Converting a BGR bitmap to RGB

问题:

I am getting bitmaps from a buffer from a net cam. When I assign these to a PictureBox the red and blue are reversed.

What can I do to the bitmap or to the PictureBox to get the red and blue in their proper places?

回答1:

The following code does the needed conversion:

public static void RGBtoBGR(Bitmap bmp)
{
    BitmapData data = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height),
                                   ImageLockMode.ReadWrite, bmp.PixelFormat);

    int length = Math.Abs(data.Stride) * bmp.Height;

    unsafe
    {
        byte* rgbValues = (byte*)data.Scan0.ToPointer();

        for (int i = 0; i < length; i += 3)
        {
            byte dummy = rgbValues[i];
            rgbValues[i] = rgbValues[i + 2];
            rgbValues[i + 2] = dummy;
        }
    }

    bmp.UnlockBits(data);
}

LockBits locks the bitmap in memory so that you can access and change the contents directly. If you don't want to have an unsafe context you can create a byte array and use Marshal.Copy to copy the data into it and back to the bitmap after manipulating. Using LockBits is the fastest option to manipulate bitmap data (much faster than GetPixel or SetPixel).

The loop iterator (i += 3) depends on the PixelFormat of the bitmap. Here I am assuming it is PixelFormat.Format24bppRgb. For Format32bppArgb it would be i += 4.