Drawing Lines - Silverlight WriteableBitmap Extensions II

The WriteableBitmap class is a nice feature that was added in Silverlight 3. It could be used to generate fast procedural images by drawing directly to a bitmap. The WriteableBitmap API is very minimalistic and there's only the raw Pixels array for such operations. This Property stores a 32 bit integer as color value for each pixel of the WriteableBitmap.

A couple of months ago I've written a handful of SetPixel methods that made it easier to use the WriteableBitmap, but they only provided a better interface for the Pixels Property and had no real functionality. This time I've written a more algorithmic method which performs a rasterization of a line: The famous DrawLine(). As the name implies is it used to draw a line between two points in the bitmap. To say it in advance: This is the way for drawing huge amounts of lines in Silverlight. I've used the elegant C# 3.0 extension methods again to add this functionality to the existing WriteableBitmap class.

Live

The application includes various scenarios where line-drawing is performed and it is also possible to set the number of lines to control the work load. The Silverlight frame rate counter at the upper left side shows the current FPS in the left-most column. You can find some details about the other parameters in this excellent blog post by András Velvárt(@vbandi).
For comparison I've also added a randomized line drawing scenario using the UIElement Line.

How it works
I've implemented two common line-drawing algorithms: The well-known Bresenham algorithm that uses only cheap integer arithmetic and a Digital Differential Analyzer (DDA) which is based upon floating point arithmetic. I don't want to explain the algorithmic details that were already explained several times in the literature or on the web. The linked Wikipedia articles are a good starting point if you are interested.

The signature of the extension methods

DrawLine(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, Color color);
DrawLine(this WriteableBitmap bmp, int x1, int y1, int x2, int y2, int color);

DrawLineBresenham(this WriteableBitmap bmp, 
                  int x1, int y1, int x2, int y2, int color);

Clear(this WriteableBitmap bmp, Color color);  

The default DrawLine() method uses a DDA algorithm and is available for the Color structure and an integer value as line color. Furthermore it needs the x and y coordinate of the start point (x1, y1) and the end point (x2, y2) of the line.
I have also added a Clear() method that fills the whole WriteableBitmap with a Color.

Usage

// Initialize the WriteableBitmap with size 512x512
WriteableBitmap writeableBmp = new WriteableBitmap(512, 512);

// Set it as source of an Image control
ImageControl.Source = writeableBmp;


// Fill the WriteableBitmap with white color
writeableBmp.Clear(Colors.White);

// Draw a black line from P1(10, 5) to P2(20, 40)
writeableBmp.DrawLine(10, 5, 20, 40, Colors.Black);

// Render it!
writeableBmp.Invalidate();

Results
The WriteableBitmap line-drawing approach is more than 20-30 times faster as UIElement Line. So if you need to draw many lines and don't need anti-aliasing or other UIELement properties, the DrawLine() extensions methods are the right choice.
One interesting fact: The Bresenham algorithm that uses only simple integer operations is not the fastest line-drawing algorithm anymore. It was one of the earliest computer graphics algorithms invented in the 1960s. Since then the hardware has changed dramatically and a simple floating point DDA technique gives almost the same results on modern hardware.
Please keep in mind that these results may differ depending on the used hardware.

Source code
You can download the sample application as a Visual Studio 2008 solution including the complete and documented ready-to-use WriteableBitmapExtensions class from here. Check out my Codeplex project WriteableBitmapEx for an up to date version of the extension methods.
Have fun and let me know it if you do some cool stuff with it.

To be continued...
I'm planning to write more shape extensions for the WriteableBitmap like DrawRectangle(), DrawEllipse(), DrawPolyline(), etc. I will publish them in follow-up blog posts.

Update 11-06-2009
Goto Drawing Shapes - Silverlight WriteableBitmap Extensions III

Silverlight Live Streaming being discontinued. What now?

The Microsoft Live team announced in a blog post that the Silverlight Streaming service is being discontinued. The Silverlight Streaming service offered free hosting for Silverlight applications and videos.




The blog post states:

"Microsoft Silverlight Streaming by Windows Live Beta is being discontinued and will eventually be taken down.
A new Windows® Azure(TM)-based hosting and delivery service will be launched by the end of 2009, though this is not a direct replacement for Silverlight Streaming and will have costs associated with its use."

Further they write:

"Don't panic! All your current content is safe and you will receive sufficient notice for you to make an informed decision on where to host your Silverlight content and applications. However, in the interim, we would like you to be aware of the following:

Effective immediately, no new sign-ups are permitted for the Silverlight Streaming service. In addition, the Silverlight Streaming publishing plug-in for Expression Encoder will no longer be available for download.
The new Windows Azure functionality will not be a direct replacement for the Silverlight Streaming service and will be a paid subscription service."

The blog post also gives some instructions on how to retrieve the hosted Silverlight content. One can't really say that the offered (Azure) "migration process" is comfortable and frictionless. Actually, as far as I know there is no real Azure migration service at the moment, one can only access the Silverlight Streaming file system and copy the content.

What now?
In my opinion this is not a very nice move of Microsoft for bloggers like me. I have used Silverlight Streaming to provide live examples for my Silverlight posts. I mainly blog about Silverlight development and provide content and source code for free, therefore I help Microsoft to spread their technology - for free. It's a shame that a free streaming service at least for personal / educational purposes like mine isn't offered anymore. I might see things too naive and got the intention of Silverlight Streaming wrong, but from my point of view it's definitely not nice.
For now I have uploaded the samples and Videos to my public Dropbox folder and stream them from there. Dropbox uses Amazon's S3 storage system to store files.
Timothy Parez (@delegatevoid) pointed me on this alternative and it actually works fine. Thanks Timothy!
I hope this blog post from the Silverlight Streaming team does not reflect the final decisions for Microsoft's Silverlight hosting. At least I can hope...

Read Between The Pixels - HLSL Kill Pixel Shader

A couple of days ago William Moore (@codenenterp) asked me via Twitter if I know a pixel shader that makes odd pixels black. I didn't know were such a shader is available, but I knew that it should be not hard to write one in HLSL. So I fired up the Shazzam tool and wrote two shaders.






I have started with a pixel shader that kills the color in every odd row of the texture and makes every second scanline transparent. If a black rectangle is overlaid with such a shaded image, each odd scanline will be seen as black, thus resulting in the desired black scanline effect.

An alternating line effect could be realized if an image instead of a black rectangle is overlaid.

After that I've extended the pixel shader to kill every odd pixel to produce the checkerboard effect William Moore wanted.

This is how a Silverlight Button could look like if the shader is applied to it:

How it works
The pixel shaders are pretty simple and small. As always, I've commented the HLSL code, but if you have any further questions, feel free to leave a comment.
Here is the shader that kills every odd scanline:

// Parameter
float2 TextureSize : register(C0);

// Sampler
sampler2D TexSampler : register(S0);

// Shader
float4 main(float2 texCoord : TEXCOORD) : COLOR 
{ 
   // Default color is fully transparent
   float4 color = 0;

   // Scale to int texture size
   float row = texCoord.y * TextureSize.y * 0.5f; 

   // Calc diff between rounded half and half to get 0 or 0.5
   float diff = round(row) - row;
   float diffSq = diff * diff;

   // Even or odd? Only even lines are sampled
   if(diffSq < 0.1)
   {
      color = tex2D(TexSampler, texCoord);
   } 
   return color;
}

And here the shader that kills every odd pixel:

// Parameter
float2 TextureSize : register(C0);

// Sampler
sampler2D TexSampler : register(S0);

// Shader
float4 main(float2 texCoord : TEXCOORD) : COLOR 
{ 
   // Default color is fully transparent
   float4 color = 0;

   // Scale to int texture size, add x and y
   float2 vpos = texCoord * TextureSize * 0.5f; 
   float vposSum = vpos.x + vpos.y;
 
   // Calc diff between rounded half and half to get 0 or 0.5
   float diff = round(vposSum) - vposSum;
   float diffSq = diff * diff;

   // Even or odd? Only even pixels are sampled
   if(diffSq < 0.1)
   {
      color = tex2D(TexSampler, texCoord);
   } 
   return color;
}

As you can see the only difference is that the kill pixel shader uses the sum of the texture coordinate's x and y component to determine if the pixel is even or odd.
For the effect of the uppermost blog post image (mandrill), I've used the product of x and y instead of the sum.
The size of the killed pixel group could be changed with the TextureSize parameter. The following example uses an eight of the original picture size as TextureSize.

Source code
I have also written a small Silverlight application that shows an image with the pixel shader attached. You can download the Visual Studio 2008 solution including the pixel shaders from here.

Photos from the USC-SIPI Image Database