WPF WriteableBitmap and effects

Question

Im trying to look into using the WPF WriteableBitmap class to allow my application to apply an opacity mask to an image.

Basically I have a blue rectangle as an image, and another 100% transparent green rectangle image over the top of the blue one.

When the user moves their mouse over the green (transparent) image, I want to apply the opacity mask (perhaps using a simple ellipse) so that it looks like a green glow is occurring.

Im purposefully not doing this is XAML and standard WPF effects because I really need it to be super performant and I will eventually swap out the ellipse with a more advance blob...

Any thoughts??

Thanks!

Answer 1

Hi Mark.

I'm sorry, I don't quite understand your intentions. Maybe if I could see the image, I could answer correctly from start, but here is my first-maybe-wrong answer.

If you say super-performant, you probably want to look at pixel shaders. They are processed by GPU, supported by WPF in a form of a custom effect and are easy to implement. Also you can apply shaders to playing video, while it's hard to do with WritableBitmap.

To write a pixel shader, you need to have FX Compiler (fxc.exe) from DirectX SDK and Shazzam tool - WYSIWYG WPF Shaders compiler by Walt Ritscher.

When you get them both, go ahead and try the following HLSL code

float X : register(C0); // Mouse cursor X position
float Y : register(C1); // Mouse cursor Y position
float4 Color : register(C2); // Mask color
float R : register(C3); // Sensitive circle radius.

sampler2D implicitInputSampler : register(S0);


float4 main(float2 uv : TEXCOORD) : COLOR
{
    float4 finalColor = tex2D(implicitInputSampler, uv);
    if ( (uv.x - X) * (uv.x - X) + (uv.y - Y) * (uv.y - Y) < R*R)
    {
        finalColor = Color; // Blend/Change/Mask it as you wish here.
    }
    return finalColor;
}

This gives you the following C# effect:

namespace Shazzam.Shaders {
    using System.Windows;
    using System.Windows.Media;
    using System.Windows.Media.Effects;


    public class AutoGenShaderEffect : ShaderEffect {

        public static DependencyProperty InputProperty = ShaderEffect.RegisterPixelShaderSamplerProperty("Input", typeof(AutoGenShaderEffect), 0);

        public static DependencyProperty XProperty = DependencyProperty.Register("X", typeof(double), typeof(AutoGenShaderEffect), new System.Windows.UIPropertyMetadata(new double(), PixelShaderConstantCallback(0)));

        public static DependencyProperty YProperty = DependencyProperty.Register("Y", typeof(double), typeof(AutoGenShaderEffect), new System.Windows.UIPropertyMetadata(new double(), PixelShaderConstantCallback(1)));

        public static DependencyProperty ColorProperty = DependencyProperty.Register("Color", typeof(System.Windows.Media.Color), typeof(AutoGenShaderEffect), new System.Windows.UIPropertyMetadata(new System.Windows.Media.Color(), PixelShaderConstantCallback(2)));

        public static DependencyProperty RProperty = DependencyProperty.Register("R", typeof(double), typeof(AutoGenShaderEffect), new System.Windows.UIPropertyMetadata(new double(), PixelShaderConstantCallback(3)));

        public AutoGenShaderEffect(PixelShader shader) {
            // Note: for your project you must decide how to use the generated ShaderEffect class (Choose A or B below).
            // A: Comment out the following line if you are not passing in the shader and remove the shader parameter from the constructor

            PixelShader = shader;

            // B: Uncomment the following two lines - which load the *.ps file
            // Uri u = new Uri(@"pack://application:,,,/glow.ps");
            // PixelShader = new PixelShader() { UriSource = u };

            // Must initialize each DependencyProperty that's affliated with a shader register
            // Ensures the shader initializes to the proper default value.
            this.UpdateShaderValue(InputProperty);
            this.UpdateShaderValue(XProperty);
            this.UpdateShaderValue(YProperty);
            this.UpdateShaderValue(ColorProperty);
            this.UpdateShaderValue(RProperty);
        }

        public virtual System.Windows.Media.Brush Input {
            get {
                return ((System.Windows.Media.Brush)(GetValue(InputProperty)));
            }
            set {
                SetValue(InputProperty, value);
            }
        }

        public virtual double X {
            get {
                return ((double)(GetValue(XProperty)));
            }
            set {
                SetValue(XProperty, value);
            }
        }

        public virtual double Y {
            get {
                return ((double)(GetValue(YProperty)));
            }
            set {
                SetValue(YProperty, value);
            }
        }

        public virtual System.Windows.Media.Color Color {
            get {
                return ((System.Windows.Media.Color)(GetValue(ColorProperty)));
            }
            set {
                SetValue(ColorProperty, value);
            }
        }

        public virtual double R {
            get {
                return ((double)(GetValue(RProperty)));
            }
            set {
                SetValue(RProperty, value);
            }
        }
    }
}

Now you can track mouse position, and set corresponding properties of your effect to trigger changes. One thing to note here: X and Y in HLSL code are ranged from 0 to 1. So you'll have to convert actual coordinates to percentages, before passing them to shader.

Things to read more about pixel shaders and WPF:

• Reference for HLSL on MSDN.
• Writing custom GPU-based Effects for WPF by Greg Schechter.

Hope this helps :)