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Confused about LockBits, BitmapData and PixelFormat.Format48bppRgb

Consider the following code:
public static Bitmap Create3x3Bitmap(PixelFormat pixelFormat)
{
var bmp = new Bitmap(3, 3, pixelFormat);
// I know SetPixel does not perform well, this is strictly for learning purposes
bmp.SetPixel(0, 0, Color.Red);
bmp.SetPixel(1, 0, Color.Lime);
bmp.SetPixel(2, 0, Color.Blue);
bmp.SetPixel(0, 1, Color.White);
bmp.SetPixel(1, 1, Color.Gray);
bmp.SetPixel(2, 1, Color.Black);
bmp.SetPixel(0, 2, Color.Cyan);
bmp.SetPixel(1, 2, Color.Fuchsia);
bmp.SetPixel(2, 2, Color.Yellow);
return bmp;
}
The code above should produce a 3 x 3 Bitmap instance which, if magnified, should look like this:
I figured out that I could "scan" the bitmap's pixels information using the Bitmap.LockBits method. I succeeded doing just that using the 24 and 32 bits based PixelFormat values as the pixelFormat argument.
However, I have yet to understand how to work out pixel informations when PixelFormat.Format48bppRgb is used instead:
public void Test()
{
using (var bmp = Create3x3Bitmap(PixelFormat.Format48bppRgb))
{
var lockRect = new Rectangle(0, 0, bmp.Width, bmp.Height);
var data = bmp.LockBits(lockRect, ImageLockMode.ReadWrite, bmp.PixelFormat);
var absStride = Math.Abs(data.Stride); // will be equal to 20
var size = absStride * data.Height; // will be equal to 60
byte[] scanData = new byte[size];
Marshal.Copy(data.Scan0, scanData, 0, size);
// ...
// more stuff here, irrelevant for the actual question
// ...
bmp.UnlockBits(bitmapData);
}
}
If I run the code above using the debugger and break right after the call to Marshal.Copy, I can see that the scanData byte array contains 60 bytes. My understanding is that for each of the three RGB channels, two bytes are required. This means 6 bytes per pixel. There's also two additional unused bytes for each "row" on the y-axis, which in this case is 3 rows.
So if I'm getting this right, here is how I should interpret the array's content for the first row:
Now what confuses me is how I'm supposed to interpret each channel's pair of bytes and translate that back into the original color. It makes sense that for the first pixel (which is red), a pair of 0s would be found for both the blue and green channel. I wasn't so sure what to make of 0 and 32 as a pair that's supposed to mean "full-on red", but looking around the net I came to understand that the range is 0 to 8192 rather than 0 to 65535, due to a limitation of GDI+.
And sure enough, using BitConverter.ToUInt16 got me a value of 8192 for that pair of bytes. So the result for the red pixel makes sense.
However, it also gives 1768 for the "232, 6" pairs found on each channel of the gray pixel (indexes 26 to 31 on the image above). And that's where I am confused. Since gray color's 8 bits channels are midway in the range of 0 to 255, I would have expected something like 4095 or 4096, which is midway between 0 to 8192. 🤷‍♂️
Is my understanding on byte pairs representing channels even correct? If so then how come I got these channel values for gray color?

Minor nit: you wrote this:
There's also two additional empty pixels for each "row" on the y-axis, which in this case is 3 rows.
There are two additional bytes for each row, making the stride 20 bytes instead of the 18 bytes that would be needed for three 6-byte pixel values. This fulfills the GDI+ requirement that the bitmap stride be a multiple of 4.
As far as the question itself goes…
As noted in a comment, the Q&A Why are RGB values of a PixelFormat.Format48bppRgb image only from 0 to 255? should be helpful to you. Indeed, this statement has at least half of the answer to your puzzle:
GDI+ allows only values from 0 to 8192 in the color channel.
This means that when you set a pixel to, for example, red, which has a 24-bit RGB value of (255,0,0), this is extended to 48-bits by creating a value of (8192,0,0). This is different from the (65535,0,0) one might expect with 16 bits per pixel.
The other part is that the pixel channels are stored as two-byte, little-endian, 16-bit values. When you see a byte 0 (0x00), followed by a byte 32 (0x20), the way to interpret that is to store the first byte in a ushort variable, then shift the second byte left by 8 bits and combine that with the first byte in the same variable. E.g.:
byte[] redChannelBytes = { 0x00, 0x20 };
ushort redChannel = redChannelBytes[0] | (redChannelBytes[1] << 8);
Or you can just call BitConverter.ToUInt16(byte[], int)
byte[] redChannelBytes = { 0x00, 0x20 };
ushort redChannel = BitConverter.ToUInt16(redChannelBytes, 0);

Maybe I'm a bit late but I think I can add some useful info to the accepted answer.
Since gray color's 8 bits channels are midway in the range of 0 to 255, I would have expected something like 4095 or 4096, which is midway between 0 to 8192.
It's because the transformation between 'ordinary' and 'wide' pixel formats is not linear. The regular pixel formats (and the Color struct as well) represent colors with gamma correction γ = 2.2, whereas the wide pixel formats have no gamma correction (γ = 1.0).
To get the correct 13bpp level, you can use the following formula:
outputLevel = 8192 * Math.Pow(inputLevel / 255d, 1d / gamma)
which gives you 1770 for inputLevel = 128 and gamma = 0.45 (= 1 / 2.2), which is very close to 1768.
Actually Windows is cheating a bit as you can notice for low inputLevel values: the formula above gives you 0 for input levels < 5, though try SetPixel with 48/64 bpp format and low RGB color values and you can see that the raw RGB components are never zero and they are always different. So using a proper gamma correction may ridiculously cause loss of information when widening the pixel format... That's why (besides performance) I use lookup tables instead when translating colors. The linked source points to the 48-bit color transformations.
To make things even more complicated all these apply only when using GDI+ on Windows. The ReactOS implementation uses a simple linear transformation for wide formats, and also uses the full 16 bit range as opposed to the Windows' 13 bpp range. And in Mono the libgdiplus implementation simply does not support the wide formats at all.
That's why the linked source firstly checks whether to use the lookup tables. It always respects the actual underlying behavior and can also use the full 16bpp range depending on the actual GDI+ implementation. Feel free to use the library if you wish. One of the main motivations was to simplify the complexity and also to provide fast solutions for manipulating bitmaps of any PixelFormat. It also allows to access the actual underlying raw data in a managed way (see the ReadRaw/WriteRaw examples under the last link).

Number of channels in System.Drawing.Bitmap C#

I'm coding in C#, and loading images this way :
// loading image
string imageFileName = "myImage.jpg";
Bitmap bmp = new Bitmap(imageFileName, false);
// trying to display color so that I know how many channels I have
// except it always displays 4 values, whether I have 1, 3 or 4 channels
Color color = bmp.GetPixel(0, 0);
Console.WriteLine(color.ToString());
// locking the bitmap's bits
System.Drawing.Imaging.BitmapData bmpData = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat);
// doing stuff that requires me to know the number of channels of the image, amongst other things
// unlocking the bitmap's bits
bmp.UnlockBits(bmpData);
I need to know the number of channels in my image (usually they are grayscale (1), RGB (3) or RGBA (4)), and I don't know how is that information storred.
EDIT :
I'm not looking to force a pixel format. I'm trying to load an image, and figure out procedurally what is the number of channel in the image I loaded.

There is a PixelFormat property, you can read about in on MSDN.
You can combine with: GetPixelFormatSize it to get the bytes per pixel if you want.

Creation:
You need to use an overload of the Bitmap contructor that takes a PixelFormat parameter.
Usage:
The the bitmap.PixelFormat property will tell you what you have.
Here is more info about the PixelFormats

c# faster way of setting pixel colours

I've picked up a project that creates a noise map using setPixel(). The majority of the runtime of this application is spent within the setPixel() function so I was looking to increase the speed at which the function executes.
I have done some research into this and found that this:
int index = x + (y * Width);
int col = color.ToArgb();
if (this.Bits == null)
{
this.Bits = new Int32[Width * Height];
}
Bits[index] = col;
has been recommended as a quicker approach. However, this is generating a completely black image.
I don't understand 100% how image manipulation and memory pointers work to be able to understand the code completely and refactor it to something better.
Here's the original code the person before implemented:
unsafe
{
var scan0 = (byte*)Iptr;
int bitmapStride = Stride;
int bitmapPixelFormatSize = Depth / 8;
index = (bitmapStride * y) + (x * bitmapPixelFormatSize);
if (bitmapPixelFormatSize == 4)
{
scan0[index + 3] = color.A;
scan0[index + 2] = color.R;
scan0[index + 1] = color.G;
scan0[index] = color.B;
}
else if (bitmapPixelFormatSize == 1)
{
scan0[index] = color.R;
}
else
{
scan0[index + 2] = color.R;
scan0[index + 1] = color.G;
scan0[index] = color.B;
}
}
Iptr is just an IntPtr
Stride is an int, the only place I can find this being set is Stride = (PixelCount/Height) * (Depth / 8)
x is width
y is height
Would I be able to get an explanation of what is happening in the original block of code and possibly some help in understanding how to convert that to something that executes quicker, currently it takes around 500,000ms to finish this function due to a nested for loop of width * height.

Note: The following information was originally created by Bob Powell. The original link is no longer functional, so I have copied this information from the Internet Archive at https://web.archive.org/web/20120330012542/http://bobpowell.net/lockingbits.htm. It's a bit long, but I think it's worth preserving.
I'm not sure if this will serve as a direct answer to your question, but perhaps it will help you in finding a solution.
Using the LockBits method to access image data
Many image processing tasks and even file type conversions, say from 32 bit-per-pixel to 8 bit-per-pixel can be speeded up by accessing the pixel data array directly, rather than relying on GetPixel and SetPixel or other methods.
You will be aware that .NET is a managed code system which most often uses managed data so it's not often that we need to gain access to bytes stored in memory anymore however, image manipulation is one of the few times when managed data access is just too slow and so we need to delve once again into the knotty problems of finding the data and manipulating it.
Before I start on the subject in hand, I'll just remind you that the methods used to access any unmanaged data will be different depending on the language in which your program is written. C# developers have the opportunity, via the unsafe keyword and use of pointers, to access data in memory directly. Visual basic programmers should access such data through the Marshal class methods which may also show a small performance loss.
Lock up your bits
The Bitmap class provides the LockBits and corresponding UnlockBits methods which enable you to fix a portion of the bitmap pixel data array in memory, access it directly and finally replace the bits in the bitmap with the modified data. LockBits returns a BitmapData class that describes the layout and position of the data in the locked array.
The BitmapData class contains the following important properties;
Scan0 The address in memory of the fixed data array
Stride The width, in bytes, of a single row of pixel data. This width
is a multiple, or possibly sub-multiple, of the pixel dimensions of
the image and may be padded out to include a few more bytes. I'll
explain why shortly.
PixelFormat The actual pixel format of the data. This is important
for finding the right bytes
Width The width of the locked image
Height The height of the locked image
The relationship of Scan0 and Stride to the array in memory is shown in figure1.
The Stride property, as shown in figure 1, holds the width of one row in bytes. The size of a row however may not be an exact multiple of the pixel size because for efficiency, the system ensures that the data is packed into rows that begin on a four byte boundary and are padded out to a multiple of four bytes. This means for example that a 24 bit per pixel image 17 pixels wide would have a stride of 52. The used data in each row would take up 317 = 51 bytes and the padding of 1 byte would expand each row to 52 bytes or 134 bytes. A 4BppIndexed image of 17 pixels wide would have a stride of 12. Nine of the bytes, or more properly eight and a half, would contain data and the row would be padded out with a further 3 bytes to a 4 byte boundary.
The data carrying portion of the row, as has been suggested above, is laid out according to the pixel format. A 24 bit per pixel image containing RGB data would have a new pixel every 3 bytes, a 32 bit per pixel RGBA every four bytes. Pixel formats that contain more than one pixel per byte, such as the 4 bit per pixel Indexed and 1 bit per pixel indexed, have to be processed carefully so that the pixel required is not confused with it's neigbour pixels in the same byte.
Finding the right byte.
Because the stride is the width of a row, to index any given row or Y coordinate you can multiply the stride by the Y coordinate to get the beginning of a particular row. Finding the correct pixel within the row is possibly more difficult and depends on knowing the layout of the pixel formats. The following examples show how to access a particular pixel for a given pixel format.
Format32BppArgb Given X and Y coordinates, the address of the first
element in the pixel is Scan0+(y * stride)+(x*4). This Points to the
blue byte. The following three bytes contain the green, red and alpha
bytes.
Format24BppRgb Given X and Y coordinates, the address of the first
element in the pixel is Scan0+(yStride)+(x3). This points to the
blue byte which is followed by the green and the red.
Format8BppIndexed Given the X and Y coordinates the address of the
byte is Scan0+(y*Stride)+x. This byte is the index into the image
palette.
Format4BppIndexed Given X and Y coordinates the byte containing the
pixel data is calculated as Scan0+(y*Stride)+(x/2). The corresponding
byte contains two pixels, the upper nibble is the leftmost and the
lower nibble is the rightmost of two pixels. The four bits of the
upper and lower nibble are used to select the colour from the 16
colour palette.
Format1BppIndexed Given the X and Y coordinates, the byte containing
the pixel is calculated by Scan0+(y*Stride)+(x/8). The byte contains
8 bits, each bit is one pixel with the leftmost pixel in bit 8 and
the rightmost pixel in bit 0. The bits select from the two entry
colour palette.
Iterating through the pixels
For pixel formats with one or more bytes per pixel, the formula is simple and can be accomplished by looping through all Y and X values in order. The code in the following listings sets the blue component of a 32 bit per pixel image to 255. In both cases bm is a bitmap previously created.
BitmapData bmd=bm.LockBits(new Rectangle(0, 0, 10, 10), System.Drawing.Imaging.ImageLockMode.ReadOnly, bm.PixelFormat);
int PixelSize=4;
for(int y=0; y<bmd.Height; y++)
{
byte* row = (byte *)bmd.Scan0+(y*bmd.Stride);
for(int x = 0; x<bmd.Width; x++)
{
row[x * PixelSize] = 255;
}
}
In VB this operation would be treated a little differently because VB has no knowledge of pointers and requires the use of the marshal class to access unmanaged data.
Dim x As Integer
Dim y As Integer
Dim PixelSize As Integer = 4
Dim bmd As BitmapData = bm.LockBits(new Rectangle(0, 0, 10, 10), System.Drawing.Imaging.ImageLockMode.ReadOnly, bm.PixelFormat)
For y = 0 To bmd.Height - 1
For x = 0 To bmd.Width - 1
Marshal.WriteByte(bmd.Scan0, (bmd.Stride * y) + (4 * x) , 255)
Next
Next
Sub-byte pixels.
The Format4BppIndexed and Format1BppIndexed pixel formats mentioned earlier both have more than one pixel stored in a byte. In such cases, it's up to you to ensure that changing the data for one pixel does not effect the other pixel or pixels held in that byte.
The method for indexing a 1 bit per pixel image relies on using bitwise logical operations And and Or to reset or set specific bits in the byte. After using the formula shown above for 1 bit per pixel images, the lower 3 bits of the X coordinate is used to select the bit required. The listings below show this process in C# and VB. In both examples bmd is a bitmap data extracted from a 1 bit per pixel image.
C# code uses pointers and requires compiling with unsafe code
byte* p=(byte*)bmd.Scan0.ToPointer();
int index=y*bmd.Stride+(x>>3);
byte mask=(byte)(0x80>>(x&0x7));
if(pixel)
p[index]|=mask;
else
p[index]&=(byte)(mask^0xff);
VB code uses the marshal class
Dim mask As Byte = 128 >> (x And 7)
Dim offset As Integer = (y * bmd.Stride) + (x >> 3)
Dim currentPixel As Byte = Marshal.ReadByte(bmd.Scan0, offset)
If pixel = True Then
Marshal.WriteByte(bmd.Scan0, offset, currentPixel Or mask)
Else
Marshal.WriteByte(bmd.Scan0, offset, CByte(currentPixel And (mask Xor 255)))
End If
Note, it's quite valid to use the Marshal class from C# code. I used pointers because it offers the best performance.
Accessing individual pixels in a 4 bit per pixel image is handled in a similar manner. The upper and lower nibble of the byte must be dealt with separately and changing the contents of the odd X pixels should not effect the even X pixels. The code below shows how to perform this in C# and VB.
C#
int offset = (y * bmd.Stride) + (x >> 1);
byte currentByte = ((byte *)bmd.Scan0)[offset];
if((x&1) == 1)
{
currentByte &= 0xF0;
currentByte |= (byte)(colorIndex & 0x0F);
}
else
{
currentByte &= 0x0F;
currentByte |= (byte)(colorIndex << 4);
}
((byte *)bmd.Scan0)[offset]=currentByte;
VB
Dim offset As Integer = (y * bmd.Stride) + (x >> 1)
Dim currentByte As Byte = Marshal.ReadByte(bmd.Scan0, offset)
If (x And 1) = 1 Then
currentByte = currentByte And &HF0
currentByte = currentByte Or (colorIndex And &HF)
Else
currentByte = currentByte And &HF
currentByte = currentByte Or (colorIndex << 4)
End If
Marshal.WriteByte(bmd.Scan0, offset, currentByte)
Using LockBits and UnlockBits
The LockBits method takes a rectangle which may be the same size or smaller than the image being processed, a PixelFormat which is usually the same as that of the image being processed and a ImageLockMode value that specifies whether the data is read-only, write-only, read-write or a user allocated buffer. This last option cannot be used from C# or VB because the method overload for LockBits that specifies a user buffer is not included in the GDI+ managed wrapper.
It is very important that when all operations are complete the BitmapData is put back into the bitmap with the UnlockBits method. The snippet of code below illustrates this.
Dim bmd As BitmapData = bm.LockBits(New Rectangle(0, 0, 10, 10), ImageLockMode.ReadWrite, bm.PixelFormat)
' do operations here
bm.UnlockBits(bmd)
Summary
That just about covers the aspects of accessing the most popular and most difficult pixel formats directly. Using these techniques instead of the GetPixel and SetPixel methods provided by Bitmap will show a marked performance boost to your image processing and image format conversion routines.

Byte of pixel data to image

I have learnt how to export pixel data of an image to byte array, here is my code
void Button2Click(object sender, EventArgs e)
{
Bitmap img = new Bitmap (#"24x30.bmp");
var BitmapData = img.LockBits( new Rectangle(0,0,img.Width,img.Height),ImageLockMode.ReadOnly,img.PixelFormat);
var length = BitmapData.Stride * BitmapData.Height;
MessageBox.Show(BitmapData.Width.ToString());
byte[] bytes = new byte[length];
Marshal.Copy(BitmapData.Scan0, bytes, 0, length);
img.UnlockBits(BitmapData);
string test = ByteArrayToBinary(bytes);
}
I convert the bytes to string bit but lets ignore it. What I want to know is, how can I convert the byte of pixel data to an image? Please share the code and the reference.
I have read many references but I don't get it until now.
EDIT:
This summary of my case, i have Stride, Width, Height, and Byte[] of Pixel data. How can i reconstruct it to image again thanks

Same code, but copy the other way. (You can read a dummy image or use another image constructor.)

Obviously, you first step should be to somehow convert your text back to a byte array, but then you'll see that you can't actually create an image from just that data.
As you mentioned in comments already, your dumped binary block is missing all header data. You need the original width, height and pixel format before you can convert the data back to an image.
Also, if this is an indexed image, there will be a colour palette, and you don't save that either.
And finally, you copy all data in the image memory. Images are kept in memory per line of pixels, but these lines are usually rounded to the next multiple of four bytes (except in some indexed pixel formats, I think). This means that unless your image uses four bytes per pixel (32 bits per pixel), the bytes you end up with may contain junk data at the end of each line. You don't trim that off in any way, meaning you not only need the width and height, but the stride as well, before you can reconstruct the image.
As for how to build the image, the method is pretty much the same. You make a new image with the correct dimensions and pixel format, open its backing memory using LockBits but in WriteOnly mode, and copy your data into it.
I posted a full method for that on this site before, so feel free to check it out.

C# Getting the pixel data efficiently from System.Drawing.Bitmap

I have several (~2GB) raw 24bpp RGB files on HDD.
Now I want to retrieve a portion of it and scale it to the desired size.
(The only scales allowed are 1, 1/2, 1/4, 1/8, ..., 1/256)
So I'm currently reading every line from the rectangle of interest into an array, which leaves me with a bitmap which has correct height but wrong width.
As the next step I'm creating a Bitmap from the newly created array.
This is done with by using a pointer so there is no copying of data involved.
Next I'm calling GetThumbnailImage on the Bitmap, which creates a new bitmap with the correct dimensions.
Now I want to return the raw pixel data (as a byte array) of the newly created bitmap.
But to achieve that I'm currently copying the data using LockBits into a new array.
So my question is: Is there a way to get the pixel data from a Bitmap into a byte array without copying it?
Something like:
var bitmapData = scaledBitmap.LockBits(...)
byte[] rawBitmapData = (byte[])bitmapData.Scan0.ToPointer()
scaledBitmap.UnlockBits(bitmapData)
return rawBitmapData
I'm well aware that this doesn't work, it is just an example to what I basically want to achieve.

I think this is your best bet.
var bitmapData = scaledBitmap.LockBits(...);
var length = bitmapData.Stride * bitmapData.Height;
byte[] bytes = new byte[length];
// Copy bitmap to byte[]
Marshal.Copy(bitmapData.Scan0, bytes, 0, length);
scaledBitmap.UnlockBits(bitmapData);
You have to copy it, if you want a pass around a byte[].
You don't have to delete the bytes that were allocated, you just need to Dispose of the original Bitmap object when done as it implements IDisposable.