使用您可以使用 Graphics.DrawImage 接受 ImageAttributes 参数的重载.
ImageAttributes.SetColorMatrix() 设置颜色调整矩阵，可以选择针对特定类别(位图，笔，画笔等)，并且可以指示其跳过灰色"颜色，仅修改灰色"颜色或修改所有颜色".

You can use the Graphics.DrawImage overload that accepts an ImageAttributes argument.
ImageAttributes.SetColorMatrix() sets the color-adjustment matrix, optionally targeting a specific category (Bitmap, Pen, Brush etc.) and can be instructed to skip the Gray Colors, modify the Gray colors only or all Colors.

ImageAttributes.SetThreshold() 方法允许调节颜色"截止点(阈值)以微调亮度.
它接受从 0 到 1 的值.
设置为 0 时，图像全为白色，设置为 1 时全为黑色(请参阅有关此文档的文档).

The ImageAttributes.SetThreshold() method allows to regulate the Colors cutoff point (threshold) to fine tune the Brightness.
It accepts values from 0 to 1.
When set to 0, an image is all white, all black when set to 1 (see the Docs about it).

还认为倒置"是指取决于原始的位图颜色模式，因此请尝试其他方法.有时，将亮度反转可以使您获得更好的效果，有时却不能.

Also consider that the "Inversion" depends on the original bitmap color pattern, so try different approaches. Sometimes, inverting the brightness can give you a better result, sometime it doesn't.

您的OCR必须经过培训"，以验证哪种值更适合它.

You OCR must be "trained", to verify what values suits it better.

看看这些文章:
重新着色(MSDN)
ASCII艺术生成器(CodeProject)

亮度矩阵:
R =红色G =绿色B =蓝色A = Alpha通道W =白色(亮度)

修改亮度组件以获得反转"

Modify the Brightness component to obtain an "Inversion"

    R  G  B  A  W
R  [1  0  0  0  0]
G  [0  1  0  0  0]
B  [0  0  1  0  0]
A  [0  0  0  1  0]
W  [b  b  b  0  1]    <= Brightness

using System.Drawing;
using System.Drawing.Imaging;

// ...

Image colorImage = Clipboard.GetImage();
// Default values, no inversion, no threshold adjustment
var bmpBlackWhite = BitmapToBlackAndWhite(colorImage);
// Inverted, use threshold adjustment set to .75f
var bmpBlackWhite = BitmapToBlackAndWhite(colorImage, true, true, .75f);

// ...

private Bitmap BitmapToBlackAndWhite(Image image, bool invert = false, bool useThreshold = false, float threshold = .5f)
{
    var mxBlackWhiteInverted = new float[][]
    {
        new float[] { -1, -1, -1,  0,  0},
        new float[] { -1, -1, -1,  0,  0},
        new float[] { -1, -1, -1,  0,  0},
        new float[] {  0,  0,  0,  1,  0},
        new float[] {  1,  1,  1,  0,  1}
    };

    var mxBlackWhite = new float[][]
    {
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 1,  1,  1,  0,  0},
        new float[] { 0,  0,  0,  1,  0},
        new float[] {-1, -1, -1,  0,  1}
    };

    var bitmap = new Bitmap(image.Width, image.Height);
    using (var g = Graphics.FromImage(bitmap))
    using (var attributes = new ImageAttributes()) {

        attributes.SetColorMatrix(new ColorMatrix(invert ? mxBlackWhiteInverted : mxBlackWhite));
        // Adjust the threshold as needed
        if (useThreshold) attributes.SetThreshold(threshold);
        var rect = new Rectangle(Point.Empty, image.Size);
        g.DrawImage(image, rect, 0, 0, image.Width, image.Height, GraphicsUnit.Pixel, attributes);
        return bitmap;
    }
}