Question

I’m using this code to capture the screen:

public Bitmap CaptureWindow(IntPtr handle)
{
    // get te hDC of the target window
    IntPtr hdcSrc = User32.GetWindowDC(handle);
    // get the size
    User32.RECT windowRect = new User32.RECT();
    User32.GetWindowRect(handle, ref windowRect);
    int width = windowRect.right - windowRect.left;
    int height = windowRect.bottom - windowRect.top;
    // create a device context we can copy to
    IntPtr hdcDest = GDI32.CreateCompatibleDC(hdcSrc);
    // create a bitmap we can copy it to,
    // using GetDeviceCaps to get the width/height
    IntPtr hBitmap = GDI32.CreateCompatibleBitmap(hdcSrc, width, height);
    // select the bitmap object
    IntPtr hOld = GDI32.SelectObject(hdcDest, hBitmap);
    // bitblt over
    GDI32.BitBlt(hdcDest, 0, 0, width, height, hdcSrc, 0, 0, GDI32.SRCCOPY);
    // restore selection
    GDI32.SelectObject(hdcDest, hOld);
    // clean up 
    GDI32.DeleteDC(hdcDest);
    User32.ReleaseDC(handle, hdcSrc);
    // get a .NET image object for it
    Bitmap img = Image.FromHbitmap(hBitmap);
    // free up the Bitmap object
    GDI32.DeleteObject(hBitmap);
    return img;
}

I then want to convert the bitmap to 256 colors (8 bit). I tried this code but get an error about not being able to create an Image from an indexed bitmap format:

Bitmap img8bit = new Bitmap(img.Width,img.Height,
                           System.Drawing.Imaging.PixelFormat.Format8bppIndexed);
Graphics g = Graphics.FromImage(img8bit);
g.DrawImage(img,new Point(0,0));

I did see some examples to convert bitmaps between different formats, but in my case I’m looking for the best way to do this while capturing from the screen. For example, if there is a method that will work better by creating an 8-bit bitmap to begin with and then blit the screen to that, that would be preferred over caputring screen to comptible bitmap first and then converting it. Unless it’s better to capture then convert anyway.

I have a program written in C++ using Borland Builder 6.0 VCL, and I’m trying to memic that. In that case it is a simple matter of setting the pixel format for VCL’s TBitmap object. I notice Bitmap.PixelFormat is read-only in .NET, ugh.

Update: In my case I don’t think the answer is as complex as some other usage that requires figuring out the best palette entries, because Graphics.GetHalftonePalette using the screen DC should be fine, since my original bitmap comes from the screen, not just any random bitmap that might come from a file/email/download/etc. I beleive there is something that can be done with maybe 20 lines of code that involves DIBs and GetHalftonePalette — just can’t find it yet.

Answer 1

Converting a full color bitmap to 8bpp is a difficult operation. It requires creating a histogram of all the colors in the image and creating a palette that contains an optimized set of colors that best map to the original colors. Then using a technique like dithering or error diffusion to replace the pixels whose colors don’t have an exact match with the palette.

This is best left to a professional graphics library, something like ImageTools. There is one cheap way that can be tricked in the .NET framework. You can use the GIF encoder, a file format that has 256 colors. The result isn’t the greatest, it uses dithering and that can be pretty visible sometimes. Then again, if you really cared about image quality then you wouldn’t use 8bpp anyway.

    public static Bitmap ConvertTo8bpp(Image img) {
        var ms = new System.IO.MemoryStream();   // Don't use using!!!
        img.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
        ms.Position = 0;
        return new Bitmap(ms);
    }