Downsampling and Lanczos

asked 2019-04-05 09:03:42 -0500

Koray gravatar image

If I'm not understanding it wrong, according to this Lanczos should generate good results for downscaling. However my tries and this shows that it does not. Why?

Can you recommend me a good way to downscale an image which would produces nice looking thumbnails. Should I use Area Interpolation? I there any other alternative in OpenCV?

image description cv2 resize with Lanczos4

image descriptioncv2 resize with Area

And using PryDown first I've got a smoother result:

image description

Code:

public static void Resize_PreserveAspectRatio(this Mat mat, Mat dst, int length, InterpolationFlags st = InterpolationFlags.Cubic, bool changeMaxLength = true)
        {
            double w = mat.Width;
            double h = mat.Height;

            double len2x = length * 2d;
            double div = changeMaxLength ? Math.Max(w, h) : Math.Min(w, h);
            if (div > len2x)
            {
                using (Mat mat1 = mat.Clone())
                {
                    while (div > len2x)
                    {
                        Cv2.PyrDown(mat1, mat1);
                        w = mat1.Width;
                        h = mat1.Height;
                        div = changeMaxLength ? Math.Max(w, h) : Math.Min(w, h);
                    }
                    double w1 = (w / div) * length;
                    double h1 = (h / div) * length;
                    Cv2.Resize(mat1, dst, new Size(w1, h1), 0d, 0d, st);
                }
            }
            else
            {
                double w1 = (w / div) * length;
                double h1 = (h / div) * length;
                Cv2.Resize(mat, dst, new Size(w1, h1), 0d, 0d, st);
            }
        }
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Comments

and the message is: don't believe arbitrary blogposts ?

berak gravatar imageberak ( 2019-04-09 02:11:56 -0500 )edit