How do I draw transparent polygons?

This is a discussion on How do I draw transparent polygons? within the C# Programming forums, part of the General Programming Boards category; Hello, Suppose I have two Point / PointF arrays P1 and P2. How can I draw two polygons on a ...

  1. #1
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    How do I draw transparent polygons?

    Hello,

    Suppose I have two Point / PointF arrays P1 and P2. How can I draw two polygons on a panel, one from P1 and the other from P2, such that the two are transparent? Meaning, in case they overlap, I can also see the one that's under.

    From what I've seen on searches I have to use DirectX directly or something like that, but I haven't seen any clear examples. Could someone provide an example?

  2. #2
    and the hat of wrongness Salem's Avatar
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    So what functions do you use to draw opaque polygons?

    Look through the documentation for say "alpha". If you're familiar with RGB for setting a colour, then RGBA is the advanced form.
    Eg.
    RGB=( 255, 0, 0 ); // red

    then
    RGBA=( 255, 0, 0, 255 ); // solid red
    RGBA=( 255, 0, 0, 128 ); // semi-transparent red
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  3. #3
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    Well a simple solution for you would be to alter the color's opacity by setting the color's alpha value to something lower than 255. Example:

    Code:
    using System.Drawing;
    using System.Windows.Forms;
    
    namespace WindowsFormsApplication1
    {
        public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }
    
            private void Form1_Paint(object sender, PaintEventArgs e)
            {
                Graphics g = e.Graphics;
    
                g.FillPolygon(new SolidBrush(Color2Transparent(Color.Red)), new Point[] {
                    new Point(10, 10),
                    new Point(100, 100),
                    new Point(10, 110)
                });
    
                g.FillPolygon(new SolidBrush(Color2Transparent(Color.Blue)), new Point[] {
                    new Point(50, 10),
                    new Point(150, 100),
                    new Point(60, 110)
                });
            }
    
            private Color Color2Transparent(Color c)
            {
                return Color.FromArgb(128, c.R, c.G, c.B); // setting opacity to about 50%
            }
        }
    }
    The result will look like this: http://i45.tinypic.com/9lj3hx.jpg

  4. #4
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    Sorry Salem, I didn't see your post when I starting writing mine.

  5. #5
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    I was using the FillPolygon function to draw opaque polygons. I didn't know about the FromArgb function, that is what I was looking for.

    Thank you both!

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