How to resize image with keep length-width ratio?

[zengjie617789]

I want to resize a face with keep length-width ration and paste it into a target size image with padding zero.
I use the code below, but I donot know how to paste it into a target size image fastly

 scale_size = min_side / 640.0;
            int target_w = image_w / scale_size;
            int target_h = image_h / scale_size;
            in_resized = ncnn::Mat::from_pixels_resize(cv_mat_in.data,
                                                       ncnn::Mat::PIXEL_RGB, image_w, image_h,
                                                       target_w,
                                                       target_h);

Anyone who can help will be appreciated!