Copy bytes in memory to an Array in VB.NET

Question

Hello,

unfortunately I cannot resort to C# in my current project, so I'll have to solve this without the unsafe keyword.

I've got a bitmap, and I need to access the pixels and channel values directly. I'd like to go beyond Marshal.ReadByte() and Marshal.WriteByte() (and definitely beyond GetPixel and SetPixel).

Is there a way to put all the pixel data of the bitmap into a Byte array that works on both 32 and 64 bit systems? I want the exact same layout as the original bitmap, so the padding for each row (if it exists) also needs to be included.

Marshal doesn't seem to have something akin to:

byte[] ReadBytes(IntPtr start, int offset, int count)

Unless I totally missed it...

Any help greatly appreciated, David

ps. So far all my images are in 32BppPArgb pixelformat.

Answer 1

VB does not offer methods for direct memory access. You have two choices:

1. Use the Marshal class
2. Write a small unsafe C# (or C++/CLI) library that handles only these operations and reference it from your VB code.

Alright, there is a third option. VB.Net does not inherently support direct memory access, but it can be accomplished. It's just ugly and prone to errors. Nonetheless, if you're willing to put in the effort, you can try building a bitmap access library using these techniques combined with the approach referenced previously.

Answer 2

Would something like this do? (untested):

Public Shared Function BytesFromBitmap(ByVal Image As Drawing.Bitmap) As Byte()
   Using buffer As New IO.MemoryStream()
        image.Save(result, Drawing.Imaging.ImageFormat.Bmp)

        Using rdr As New IO.BinaryReader(buffer)
            Return rdr.ReadBytes(buffer.Length)
        End Using
    End Using
End Function

It won't let you manipulate the pixels in a Drawing.Bitmap object directly, but it will let you copy that bitmap to a byte array, as per the question title.

Another option is serialization via the BinaryFormatter, but I think that will still require you to pass it through a MemoryStream.

Answer 3

Marshal does have a Method that does exactly what you are asking. See Marshall.Copy()

public static void Copy(
    IntPtr source,
    byte[] destination,
    int startIndex,
    int length
   )

Copies data from an unmanaged memory pointer to a managed 8-bit unsigned integer array.

And there are overloads to go the other direction as well

Answer 4

shf301 is right on the money, but I'd like to add a link to a comprehensive explanation/tutorial on fast pixel data access. Rather than saving the image to a stream and accessing a file-in-memory, it would be better to lock the bitmap, copy pixel data out, access it, and copy it back in. The performance of this technique is pretty good.

Code is in c#, but the approach is language-neutral and easy to read.

http://ilab.ahemm.org/tutBitmap.html