Re: Near-opaque Alpha Values Fail to Retain Color on Copy

From: Bob Powell [MVP] (bob_at__spamkiller_bobpowell.net)
Date: 05/17/04 

Date: Mon, 17 May 2004 11:53:36 +0200

The colour value isn't changing after copy. The values in bmpCopy are
exactly the same.

The value *is* changing after you draw the copied bitmap to the graphics but
that is to be expected because you're drawing a semi opaque pixel which is
being copied using the alpha blending formula. In fact you're drawing a semi
opaque value onto another semi opaque pixel which is even more complex.

If you run your messagebox on the bmpCopy you'll see that the copy was
faithful.

This isn't a bug. 

-- 
Bob Powell [MVP]
Visual C#, System.Drawing
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"BlueMonkMN" <BlueMonkMN@email.com> wrote in message
news:f1299d49.0405160658.78ad528b@posting.google.com...
> I posted earlier about a problem with combining alpha values, but I
> have simplified the problem much more.  Take a look at the red value
> magically changing when I try to make an exact copy of an image with
> near-opaque alpha values.  This must be a GDI+ bug.  Is there a
> work-around or fix?
>
> using System;
> using System.Drawing;
> using System.Drawing.Imaging;
> using System.Drawing.Drawing2D;
> using System.Windows.Forms;
>
> namespace Alpha
> {
>    public class AlphaProblem
>    {
>       public static void Main()
>       {
>          Bitmap bmp = new Bitmap(3,3,PixelFormat.Format32bppArgb);
>          Brush bru = new SolidBrush(Color.FromArgb(254,255,0,0));
>          Graphics g = Graphics.FromImage(bmp);
>          g.FillRectangle(bru,0,0,3,3);
>          MessageBox.Show("Red = " +
>             bmp.GetPixel(1,1).R.ToString(), "Before Copy");
>          Bitmap bmpCopy = (Bitmap)bmp.Clone();
>          g.CompositingMode = CompositingMode.SourceCopy;
>          g.DrawImageUnscaled(bmpCopy,0,0);
>          MessageBox.Show("Red = " +
>             bmp.GetPixel(1,1).R.ToString(), "After Copy");
>          g.Dispose();
>          bru.Dispose();
>          bmpCopy.Dispose();
>          bmp.Dispose();
>       }
>    }
> }

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