Blending

http://www.codeproject.com/Articles/20018/Gradients-made-easy

Blending

With blending, we can define a custom gradient and therefore take control over how the gradient is rendered. Blending allows us to override the normal blending behavior and set points along the path where we define alternate drop off rates. We still go from a start color to an end color, but as you can see in the figure below, the drop off rates at the various positions allow much greater control over the rendering process.

Figure 6. Blending using LinearGradientBrush (left) and PathGradientBrush (right)

A couple of terms are in order here:

• Position - A point along the gradient path
• Factor - Drop off rate at the defined position

In Figure 6 above, I have numbered the positions, and combined with the listing below, you will get a better understanding of how blending works. This code is used to produce the right hand drawing in Figure 6.

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private void label1_Paint(object sender, PaintEventArgs e)
{
    GraphicsPath gp = new GraphicsPath();
    gp.AddEllipse(label1.ClientRectangle);    

    PathGradientBrush pgb = new PathGradientBrush(gp);
    pgb.FocusScales = new PointF(0f, 0f);

    Blend blnd = new Blend();
    blnd.Positions = new float[] { 0f, .25f, .5f, .75f, 1f };
    blnd.Factors = new float[] { 1f, 0f, 1f, 0f, 1f };
    pgb.Blend = blnd;
    e.Graphics.FillPath(pgb, gp);

    pgb.Dispose();
    gp.Dispose();
}

Both gradients were drawn using the same values, and from what we've learned so far, the outcome is predictable. The only difference being the PathGradientBrush, the positions radiate outward from the center point, and at the positions noted, you can see the transitions made.

Note: There are three things that are very important here:

1. The number of positions and factors must be the same.
2. The first position must be 0 and the last 1.
3. The number of positions, factors, and colors should not exceed the number of points in the graphics point list. If it does, I turn off color blending, or GDI+ throws an exception. (Working on this.)

If either of these conditions is not met, GDI+ will throw an exception.