C# – Create Bitmap from a byte array of pixel data

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This question is about how to read/write, allocate and manage the pixel data of a Bitmap.

Here is an example of how to allocate a byte array (managed memory) for pixel data and creating a Bitmap using it:

Size size = new Size(800, 600);
PixelFormat pxFormat = PixelFormat.Format8bppIndexed;
//Get the stride, in this case it will have the same length of the width.
//Because the image Pixel format is 1 Byte/pixel.
//Usually stride = "ByterPerPixel"*Width

//But it is not always true. More info at bobpowell.

int stride = GetStride(size.Width, pxFormat);
byte[] data = new byte[stride * size.Height];
GCHandle handle = GCHandle.Alloc(data, GCHandleType.Pinned);
Bitmap bmp = new Bitmap(size.Width, size.Height, stride,
             pxFormat, handle.AddrOfPinnedObject());

//After doing your stuff, free the Bitmap and unpin the array.
bmp.Dispose();
handle.Free();

public static int GetStride(int width, PixelFormat pxFormat)
{
    //float bitsPerPixel = System.Drawing.Image.GetPixelFormatSize(format);
    int bitsPerPixel = ((int)pxFormat >> 8) & 0xFF;
    //Number of bits used to store the image data per line (only the valid data)
    int validBitsPerLine = width * bitsPerPixel;
    //4 bytes for every int32 (32 bits)
    int stride = ((validBitsPerLine + 31) / 32) * 4;
    return stride;
}

I thought that the Bitmap would make a copy of the array data, but it actually points to the same data. Was you can see:

Color c;
c = bmp.GetPixel(0, 0);
Console.WriteLine("Color before: " + c.ToString());
//Prints: Color before: Color [A=255, R=0, G=0, B=0]
data[0] = 255;
c = bmp.GetPixel(0, 0);
Console.WriteLine("Color after: " + c.ToString());
//Prints: Color after: Color [A=255, R=255, G=255, B=255]

Questions:

Is it safe to do create a bitmap from a byte[] array (managed memory) and free() the GCHandle? If it is not safe, Ill need to keep a pinned array, how bad is that to GC/Performance?
Is it safe to change the data (ex: data[0] = 255;)?
The address of a Scan0 can be changed by the GC? I mean, I get the Scan0 from a locked bitmap, then unlock it and after some time lock it again, the Scan0 can be different?
What is the purpose of ImageLockMode.UserInputBuffer in the LockBits method? It is very hard to find info about that! MSDN do not explain it clearly!

EDIT 1: Some followup

You need to keep it pinned. Will it slow down the GC? I've asked it here. It depends on the number of images and its sizes. Nobody have gave me a quantitative answer. It seams that it is hard to determine.
You can also alloc the memory using Marshal or use the unmanaged memory allocated by the Bitmap.
I've done a lot of test using two threads. As long as the Bitmap is locked it is ok. If the Bitmap is unlock, than it is not safe! My related post about read/write directly to Scan0. Boing's answer "I already explained above why you are lucky to be able to use scan0 outside the lock. Because you use the original bmp PixelFormat and that GDI is optimized in that case to give you the pointer and not a copy. This pointer is valid until the OS will decide to free it. The only time there is a guarantee is between LockBits and UnLockBits. Period."
Yeah, it can happen, but large memory regions are treated different by the GC, it moves/frees this large object less frequently. So it can take a while to GC move this array. From MSDN: "Any allocation greater than or equal to 85,000 bytes goes on the large object heap (LOH)" … "LOH is only collected during a generation 2 collection". .NET 4.5 have Improvements in LOH.
This question have been answered by @Boing. But I'm going to admit. I did not fully understand it. So if Boing or someone else could please clarify it, I would be glad. By the way, Why I can't just directly read/write to Sca0 without locking? => You should not write directly to Scan0 because Scan0 points to a copy of the Bitmap data made by the unmanaged memory (inside GDI). After unlock, this memory can be reallocate to other stuff, its not certain anymore that Scan0 will point to the actual Bitmap data. This can be reproduced getting the Scan0 in a lock, unlock, and do some rotate-flit in the unlocked bitmap. After some time, Scan0 will point to an invalid region and you will get an exception when trying to read/write to its memory location.

Best Answer

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.
If you copy, perfectly safe.
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.
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;

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C# – Creating a byte array from a stream

It really depends on whether or not you can trust s.Length. For many streams, you just don't know how much data there will be. In such cases - and before .NET 4 - I'd use code like this:

public static byte[] ReadFully(Stream input)
{
    byte[] buffer = new byte[16*1024];
    using (MemoryStream ms = new MemoryStream())
    {
        int read;
        while ((read = input.Read(buffer, 0, buffer.Length)) > 0)
        {
            ms.Write(buffer, 0, read);
        }
        return ms.ToArray();
    }
}

With .NET 4 and above, I'd use Stream.CopyTo, which is basically equivalent to the loop in my code - create the MemoryStream, call stream.CopyTo(ms) and then return ms.ToArray(). Job done.

I should perhaps explain why my answer is longer than the others. Stream.Read doesn't guarantee that it will read everything it's asked for. If you're reading from a network stream, for example, it may read one packet's worth and then return, even if there will be more data soon. BinaryReader.Read will keep going until the end of the stream or your specified size, but you still have to know the size to start with.

The above method will keep reading (and copying into a MemoryStream) until it runs out of data. It then asks the MemoryStream to return a copy of the data in an array. If you know the size to start with - or think you know the size, without being sure - you can construct the MemoryStream to be that size to start with. Likewise you can put a check at the end, and if the length of the stream is the same size as the buffer (returned by MemoryStream.GetBuffer) then you can just return the buffer. So the above code isn't quite optimised, but will at least be correct. It doesn't assume any responsibility for closing the stream - the caller should do that.

See this article for more info (and an alternative implementation).

C# – How to convert a byte array to a hexadecimal string, and vice versa

You can use Convert.ToHexString starting with .NET 5.
There's also a method for the reverse operation: Convert.FromHexString.

For older versions of .NET you can either use:

public static string ByteArrayToString(byte[] ba)
{
  StringBuilder hex = new StringBuilder(ba.Length * 2);
  foreach (byte b in ba)
    hex.AppendFormat("{0:x2}", b);
  return hex.ToString();
}

or:

public static string ByteArrayToString(byte[] ba)
{
  return BitConverter.ToString(ba).Replace("-","");
}

There are even more variants of doing it, for example here.

The reverse conversion would go like this:

public static byte[] StringToByteArray(String hex)
{
  int NumberChars = hex.Length;
  byte[] bytes = new byte[NumberChars / 2];
  for (int i = 0; i < NumberChars; i += 2)
    bytes[i / 2] = Convert.ToByte(hex.Substring(i, 2), 16);
  return bytes;
}

Using Substring is the best option in combination with Convert.ToByte. See this answer for more information. If you need better performance, you must avoid Convert.ToByte before you can drop SubString.

Best Answer

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