I’ve been playing around with image processing lately and since my last post about loading thumbnail images from files I couldn’t help myself from trying to roll my own “Web 2.0 reflection effect” directly in .NET 2.0 with no 3D support whatsoever. Actually, I think was more inspired by Windows Vista’s thumbnails (to the right) than the web.

This is what I eventually came up with:

Although this is all easy – since there were a few things that couldn’t be done in “pure” GDI+ and then some uncommon approaches involved in my solution, I think that there may be some people out there who don’t find this totally trivial. So I thought that it might be worth writing this down.

From the original picture I work through four steps:

1. The first step merely shrinks the original picture to the desired size and puts a frame around it. This is trivial:

	protected virtual Bitmap createFramedBitmap( Bitmap bmpSource, Size szFull )
	{
		Bitmap bmp = new Bitmap( szFull.Width, szFull.Height );
		using ( Graphics g = Graphics.FromImage( bmp ) )
		{
			g.FillRectangle( FrameBrush, 0, 0, szFull.Width, szFull.Height );
			g.DrawRectangle( BorderPen, 0, 0, szFull.Width - 1, szFull.Height - 1 );
			g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
			g.DrawImage(
				bmpSource,
				new Rectangle( FrameWidth, FrameWidth, szFull.Width - FrameWidth * 2, szFull.Height - FrameWidth * 2 ),
				new Rectangle( Point.Empty, bmpSource.Size ),
				GraphicsUnit.Pixel );
		}
		return bmp;
	}

(Note the InterpolationMode property. It is important in order to resize the image with good quality!)

2. The second step is substantially more involved. It takes the result from step 1 and does this, all in one go:

1. Flip the image upside down (omitting the upper and lower parts of frame, since we don’t want them to be present in the reflection).
2. Apply a Gaussian blur convolution effect to make the image look…, well, blurred… 🙂
3. Wash out some color to make the reflection a little bit grayish.
4. Apply an alpha blend fall out.

Flipping the image and the color wash-out can both be done directly in GDI+. Flip either with Bitmap.RotateFlip or with a transformation matrix and use a ColorMatrix to alter the colors. But since neither a blur effect nor an alpha blend can be done without direct pixel manipulation I did it all in one go. For the blur effect, see Christian Graus excellent article series Image Processing for Dummies with C# and GDI+. I’ve blogged earlier on how to perform alpha blending previously by drawing the blend using a PathGradientBrush or a LinearGradientBrush in Soft edged images in GDI+. This time I will calculate the alpha value instead. The calculation is done once for every scan line and is located in a virtual function so that this formula can be overridden.

All four “effects” are handled in this loop:

	for ( int y = height-1 ; y >= 0 ; y-- )
	{
		byte alpha = (byte)(255 * calculateAlphaFallout( (double)(height - y) / height ));
		Pixel* pS = (Pixel*)bdS.Scan0.ToPointer() + bdS.Width * (bdS.Height - y - FrameWidth - 1);
		Pixel* pD = (Pixel*)bdD.Scan0.ToPointer() + bdD.Width * y;
		for ( int x = bdD.Width ; x > 0 ; x--, pD++, pS++ )
		{
			int R = gaussBlur( &pS->R, nWidthInPixels );
			int G = gaussBlur( &pS->G, nWidthInPixels );
			int B = gaussBlur( &pS->B, nWidthInPixels );
			pD->R = (byte)((R * 3 + G * 2 + B * 2) / 7);
			pD->G = (byte)((R * 2 + G * 3 + B * 2) / 7);
			pD->B = (byte)((R * 2 + G * 2 + B * 3) / 7);
			pD->A = alpha;
		}
	}

Flipping happens on line 4, blurring on lines 8-10 (the gaussBlur function is just a one-liner). Color wash-out is done on lines 11-13 and the alpha fall-out on lines 3 and 14.

The very observant reader will notice that I let the blurring wrap from one edge to another. This is a hack, but it works since the left and right edges are always exactly identical. In production code, it might be a good idea to make the blurring optional (or even to provide a user-defined convolution matrix) and also to do the same from the color wash-out which currently uses hard-coded values.

3. Create the “half sheared” bitmap:

This transform cannot be accomplished using a linear transformation, but (after taking quite a a detour on this) I realized that it can be done embarrassingly simple:

	using ( Graphics g = Graphics.FromImage( Thumbnail ) )
		for ( int x = 0 ; x < sz.Width ; x++ )
			g.DrawImage(
				bmpFramed,
				new RectangleF( x, 0, 1, sz.Height - Skew * (float)(sz.Width - x) / sz.Width ),
				new RectangleF( x, 0, 1, sz.Height ),
				GraphicsUnit.Pixel );
&#91;/sourcecode&#93;

I simply use DrawImage to draw each column by its own, transferring one column from the framed image from step 1 to a column of different height. Note that it is extremely important that we pass <strong>float</strong>s and not <strong>int</strong>s - in the latter case the result will be a disaster.

<span style="font-size:x-large;">4.</span> Draw the reflection image through a shear transform, like this:


	using ( Graphics g = Graphics.FromImage( Thumbnail ) )
	{
		System.Drawing.Drawing2D.Matrix m = g.Transform;
		m.Shear( 0, (float)Skew / sz.Width);
		m.Translate( 0, sz.Height - Skew - 1 );
		g.Transform = m;
		g.DrawImage( bmpReflection, Point.Empty );
	}
 

Download demo source code

A Lesson Learned

At first, I tried to do the shearing in both step 3 & 4 myself using code similar to this (still one column at a time):

	// this was a bad idea
	private static void paintRowWithResize(
		BitmapData bdDst,
		BitmapData bdSrc,
		int nDstColumn,
		int nSrcColumn,
		int nDstRow,
		double dblSrcRow,
		int nRows,
		double dblStep )
	{
		unchecked
		{
			unsafe
			{
				Pixel* pD = (Pixel*)bdDst.Scan0.ToPointer() + nDstColumn + nDstRow * bdDst.Width;
				Pixel* pS = (Pixel*)bdSrc.Scan0.ToPointer() + nSrcColumn;
				while ( nRows -- > 0 )
				{
					int nYSrc = (int)dblSrcRow;
					Pixel p1 = pS[nYSrc * bdSrc.Width];
					Pixel p2 = p2 = pS[(nYSrc + 1) * bdSrc.Width];
					double frac2 = dblSrcRow - nYSrc;
					double frac1 = 1.0 - frac2;
					pD->R = (byte)(p1.R * frac1 + p2.R * frac2);
					pD->G = (byte)(p1.G * frac1 + p2.G * frac2);
					pD->B = (byte)(p1.B * frac1 + p2.B * frac2);
					pD->A = (byte)(p1.A * frac1 + p2.A * frac2);

					dblSrcRow += dblStep;
					pD += bdDst.Width;
				}
			}
		}
	}

The result looked perfectly good, but after thinking about it for awhile I realized that this piece of code actually is completely and utterly wrong in the general case: it only works when the alpha values of the two adjacent pixels are very close. In other cases the result will be poor.

Perhaps the easiest way to see this is to think about what happens when we want a 50% mix of a completely transparent pixel and a completely opaque while pixel. Intuitively I think it’s clear that we want the result to be a white pixel with 50% transparency. However, if we represent the transparent pixel with (0,0,0,0) (the most common value I’d guess, although (0,x,x,x) is transparent regardless of the value of x) we get a gray half transparent pixel instead (127,127,127,127). Not right at all. The reason I thought my attempt looked good in the first place was just because I had a gray border around the images!

So how do we mix pixels with alpha values? Obviously “normal” alpha blending is not sufficient when we have alpha values on both pixels… after thinking about this for a few minutes,  I said to myself “why not ask someone who knows instead?”. That someone is of course Graphics.DrawImage, and so I ended up with much cleaner code. And although I never bothered to figure out how to mix and blend pixels when both pixels contain alpha values I ended up realizing this:

Graphics.DrawImage has quite a bit of work to do when we draw a 32-bit bitmap on top of another. If we have some a-priori knowledge of the nature of the bitmaps we’re working with (are any of them totally opaque?) then it is actually possible to do this ourselves much faster than Graphics.DrawImage has a chance to, because it is forced to work with the general case: both bitmaps may be semi-transparent.

I will get back on how we in some cases can outperform Graphics.DrawImage (when it comes to speed), and hopefully a real life case when we actually bother. Stay tuned. Ekeforshus

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