
                    const float4 c = tex2D(tex_preint, sf[ix], sb);


            if(preInt)
            {
                const float sb = volume(vidx, line-iPosY, slice, principal);
                if(sf >= 0.f)
                {
                    const float4 c = tex2D(tex_preint, sf, sb);

