downsize

        // determine padding for scanlines
        int old_padding = (4 - (bi.biWidth * sizeof(RGBTRIPLE)) % 4) % 4;
        
        float newbiWidth = newbi.biWidth; //make new width a float
        newbiWidth *= nn;                 //get downsize number
        newbiWidth = round(newbiWidth);   //round downsize number   
        int inewbiWidth = newbiWidth;

        int horizontal_pixels = round(bi.biWidth/newbiWidth);

        float newbiHeight = newbi.biHeight; //make new height a float
        newbiHeight *= nn;                  // get downsize number
        newbiHeight = round(newbiHeight);   // round downsize number 
        int inewbiHeight = newbiHeight;     // make downsize number int

        int vertical_pixels = round(bi.biHeight/inewbiHeight);
        
        // determine padding for scanlines        
        int new_padding = (4 - (inewbiWidth * sizeof(RGBTRIPLE)) % 4) % 4;
        
        newbi.biWidth = inewbiWidth;
        newbi.biHeight = inewbiHeight;
        newbi.biSizeImage = ((sizeof(RGBTRIPLE) * newbi.biWidth) + new_padding) * abs(newbi.biHeight);
        newbf.bfSize = newbi.biSizeImage + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
    
        // write outfile's BITMAPFILEHEADER
        fwrite(&newbf, sizeof(BITMAPFILEHEADER), 1, outptr);
    
        // write outfile's BITMAPINFOHEADER
        fwrite(&newbi, sizeof(BITMAPINFOHEADER), 1, outptr);
    
        // iterate over infile's scanlines
        for (int i = 0, biHeight = (abs(bi.biHeight)); i < biHeight; i++)
        {
            int realer_pixel = i % vertical_pixels; // row mod (infile height / new height) to choose row to write 
            if (realer_pixel == 0)  
            {
            
                // iterate over pixels in scanline
                for (int j = 0; j < ((bi.biWidth)); j++)
                {
                        // temporary storage
                        RGBTRIPLE triple;
            
                        // read RGB triple from infile
                        fread(&triple, sizeof(RGBTRIPLE), 1, inptr);
                                                
                        int real_pixel = j % horizontal_pixels;
                        if (real_pixel == 0)
                        {
                            // write RGB triple to outfile
                            fwrite(&triple, sizeof(RGBTRIPLE), 1, outptr);
                        }
                }
                // put padding
                for (int k = 0; k < new_padding; k++)
                {
                    fputc(0x00, outptr);
                }                   
            // skip over padding, if any
            fseek(inptr, old_padding, SEEK_CUR);
         
            } else {
              // skip over padding, if any
              fseek(inptr, ((bi.biWidth) * 3) + old_padding, SEEK_CUR);
            }
        }