How I use .NET ColorMatrix to change colors?

I hav开发者_Python百科e an image that I would like to set pixels to White if pixel(x,y).R < 165.

After that I would like to set Black all the pixels that aren't White.

Can I do this using ColorMatrix?

You can't do that with a colormatrix. A colormatrix is good for linear transforms from one color to another. What you need is not linear.

A good way to do these relatively simple image manipulations is to get directly at the bitmap data yourself. Bob Powell has written an article on this at https://web.archive.org/web/20141229164101/http://bobpowell.net/lockingbits.aspx. It explains how to lock a bitmap and access its data via the Marshal class.

It's good to have a struct along these lines:

[StructLayout(LayoutKind.Explicit)]
public struct Pixel
{
    // These fields provide access to the individual
    // components (A, R, G, and B), or the data as
    // a whole in the form of a 32-bit integer
    // (signed or unsigned). Raw fields are used
    // instead of properties for performance considerations.
    [FieldOffset(0)]
    public int Int32;
    [FieldOffset(0)]
    public uint UInt32;
    [FieldOffset(0)]
    public byte Blue;
    [FieldOffset(1)]
    public byte Green;
    [FieldOffset(2)]
    public byte Red;
    [FieldOffset(3)]
    public byte Alpha;


    // Converts this object to/from a System.Drawing.Color object.
    public Color Color {
        get {
            return Color.FromArgb(Int32);
        }
        set {
            Int32 = Color.ToArgb();
        }
    }
}

Just create a new Pixel object, and you can set its data via the Int32 field and read back/modify the individual color components.

Pixel p = new Pixel();
p.Int32 = pixelData[pixelIndex]; // index = x + y * stride
if(p.Red < 165) {
    p.Int32 = 0; // Reset pixel
    p.Alpha = 255; // Make opaque
    pixelData[pixelIndex] = p.Int32;
}