Create Bitmap from a byte array of pixel data

Concerning your question 4: The ImageLockMode.UserInputBuffer can give you the control of the allocating process of those huge amount of memory that could be referenced into a BitmapData object.

If you choose to create yourself the BitmapData object you can avoid a Marshall.Copy. You will then have to use this flag in combinaison with another ImageLockMode.

Beware that it is a complicated business, specially concerning Stride and PixelFormat.

Here is an example that would get in one shot the content of 24bbp buffer onto a BitMap and then in one another shot read it back into another buffer into 48bbp.

Size size = Image.Size;
Bitmap bitmap = Image;
// myPrewrittenBuff is allocated just like myReadingBuffer below (skipped for space sake)
// But with two differences: the buff would be byte [] (not ushort[]) and the Stride == 3 * size.Width (not 6 * ...) because we build a 24bpp not 48bpp
BitmapData writerBuff= bm.LockBits(new Rectangle(0, 0, size.Width, size.Height), ImageLockMode.UserInputBuffer | ImageLockMode.WriteOnly, PixelFormat.Format24bppRgb, myPrewrittenBuff);
// note here writerBuff and myPrewrittenBuff are the same reference
bitmap.UnlockBits(writerBuff);
// done. bitmap updated , no marshal needed to copy myPrewrittenBuff 

// Now lets read back the bitmap into another format...
BitmapData myReadingBuffer = new BitmapData();
ushort[] buff = new ushort[(3 * size.Width) * size.Height]; // ;Marshal.AllocHGlobal() if you want
GCHandle handle= GCHandle.Alloc(buff, GCHandleType.Pinned);
myReadingBuffer.Scan0 = Marshal.UnsafeAddrOfPinnedArrayElement(buff, 0);
myReadingBuffer.Height = size.Height;
myReadingBuffer.Width = size.Width;
myReadingBuffer.PixelFormat = PixelFormat.Format48bppRgb;
myReadingBuffer.Stride = 6 * size.Width;
// now read into that buff
BitmapData result = bitmap.LockBits(new Rectangle(0, 0, size.Width, size.Height), ImageLockMode.UserInputBuffer | ImageLockMode.ReadOnly, PixelFormat.Format48bppRgb, myReadingBuffer);
if (object.ReferenceEquals(result, myReadingBuffer)) {
    // Note: we pass here
    // and buff is filled
}
bitmap.UnlockBits(result);
handle.Free();
// use buff at will...

If you use ILSpy you'll see that this method link to GDI+ and those methods helps are more complete.

You may increase performance by using your own memory scheme, but beware that Stride may need to have some alignment to get the best performance.

You then will be able to go wild for example allocating huge virtual memory mapped scan0 and blit them quite efficiently. Note that pinning huge array (and especially a few) won't be a burden to the GC and will allow you to manipulate the byte/short in a totally safe way (or unsafe if you seek speed)

I'm not sure if there is a reason you're doing it the way you are. Maybe there is. It seems like you're off the beaten path enough so that you might be trying to do something more advanced than what the title of your question implies...

However, the traditional way of creating a Bitmap from a Byte array is:

using (MemoryStream stream = new MemoryStream(byteArray))
{
     Bitmap bmp = new Bitmap(stream);
     // use bmp here....
}

1. Its safe if you marshal.copy data rather than setting scan0 (directly or via that overload of BitMap()). You don't want to keep managed objects pinned, this will constrain the garbage collector.
2. If you copy, perfectly safe.
3. The input array is managed and can be moved by the GC, scan0 is an unmanaged pointer that would get out of date if the array moved. The Bitmap object itself is managed but sets the scan0 pointer in Windows via a handle.
4. ImageLockMode.UserInputBuffer is? Apparently it can be passed to LockBits, maybe it tells Bitmap() to copy the input array data.

Example code to create a greyscale bitmap from array:

    var b = new Bitmap(Width, Height, PixelFormat.Format8bppIndexed);

    ColorPalette ncp = b.Palette;
    for (int i = 0; i < 256; i++)
        ncp.Entries[i] = Color.FromArgb(255, i, i, i);
    b.Palette = ncp;

    var BoundsRect = new Rectangle(0, 0, Width, Height);
    BitmapData bmpData = b.LockBits(BoundsRect,
                                    ImageLockMode.WriteOnly,
                                    b.PixelFormat);

    IntPtr ptr = bmpData.Scan0;

    int bytes = bmpData.Stride*b.Height;
    var rgbValues = new byte[bytes];

    // fill in rgbValues, e.g. with a for loop over an input array

    Marshal.Copy(rgbValues, 0, ptr, bytes);
    b.UnlockBits(bmpData);
    return b;