Clicking a control and causing it to shade

Question

In windows when you click on an icon on your desktop, the icon darkens with a shade that is based on your windows theme that is currently used.

I have a custom control that displays an image. I would like to have the same functionality as the windows icon click. How do I obtain the same result in WinForms by selecting my custom control?

Answer 1

You can use System.Drawing.KnownColor to get the proper colors for the user's theme.

Answer 2

Windows implements alpha-blending for selected icons since Windows XP. If you want to achieve similar look you must draw your image with alpha blending:

public static void DrawBlendImage(Graphics canvas, Image source, Color blendColor, float blendLevel, int x, int y)
{
  Rectangle SourceBounds = new Rectangle(x, y, source.Width, source.Height);

  ColorMatrix MaskMatrix = new ColorMatrix();
  MaskMatrix.Matrix00 = 0f;
  MaskMatrix.Matrix11 = 0f;
  MaskMatrix.Matrix22 = 0f;
  MaskMatrix.Matrix40 = (float)blendColor.R / byte.MaxValue;
  MaskMatrix.Matrix41 = (float)blendColor.G / byte.MaxValue;
  MaskMatrix.Matrix42 = (float)blendColor.B / byte.MaxValue;
  ImageAttributes MaskAttributes = new ImageAttributes();
  MaskAttributes.SetColorMatrix(MaskMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);

  ColorMatrix TransparentMatrix = new ColorMatrix();
  TransparentMatrix.Matrix33 = blendLevel;
  ImageAttributes TransparentAttributes = new ImageAttributes();
  TransparentAttributes.SetColorMatrix(TransparentMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);

  canvas.DrawImage(source, SourceBounds, 0, 0, source.Width, source.Height, GraphicsUnit.Pixel, MaskAttributes);
  canvas.DrawImage(source, SourceBounds, 0, 0, source.Width, source.Height, GraphicsUnit.Pixel, TransparentAttributes);
}

In your case you can use SystemColors.Highlight as blendColor.

Answer 3

Right now I'm using the following code... if anyone has something better, I'll be glad to change it!

  private void drawAndShadeTheImage(Graphics g)
  {
     //if the image is null then there is nothing to do.
     if (Image != null)
     {
        Bitmap bitMap = new Bitmap(Image);

        //if this control is selected, shade the image to allow the user to
        //visual identify what is selected.
        if (ContainsFocus)
        {               
           //The delta is the percentage of change in color shading of 
           //the image.
           int delta = 70;

           //zero is the lowest value (0 - 255) that can be represented by
           //a color component.
           int zero = 0;

           //Get each pixel in the image and shade it.
           for (int y = 0; y < bitMap.Height; y++)
           {
              for (int x = 0; x < bitMap.Width; x++)
              {
                 Color oColor = bitMap.GetPixel(x, y);

                 //Lime is the background color on the image and should
                 //always be transparent, if this check is removed the
                 //background will be displayed.
                 if (oColor.ToArgb() != Color.Lime.ToArgb())
                 {
                    int oR = oColor.R - delta < zero ? zero : 
                       oColor.R - delta;
                    int oG = oColor.G - delta < zero ? zero : 
                       oColor.G - delta;
                    int oB = oColor.B - delta < zero ? zero : 
                       oColor.B - delta;
                    int oA = oColor.A - delta < zero ? zero : 
                       oColor.A - delta;

                    Color nColor = Color.FromArgb(oA, oR, oG, oB);

                    bitMap.SetPixel(x, y, nColor);
                 }
              }
           }
        }

        //Make the background of the image transparent.
        bitMap.MakeTransparent();

        g.DrawImage(bitMap, this.ClientRectangle);            
     }
  }