shrinking with vips (in c++)

    #define PROGRESSIVE 1
    string interp_type("bicubic");
    string ifilename("inputimage.png");
    string ofilename("outputimage.jpg");

    vips::VIMask sharpen_filter(MASK_XSIZE, MASK_YSIZE, MASK_SCALE, MASK_OFFSET,
                            std::vector<int> MASK_COEFF);

    /* Get target_height from the user */
    vips::VImage in(ifilename.c_str());

    input_height = in.Ysize();
    aspect_ratio = double(in.Xsize()) / double(in.Ysize());
    target_width = int(target_height * aspect_ratio);

    scale = target_height / (double) input_height;

        /* Shrink to integer factor first - This is faster */
        int shrink_factor = floor(1.0 / scale);
        if ((1.0 / scale) > 2.0) {
            out = in.shrink(double(shrink_factor), double(shrink_factor));
        }

        /* Use affine transform to get to the required size */
        int height_aft_shrink = floor(input_height / shrink_factor);
        scale = target_height / (double) height_aft_shrink;
        out = out.affinei((char *) interp_type.c_str(), scale, 0, 0, scale,
                                        0, 0, 0, 0, target_width, target_height);

        /* Apply Sharpening Filter */
        if (target_width < 150 || target_height < 150) {
            out = out.conv(sharpen_filter);
        }

    if ( vips_foreign_save((VipsImage *) (out.image()),
                (char *) ofilename.c_str(),
                "interlace", PROGRESSIVE ,
                NULL) ) {
            cout << "Error in saving file-" << ofilename
                    << endl;
        }