Question

It’s a little hard to explain what I need but i’ll try:

I need to write application (winform) which will be some kind of filter to image/other forms behind it. With one exception – all behind form should looks as is except of red (for example) color, which have to be replaced to any other specified color, white for example.

So let’s imagine I have opened windows Word with few lines of text. With red and black letters.
So when i place my application above this text – it should “filter” red symbols and fill them to white.

So as i understand this task: i have to snap area behind the form, then process it (replace colors) and after draw this image on my form body.

Any links or keywords for solution?

UPD:

so – this is my final solution:

1. do form transparent (using TransparencyKey and BackColor properties)
2. place picturebox over the form
3. when we need to update image in picturebox – we replace current image with pictureBox1.Image = null;, then refreshing form with (this.Refresh()) and do new snapshot

thanks for all 😉

UPD 2:
sample http://dl.dropbox.com/u/4486681/result.png

UPD 3:
here are sources

Answer 1

you can create a snapshot of the desktop using the following code:

public Bitmap CaptureScreen()
{
    Bitmap b = new Bitmap(SystemInformation.VirtualScreen.Width, SystemInformation.VirtualScreen.Height);
    Graphics g = Graphics.FromImage(b);
    g.CopyFromScreen(0, 0, 0, 0, b.Size);
    g.Dispose();
    return b;
}

Replace the dimensions and position with the coordinates of your form. This way you get a bitmap of what’s behind your form. Then you can do the color replacement on that bitmap.

Please note that due to settings like ClearType and other anti-aliasing mechanisms, you have to also take into account “intermediate pixels” when doing the color replacement. Otherwise things will look funny 🙂