GDI+ FAQ: Create a negative image.

The simple task of inverting the colours in an image can be accomplished using the ColorMatrix class.

The steps to perform are as follows:

• Open the original image from disc
• Create an image in memory the same size as the original.
• Create an inverting matrix and attach it to an ImageAttributes object.
• Get a Graphics object for the image.
• Draw the original image to the copy using the ColorMatrix to invert the colours.

The C# code for this process is shown in the following listing.

Image img=Image.FromFile(<filename>);

Bitmap copy=new Bitmap(img.Width,img.Height);

ImageAttributes ia = new ImageAttributes();

ColorMatrix cm=new ColorMatrix();

cm.Matrix00=

cm.Matrix11=

cm.Matrix22=-1;

ia.SetColorMatrix(cm);

Graphics g=Graphics.FromImage(copy);

g.DrawImage(img,new Rectangle(0, 0, img.Width, img.Height), 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, ia);

g.Dispose();

img.Dispose();

Note how the ColorMatrix properties are set so that the elements at 0,0 1,1 and 2,2 are set to -1. Setting the matrix element at 3,3 will invert the Alpha of the image also and may make it fully transparent.

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