C# Downscale image to a certain number of pixels

Question

I have some images that have more than, let's say 6.000.000 pixels and I want to scale them to be somewhere around that value.

public void downscaleByCalculateInSampleSize(string filePath, string newPath)
    {
        int reqNumberOfPixels = 6000000;

        double inSampleSize = 1;

        using (System.Drawing.Image oImage = System.Drawing.Image.FromFile(filePath))
        {
            int newWidth = oImage.Width;
            int newHeight = oImage.Height;

            int actualNumberofPixels = oImage.Width * oImage.Height;

            if (actualNumberofPixels > reqNumberOfPixels)
            {
                inSampleSize = Math.Sqrt(actualNumberofPixels / reqNumberOfPixels);

                newWidth = Convert.ToInt32(Math.Round((float)oImage.Width / inSampleSize));
                newHeight = Convert.ToInt32(Math.Round((float)oImage.Height / inSampleSize));
            }

            var newImage = new Bitmap(newWidth, newHeight);

            Graphics graphics = Graphics.FromImage(newImage);

            graphics.DrawImage(oImage, 0, 0, newWidth, newHeight);

            newImage.Save(newPath);
        }
    }

I've tried to downscale an image that had 6367 x 4751 pixels and 72 dpi resolution (24 bit depth) with the size of 8.03 MB. I've resized this image and I was expecting to be a much more smaller one in size (bellow 8 MB) but mine has 17. The scaled image is 2847 x 2125 (96 dpi with 32 Bit depth). Why is this happening? Is there a way to downscale an image to a requested number of pixels and the result to have the size much more smaller? I don't care about the resolution...

Thats odd, because when I do some math: 2847 x 2125 x 24bpp I get approx. 17 MB. So, you might be saving as a jpeg file, the file size suggest it's a bmp. — Stefan, Nov 06 '17 at 19:04
Maybe this will give some more info: https://stackoverflow.com/questions/21218017/how-to-save-a-bitmap-image-as-jpeg — Stefan, Nov 06 '17 at 19:07
Forgot to mention: The first one has Bit depth 24 and the resized one 32. I'll edit my question — Dana, Nov 06 '17 at 19:07
Possible duplicate of [Saving image to file](https://stackoverflow.com/questions/12909905/saving-image-to-file) — Stefan, Nov 06 '17 at 19:12
Have your tried: `newImage.Save(newPath, ImageFormat.Jpeg);`? — Stefan, Nov 06 '17 at 19:12
seeing a ".jpg" extension doesn't mean this file is in jpeg format. use `newImage.Save(newPath, ImageFormat.Jpeg)` — L.B, Nov 06 '17 at 19:18

score 1 · Accepted Answer · answered Nov 06 '17 at 19:10

You are using integer division and truncating results at:

inSampleSize = Math.Sqrt(actualNumberofPixels / reqNumberOfPixels);

Try instead:

inSampleSize = Math.Sqrt((double)actualNumberofPixels / (double)reqNumberOfPixels);

Also, save with:

newImage.Save(newPath, ImageFormat.Jpeg);

As the sizes you are getting seem much too large if you are saving with a lossy format with that many pixels

C# Downscale image to a certain number of pixels

1 Answers1