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
– Stefan, Commented Nov 6, 2017 at 19:04
Maybe this will give some more info: stackoverflow.com/questions/21218017/… — Stefan
– Stefan, Commented Nov 6, 2017 at 19:07
seeing a ".jpg" extension doesn't mean this file is in jpeg format. use newImage.Save(newPath, ImageFormat.Jpeg) — L.B
– L.B, Commented Nov 6, 2017 at 19:18

Ehz · Accepted Answer · 2017-11-06 19:10:12Z

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

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