Inverting image returns a black image

Question

I want to invert an Image object. Currently my code looks like this:

    private Image Invert(Image img)
    {
        var bmpPicture = new Bitmap(img.Width, img.Height);
        var iaPicture = new ImageAttributes();
        var cmPicture = new ColorMatrix { Matrix00 = -1, Matrix11 = -1, Matrix22 = -1 };
        iaPicture.SetColorMatrix(cmPicture);
        var gfxPicture = Graphics.FromImage(img);
        var rctPicture = new Rectangle(0, 0, img.Width, img.Height);
        gfxPicture.DrawImage(img, rctPicture, 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, iaPicture);
        return bmpPicture;
    }

However, when I run this, and show it in a PictureBox, the result is a black image. I'm running this in Visual Studio 2012 under Windows 8 Release preview. If there is a better way to do this, please let me know. Thanks.

Answer 1

Try this: http://mariusbancila.ro/blog/2009/11/13/using-colormatrix-for-creating-negative-image/

public Bitmap Transform(Bitmap source)
{
    //create a blank bitmap the same size as original
    Bitmap newBitmap = new Bitmap(source.Width, source.Height);

    //get a graphics object from the new image
    Graphics g = Graphics.FromImage(newBitmap);

    // create the negative color matrix
    ColorMatrix colorMatrix = new ColorMatrix(new float[][]
    {
        new float[] {-1, 0, 0, 0, 0},
        new float[] {0, -1, 0, 0, 0},
        new float[] {0, 0, -1, 0, 0},
        new float[] {0, 0, 0, 1, 0},
        new float[] {1, 1, 1, 0, 1}
    }); 

    // create some image attributes
    ImageAttributes attributes = new ImageAttributes();

    attributes.SetColorMatrix(colorMatrix);

    g.DrawImage(source, new Rectangle(0, 0, source.Width, source.Height),
                0, 0, source.Width, source.Height, GraphicsUnit.Pixel, attributes);

    //dispose the Graphics object
    g.Dispose();

    return newBitmap;
}

Answer 2

Fast alternative to using a ColorMatrix for pixel manipulation:

public static void BitmapInvertColors(Bitmap bitmapImage)
{
    var bitmapRead   = bitmapImage.LockBits(new Rectangle(0, 0, bitmapImage.Width, bitmapImage.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppPArgb);
    var bitmapLength = bitmapRead.Stride * bitmapRead.Height;
    var bitmapBGRA   = new byte[bitmapLength];
    Marshal.Copy(bitmapRead.Scan0, bitmapBGRA, 0, bitmapLength);
    bitmapImage.UnlockBits(bitmapRead);

    for (int i = 0; i < bitmapLength; i += 4)
    {
        bitmapBGRA[i]     = (byte)(255 - bitmapBGRA[i]);
        bitmapBGRA[i + 1] = (byte)(255 - bitmapBGRA[i + 1]);
        bitmapBGRA[i + 2] = (byte)(255 - bitmapBGRA[i + 2]);
        //        [i + 3] = ALPHA.
    }

    var bitmapWrite = bitmapImage.LockBits(new Rectangle(0, 0, bitmapImage.Width, bitmapImage.Height), ImageLockMode.WriteOnly, PixelFormat.Format32bppPArgb);
    Marshal.Copy(bitmapBGRA, 0, bitmapWrite.Scan0, bitmapLength);
    bitmapImage.UnlockBits(bitmapWrite);
}