How to resize an image in C# to a certain hard-disk size?

You can calculate an approximate information level for the image by taking the original image size divided by the number of pixels:

info = fileSize / (width * height);

I have an image that is 369636 bytes and 1200x800 pixels, so it uses ~0.385 bytes per pixel.

I have a smaller version that is 101111 bytes and 600x400 pixels, so it uses ~0.4213 bytes per pixel.

When you shrink an image you will see that it generally will contain slightly more information per pixel, in this case about 9% more. Depending on your type of images and how much you shrink them, you should be able to calculate an average for how much the information/pixel ration increases (c), so that you can calculate an approximate file size:

newFileSize = (fileSize / (width * height)) * (newWidth * newHeight) * c

From this you can extract a formula for how large you have to make an image to reach a specific file size:

newWidth * newHeight = (newFileSize / fileSize) * (width * height) / c

This will get you pretty close to the desired file size. If you want to get closer you can resize the image to the calculated size, compress it and calculate a new bytes per pixel value from the file size that you got.

I achieved this by reducing the quality until I reached my desired size.

NB: Requires you to add the System.Drawing reference.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;
using System.Drawing.Drawing2D;

namespace PhotoShrinker
{
class Program
{
/// <summary>
/// Max photo size in bytes
/// </summary>
const long MAX_PHOTO_SIZE = 409600;

static void Main(string[] args)
{
    var photos = Directory.EnumerateFiles(Directory.GetCurrentDirectory(), "*.jpg");

    foreach (var photo in photos)
    {
        var photoName = Path.GetFileNameWithoutExtension(photo);

        var fi = new FileInfo(photo);
        Console.WriteLine("Photo: " + photo);
        Console.WriteLine(fi.Length);

        if (fi.Length > MAX_PHOTO_SIZE)
        {
            using (var image = Image.FromFile(photo)) 
            {
                  using (var stream = DownscaleImage(image))
                  {
                        using (var file = File.Create(photoName + "-smaller.jpg"))
                        {
                            stream.CopyTo(file);
                        }
                  }
            }
            Console.WriteLine("File resized.");
        }
        Console.WriteLine("Done.")
        Console.ReadLine();
    }

}

private static MemoryStream DownscaleImage(Image photo)
{
    MemoryStream resizedPhotoStream = new MemoryStream();

    long resizedSize = 0;
    var quality = 93;
    //long lastSizeDifference = 0;
    do
    {
        resizedPhotoStream.SetLength(0);

        EncoderParameters eps = new EncoderParameters(1);
        eps.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, (long)quality);
        ImageCodecInfo ici = GetEncoderInfo("image/jpeg");

        photo.Save(resizedPhotoStream, ici, eps);
        resizedSize = resizedPhotoStream.Length;

        //long sizeDifference = resizedSize - MAX_PHOTO_SIZE;
        //Console.WriteLine(resizedSize + "(" + sizeDifference + " " + (lastSizeDifference - sizeDifference) + ")");
        //lastSizeDifference = sizeDifference;
        quality--;

    } while (resizedSize > MAX_PHOTO_SIZE);

    resizedPhotoStream.Seek(0, SeekOrigin.Begin);

    return resizedPhotoStream;
}

private static ImageCodecInfo GetEncoderInfo(String mimeType)
{
    int j;
    ImageCodecInfo[] encoders;
    encoders = ImageCodecInfo.GetImageEncoders();
    for (j = 0; j < encoders.Length; ++j)
    {
        if (encoders[j].MimeType == mimeType)
            return encoders[j];
    }
    return null;
}
}
}