How can I copy the pixel data from a Bitmap with negative stride?

Question

I was looking for the fastest way to convert a Bitmap to 8bpp. I found 2 ways:

1.

        public static System.Drawing.Image ConvertTo8bpp(Bitmap oldbmp)
    {
        using (var ms = new MemoryStream())
        {
            oldbmp.Save(ms, ImageFormat.Gif);
            ms.Position = 0;
            return System.Drawing.Image.FromStream(ms);
        }
    }

2. http://www.wischik.com/lu/programmer/1bpp.html

But: 1. Results in a very low quality result (bad pallet)

and 2 gives me a Bitmap with negative stride, when I try to lockbits and copy the data to a byte array I get an exception: Attempted to read or write protected memory. This is often an indication that other memory is corrupt.

        BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, bmp.PixelFormat);

        this.stride = bmpData.Stride;
        this.bytesPerPixel = GetBytesPerPixel(bmp.PixelFormat);
        int length = bmpData.Stride * bmp.Height;
        if (this.stride < 0)
            this.data = new byte[-length];
        else
            this.data = new byte[length];
        Marshal.Copy(bmpData.Scan0, data, 0, length);

        //Unlock the bitmap
        bmp.UnlockBits(bmpData);

How can I make 2 gives a positive stride? Or how can I copy data using lockbits of a negative stride??

Answer 1

The problem here is that Scan0 points to the beginning of the first scan line, not the beginning of the first byte of data. In a bottom-up bitmap, the first scan line is Stride bytes from the end of the bitmap data.

When you call Marshal.Copy to copy the data from Scan0, it tries to copy (Height*Stride) bytes, starting from position ((Height-1)*Stride). Clearly, that's going to run off into the weeds.

If you just want to copy the bitmap data, you have to calculate the starting address with Scan0 - (Height-1)*Stride. That will start you at the beginning of the bitmap data. You can pass that computed address to Marshal.Copy.

If you want to copy the scan lines in order (i.e. top, next, next, ... bottom), then you have to copy a line at a time: copy Stride bytes from Scan0, then add Stride (which is negative), copy that line, etc. Rick Brewster had the right answer there: https://stackoverflow.com/a/10360753/56778