FastBitmap | A fast C # Bitmap wrapping layer | Computer Vision library

I'm working with System.Drawing.Bitmap, and need access to individual pixels, but GetPixel() and SetPixel() are too slow for me.

Using the technique described in this answer, I'm allocating a byte[], pinning it in memory, copying my original Bitmap into it, and unpinning it. I then work with the byte[], and later I pin it, construct a new Bitmap with the buffer, and save it.

I calculate the size of the buffer in bytes as width * height * BytesPerPixel. I'm using PixelFormat.Format32bppArgb, so BytesPerPixel == 4.

This all works fine and dandy, except I apparently allocate a buffer that's too small and cause an access violation when copying the bitmap. I have to add extra space (a lot) to avoid the access violation, and everything appears to work.

How do I calculate the proper size I need for _buffer?

EDIT: I just discovered that the original image is PixelFormat.Format24bppRgb. However, that's 3 bytes per pixel and my buffer should be plenty large enough I would think.

Here is a complete program that demonstrates the problem. ExtraSpace controls the extra bytes I have to allocate...: