Dan Byström’s Bwain

Once I tried to use Image.GetThumbnailImage because I wanted to fire up small thumbnails as fast as possible. So I tried:

	// totally and completely, utterly useless
	private Bitmap getThumbnailImage( string filename, int width, int height )
	{
		using ( Image img = Image.FromFile( filename ) )
			return (Bitmap)img.GetThumbnailImage( width, height, null, IntPtr.Zero );
	}

This turned out to be completely useless, since Image.FromFile first loads the full image so that no performance is gained whatsoever. Trying to google after a solution only resulted in tons of articles saying that Image.GetThumbnailImage is pretty useless and shouldn’t be used. So I dropped it and solved my problem in a completely different way. Now I just stumbled across an overloaded version Image.FromStream which I haven’t noticed before:

Image.FromStream( Stream stream, bool useEmbeddedColorManagement, bool validateImageData )

This opens up for some interesting usages. For example it means that we really can get a thumbnail fast:

	// way, way faster, but still pretty useless
	private Bitmap getThumbnailImage( string filename, int width, int height )
	{
		using ( FileStream fs = new FileStream( filename, FileMode.Open ) )
		using ( Image img = Image.FromStream( fs, true, false ) )
			return (Bitmap)img.GetThumbnailImage( width, height, null, IntPtr.Zero );
	}

This is still pretty useless, since this way we really don’t know how to get a proportional thumbnail. This is something that seems to be lacking in GDI+: an easy way to rescale images proportionally. Quite frankly: how often are we interested in non-proportional rescales? Not that often, I’d say! Here’s a better version:

	// actually works...
	private Bitmap getThumbnailImage( string filename, int width )
	{
		using ( FileStream fs = new FileStream( filename, FileMode.Open ) )
		using ( Image img = Image.FromStream( fs, true, false ) )
			return (Bitmap)img.GetThumbnailImage(
				width,
				width * img.Height / img.Width,
				null,
				IntPtr.Zero );
	}

But if we arm ourselves with a way to rescale images proportionally, something Microsoft apparently decided to leave as an exercise for each and every programmer who wants to do even the simplest things with images in GDI+:

	public static Size adaptProportionalSize(
		Size szMax,
		Size szReal )
	{
		int nWidth;
		int nHeight;
		double sMaxRatio;
		double sRealRatio;

		if ( szMax.Width < 1 || szMax.Height < 1 || szReal.Width < 1 || szReal.Height < 1 )
			return Size.Empty;

		sMaxRatio = (double)szMax.Width / (double)szMax.Height;
		sRealRatio = (double)szReal.Width / (double)szReal.Height;

		if ( sMaxRatio < sRealRatio )
		{
			nWidth = Math.Min( szMax.Width, szReal.Width );
			nHeight = (int)Math.Round( nWidth / sRealRatio );
		}
		else
		{
			nHeight = Math.Min( szMax.Height, szReal.Height );
			nWidth = (int)Math.Round( nHeight * sRealRatio );
		}

		return new Size( nWidth, nHeight );
	}
&#91;/sourcecode&#93;

With that, we can fire up a thumbnail image fast, with a given maximum allowed size while still proportional:

&#91;sourcecode language='csharp'&#93;
	// even better...
	private Bitmap getThumbnailImage( string filename, Size szMax )
	{
		using ( FileStream fs = new FileStream( filename, FileMode.Open ) )
		using ( Image img = Image.FromStream( fs, true, false ) )
		{
			Size sz = adaptProportionalSize( szMax, img.Size );
			return (Bitmap)img.GetThumbnailImage(
				sz.Width,
				sz.Height,
				null,
				IntPtr.Zero );
		}
	}
&#91;/sourcecode&#93;

So, it appears that <strong>Image.GetThumbnailImage</strong> had its use after all! But there's more we can do with this.

Even though we managed to load thumbnail images fast and proportionally, the quality isn't particularly good. That's seems to be the main concern among those who advocate not using Image.GetThumbnailImage at all. If a thumbnail is found in the image file it has already been resized once, and resizing a resized image once more certainly won't improve the quality, especially if a crappy resizing algorithm is being used. Let's see what we can do about this. If we're working with JPG images coming from a digital camera, we can most probably find the "real" thumbnail image like this:


	private Bitmap getExifThumbnail( string filename )
	{
		using ( FileStream fs = new FileStream( filename, FileMode.Open ) )
		using ( Image img = Image.FromStream( fs, true, false ) )
		{
			foreach ( PropertyItem pi in img.PropertyItems )
				if ( pi.Id == 20507 )
					return (Bitmap)Image.FromStream( new MemoryStream( pi.Value ) );
		}
		return null;
	}

If we can retrieve a thumbnail this way, it will be in its original size and so we can skip an implicit resize. If we want to rescale it anyway, we can do it in a high quality fashion. I don’t know if this is something everybody knows – I had worked with GDI+ for quite some time before I found it – but fact is that resizing an image like this give good performance but crappy result:

	// poor image quality
	Bitmap bmpResized = new Bitmap( bmpOriginal, newWidth, newHeight );

Instead, try the following:

	// superior image quality
	Bitmap bmpResized = new Bitmap( newWidth, newHeight );
	using ( Graphics g = Graphics.FromImage( bmpResized ) )
	{
		g.InterpolationMode = InterpolationMode.HighQualityBicubic;
		g.DrawImage(
			bmpOriginal,
			new Rectangle( Point.Empty, bmpResized.Size ),
			new Rectangle( Point.Empty, bmpOriginal.Size ),
			GraphicsUnit.Pixel );
	}

With these code pieces glued together I can now get a thumbnail from a JPG image both faster and with better quality than with my original Image.ImageFromThumbnail attempt!

UPDATE: This technique is used “live” in the demo source code accompanying this post: Thumbnails with glass table reflection in GDI+.

Finally, one more thing that I just come to think of while I was typing this. I have complained in earlier posts that Image.FromFile for some obscure reason keeps the file locked until disposed of. I just realized that there is an easy way around his:

	using ( FileStream fs = new FileStream( filename, FileMode.Open ) )
		bmp = (Bitmap)Image.FromStream( fs );

Behold – now the image is loaded and the file is NOT locked! 🙂 Ekeforshus