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Using GDI+ with Windows Forms, I want to be able to draw with a pen and blend color based on the destination pixel color.

For example, if I draw a line and it passes over black pixels, I want it to be a lighter color (like white for example) so that it's visible. When that same line passes over white pixels, it should be a darker color (black for example) so that it's still clearly visible.

Is there any way to do this with GDI+?

Trevor Elliott
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  • That's going to be difficult. You could create a TextureBrush with an ImageAttributes that uses a specially crafted ColorMatrix to obtain this effect. Picking a ColorMatrix that doesn't produce undesirable color shifts is going to be tricky. – Hans Passant Dec 21 '12 at 23:01
  • @Hans: I have it working with a ColorMatrix but I have no idea what kind of ColorMatrix I can use to do what I want. Doesn't the ColorMatrix just interpolate the colors in the image that I'm drawing with the brush? I need to interpolate colors based on the background image that I've already drawn with the Graphics context. – Trevor Elliott Jan 18 '13 at 18:58

3 Answers3

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As Hans Passant proposed, you could paint using what's currently in the canvas as the image for a texture brush (you might need double buffering for that to work correctly) and use a ColorMatrix to modify the colors being painted on the canvas.

There is a color matrix that inverts the colors similar to a XOR, the problem is it won't work with the middle gray. A color matrix that inverts RGB and leaves alpha intact would be:

-1, 0, 0, 0, 0
 0,-1, 0, 0, 0
 0, 0,-1, 0, 0
 0, 0, 0, 1, 0
 1, 1, 1, 0, 1

Something similar, albeit slower would be to copy the canvas to an image and process that image pixel per pixel with rules such as if the color is brighter than 0.5, make it slightly darker else, make it slightly brighter. Then, you paint with that processed image as a texture brush. This would give a better result but it would be significantly slower than using a ColorMatrix.

Coincoin
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  • So you are saying to draw my scene to an Image buffer, then use that Image with a TextureBrush as the Brush for a Pen used to draw the lines I want. Then use a ColorMatrix on the Brush to invert the colors. But then if there are any colors near the middle gray they won't be visible? – Trevor Elliott Jan 21 '13 at 15:28
  • Precisely. Or you can try the second solution which is slower but gives you more control. – Coincoin Jan 22 '13 at 20:32
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You could try XORing the pen color. Paint.NET does this with the selection border to make it visible on any color.

Peter W.
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  • Do you have any idea how they accomplish this? I actually have source code for an older version of Paint.NET and it's not done the same way as the new version. The code I have uses two pens, a white and a black pen, and draws them both with the same dash pattern with different offsets to create an alternating dash color. – Trevor Elliott Jan 18 '13 at 19:40
  • Sorry, no idea. On their blog, they call it a context-sensitive XOR stipple pattern. – Peter W. Jan 19 '13 at 23:10
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Oh, I don't think this is too difficult. You could create a pen that automatically changes colors based on wherever it is. Simply read the pixel at where the location of the pen is (See example), Get the Alpha component and set the pen color to black or white if it's greater than or less than 255/2 respectively :)

Chibueze Opata
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