[StainlessS]

@VcMohan

As you recently posed a query on compiling for both v2.58 and v2.6a4, I assume that you want subsampling for both.
Below some code I use in RoboCrop, perhaps of use:

    myName="RoboCrop: ";

    if(vi.width==0 || vi.num_frames==0)         env->ThrowError("%sClip has no video",myName);
    # ifdef AVISYNTH_PLUGIN_25
        if(vi.IsPlanar() && vi.pixel_type != 0xA0000008) {              // Planar but NOT YV12, ie Avisynth v2.6+ ColorSpace
            // Here Planar but NOT YV12, If v2.5 Plugin Does NOT support ANY v2.6+ ColorSpaces
            env->ThrowError("%sColorSpace unsupported in v2.5 plugin",myName);
        }
    # endif

    bool isyuv=false,isplanar=false,isyuy2=false,isrgb=false,isy8=false;
    int xmod=1,ymod=1;

    if(isyuv=vi.IsYUV()) {
        if(isplanar=vi.IsPlanar()) {
            PVideoFrame src = child->GetFrame(0, env);                  // get frame 0
            int rowsizeUV=src->GetRowSize(PLANAR_U);
            if(rowsizeUV!=0) {                              // Not Y8
                const int ywid=src->GetRowSize(PLANAR_Y);
                const int yhit=src->GetHeight(PLANAR_Y);
                const int uhit=src->GetHeight(PLANAR_U);
                xmod=ywid/rowsizeUV;
                ymod=yhit/uhit;
            } else {
                isy8=true;
            }
        } else {
            isyuy2=true;
            xmod=2;
        }
    } else {
        isrgb=true;
    }

    xSubS=xmod;     // for Planar GetFrame
    ySubS=ymod;

    laced           =   args[3].AsBool(true);
    ymod = (laced)? ymod*2 : ymod;

v2.58 header does not contain GetWidthSubsampling/GetHeightSubsampling