GalleryShare/GalleryShare/ThumbnailGenerator.cs

using System;
using System.Threading.Tasks;
using System.Drawing;
using System.IO;

using System.Drawing.Drawing2D;
using System.Drawing.Imaging;

namespace GalleryShare
{
	public static class ThumbnailGenerator
	{
		public static void GenerateThumbnailPng(string imagePath, Size thumbnailBounds, Stream outputStream)
		{
			using (Bitmap rawImage = new Bitmap(imagePath)) {
				float scaleFactor = Math.Min(
					thumbnailBounds.Width / (float)rawImage.Width,
					thumbnailBounds.Height / (float)rawImage.Height
				);
				Size thumbnailSize = new Size(
					(int)(rawImage.Width * scaleFactor),
					(int)(rawImage.Height * scaleFactor)
				);

				using (Bitmap smallImage = new Bitmap(thumbnailSize.Width, thumbnailSize.Height))
				using (Graphics context = Graphics.FromImage(smallImage)) {
					context.CompositingMode = CompositingMode.SourceCopy;
					context.InterpolationMode = InterpolationMode.HighQualityBicubic;
					context.DrawImage(rawImage, new Rectangle(
						Point.Empty,
						thumbnailSize
					));

					smallImage.Save(outputStream, ImageFormat.Png);
				}
			}
		}
	}
}