Intro
I was developing Silverlight 4 application for drawing floor plan on a plane. One requirement was, there should be a grid mode, and while in this mode, drawing element should snap to the grid. So I need to display this grid on drawing surface. I could think of 2 options how to display a grid:
- Many rectangle elements
- Background brush
The problem with creating rectangle for each grid point was that it appeared to be extremely slow, considering the fact that the grid was supposed to be constantly changing. Maybe it had to do something with the implementation, as grid points were stored in ItemsControl, which was databound to constantly changing ItemsSource.
The problem with background brush is that there is no such a thing in silverlight 4 off-the-shelf. There is just extremely limited range of basic brushes (like SolidColorBrush, LinearGradientBrush). Although it is written in MSDN that you can derive from Brush class, I couldn't find example on how to do this. By definition, brush maps from a point coordinate to a color. Apparently, Brush class does much more, and since it doesn’t provide something like
public abstract class Brush
{
public abstract Color Map(Point point);
}
(apparently, for performance considerations), it seemed like too much of a hassle to mess with the internals of Silverlight to implement my custom brush.
So, after some googling, I came up with 3rd option:
- Pixel shader effect
The idea of pixel shader effect is that each UIElement can have a pixel shader effect applied to it. Pixel shader is somewhat similar to a brush: it maps a pixel from the input image to a color at a corresponding location in the destination image. So it is like a better brush, as it can obtain color at any coordinates from the source image which regular brush cannot.
Silverlight shader effects are written in High-Level Shader Language(HLSL), a C-like language, originally invented by Microsoft for Direct3D. Ideally, pixel shaders should be executed on GPU, utilizing its high parallel throughput ability (each pixel can be processed independently). However, as of now(Silverlight 4), it only utilizes CPU’s SIMD instructions. In other words, today it tends to be quite costly, but this may improve in the future.
So, my idea was, I can create a shader effect which works just like a plain old brush: mapping from source coordinates to destination color(source color can be totally ignored). Shazzam is a good to help with WPF/Silverlight effects development. Having no experience with HLSL, I wrote the simplest thing I could think of, like this:
sampler2D input : register(s0);
// Grid effect shader
/// <summary>Size of the lattice</summary>
/// <minValue>1/minValue>
/// <maxValue>10000</maxValue>
/// <defaultValue>40</defaultValue>
float LatticeSize : register(C0);
/// <summary>Size X of the texture</summary>
/// <minValue>1/minValue>
/// <maxValue>10000</maxValue>
/// <defaultValue>1024</defaultValue>
float SizeX : register(C1);
/// <summary>Size Y of the texture</summary>
/// <minValue>1/minValue>
/// <maxValue>10000</maxValue>
/// <defaultValue>1024</defaultValue>
float SizeY : register(C2);
/// <summary>point size</summary>
/// <minValue>1/minValue>
/// <maxValue>10000</maxValue>
/// <defaultValue>2</defaultValue>
float PointSize : register(C3);
/// <summary>point color</summary>
/// <defaultValue>#FF707070</defaultValue>
float4 PointColor : register(C4);
float4 main(float2 uv : TEXCOORD) : COLOR
{
float stepX = LatticeSize/SizeX;
float stepY = LatticeSize/SizeY;
float pointSizeX = (1 / SizeX)*PointSize;
float pointSizeY = (1 / SizeY)*PointSize;
float4 Color;
Color= tex2D( input, uv.xy);
float dx = uv.x % (stepX);
float dy = uv.y % (stepY);
if(stepX - dx < dx) dx = stepX - dx;
if(stepY - dy < dy) dy = stepY - dy;
if ((dx < pointSizeX)&&(dy < pointSizeY) )
{
Color = PointColor;
}
return Color;
}
and this kind of worked, but there was a problem of edge smoothing. Instead of smoothing edges of a grid point, my shader would create them of a different size, which looks very unpleasant:
Another issue was, this shader effect was also quite slow, considering the fact that it had to be applied for 10000x10000 canvas. Possibly due to all the if’s in the shader code. It is certainly possible to improve this shader so that it would smooth the point edges, and also optimize it big time.
Final solution
The final solution came to my mind after I saw it is possible to create striped brush in Silverlight using LinearGradientBrush, like this
<LinearGradientBrush StartPoint="0,0" EndPoint="0.1,0" SpreadMethod="Repeat">
<GradientStop Offset="0" Color="Red"/>
<GradientStop Offset="0.5" Color="Red"/>
<GradientStop Offset="0.5" Color="White"/>
<GradientStop Offset="1" Color="White"/>
</LinearGradientBrush>
and the idea that we can overlay one canvas on top of the another. So the solution looks like this:
<Grid>
<Canvas Width="300" Height="300">
<Canvas.Background>
<LinearGradientBrush StartPoint="0,0" EndPoint="0,0.05" SpreadMethod="Repeat" >
<GradientStop Offset="0" Color="#FF101010"/>
<GradientStop Offset="0.1" Color="#FF101010"/>
<GradientStop Offset="0.1" Color="#FFF0F0F0"/>
<GradientStop Offset="1" Color="#FFF0F0F0"/>
</LinearGradientBrush>
</Canvas.Background>
</Canvas>
<Canvas Width="300" Height="300">
<Canvas.Background>
<LinearGradientBrush StartPoint="0,0" EndPoint="0.05,0" SpreadMethod="Repeat" >
<GradientStop Offset="0" Color="Transparent"/>
<GradientStop Offset="0.1" Color="Transparent"/>
<GradientStop Offset="0.1" Color="#FFF0F0F0"/>
<GradientStop Offset="1" Color="#FFF0F0F0"/>
</LinearGradientBrush>
</Canvas.Background>
</Canvas>
</Grid>
Which produces the output:
It is still not ideal, as the grid points are somewhat blurred which becomes very apparent as point size approaches 1px on the display. This approach, however, has huge advantage over the others: it is lightning-fast! I think that this, combined with “right” resolutions, will be a reasonable solution.
, 
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