Develop Reference

Encode a Bitmap as a Jpeg

First, this question is NOT about "how to save a Bitmap as a jpeg on your disk?"
I can't find (or think of) a way of converting a Bitmap with Jpeg compression, but keep it as a Bitmap object. MSDN clearly shows how to save a Bitmap as a JPEG, but what I'm looking for is how to apply the encoding/compression to the Bitmap object, so I can still pass it around, in my code, without referencing the file.
One of the reasons behind that would be a helper class that handles bitmap, but shouldn't be aware of the persistency method used.

All images are bitmaps when loaded into program memory. Specific compressions are typically utilized when writing to disk, and decompressing when reading from disk.
If you're worried about the in-memory footprint of an image you could zip-compress the bytes and pass the byte array around internally. Zipping would be good for lossless compression of an image. Don't forget that many image compressions have different levels of losiness (sp?) In other words, the compression throws away data to store the image in the smallest number of bytes possible.
De/compression is also a performance tradeoff in that you're trading memory footprint for processing time. And in any case, unless you get really fancy, the image does need to be a bitmap if you need to manipulate it in any way.
Here is an answer for a somewhat similar question which you might find interesting.

Bitmap does not support encoded in-memory storage. It is always unencoded (see the PixelFormat enum). Problably you need to write your own wrapper class/abstraction, or give up on that idea.

var stream = new MemoryStream()
Bitmap.Save(Stream, ImageFormat)
Does it what you need?

Processing on large bitmaps (up to 3GB)

I'm working on some university project and got stuck with memory issue.
I load a bitmap which takes about 1,5GB on HDD with code below:
Bitmap bmp = new Bitmap(pathToFile);
The issue is that the newly created Bitmap object uses about 3,5GB of RAM which is something I can't understand (that's really BIG wrapper :E). I need to get to the pixel array, and the use of Bitmap class is really helpful (I use LockBits() method later, and process the array byte per byte) but in this case it's total blocker. So here is my question:
Is there any easy way to extract the pixel array without lending additional 2gb?
I'm using c# just to extract the needed array, which is later processed in c++ - maybe I can extract all needed data in c++ (but conversion issue appears here - I'm concentrating on 24bgr format)?
PS: I need to keep the whole bitmap in memory so splitting it into parts is no solution.
PS2: Just to clarify some issues: I know the difference between file extension and file format. The loaded file is uncompressed bitmap 3 bytes per pixel of size ~1.42GB (16k x 32k pixels), so why Bitmap object is more than two times bigger? Any decompressing issues and converting into other format aren't taking place.

Consider using Memory Mapped Files to access your HUGE data :).
An example focused on what you need can be found here: http://visualstudiomagazine.com/articles/2010/06/23/memory-mapped-files.aspx
It's in managed code but you might as well use it from equivalent native code.
Let me know if you need more details.

You can use this solution , Work with bitmaps faster in C#
http://www.codeproject.com/Tips/240428/Work-with-bitmap-faster-with-Csharp
Or you can use memory mapped files
http://visualstudiomagazine.com/articles/2010/06/23/memory-mapped-files.aspx

You can stop memory caching.
Instead of
Bitmap bmp = new Bitmap(pathToFile);
Use
var bmp = (Bitmap)Image.FromStream(sourceFileStream, false, false);
see https://stackoverflow.com/a/47424918/887092

Reading one bit from a pbm (portable bitmap image)

I want to read a pbm (portable bitmap format) image having depth one bit per pixel and store that data in a file, so I can perform some operation on bits.
I think in C# the smallest data that can be read is 1 byte so how can I read a bit?

You can't read a bit directly. But you can read a byte and then get the bits from that byte. Or, better yet, read a bunch of bytes into an array and then create a BitArray.
Or ... is this "portable bitmap format" supported directly by the Bitmap class? Bitmap does support a 1 bit per pixel format. It might be as simple as calling the Bitmap constructor to load from a file or a stream.
More information on your problem will get you a better answer.

How can I read and write JPEG data on a per-pixel basis?

The title pretty much explains my question. I would like to be able to read and write JPEG data on a per-pixel basis using C#.
I'm thinking something along the lines of CreateJPEG(x, y) which would set up a blank JPEG image in memory, and would give me a JPEG object, and then something like SetPixel(x, y, Color) and GetPixel(x, y) the latter of which would return a Color or something similar. You could then call an Apply() or Save() method, for example, to save the image in a standard JPEG-readable format (preferrably with compression options, but that's not necessary).
And I'm assuming some C# library or namespace makes this all a very easy process, I'd just like to know the best way to go about it.

Have a look at the Bitmap class. For advanced drawing besides manipulating single pixel you will have to use the Graphics class.
var image = new Bitmap("foo.jpg");
var color = image.GetPixel(1, 2);
image.SetPixel(42, 42, Color.White);
image.Save("bar.jpg", ImageFormat.Jpeg);
As Lasse V. Karlsen mentions in his answer this will not really manipulate the JPEG file. The JPEG file will be decompressed, this image data will be altered, and on saving a new JPEG file is created from the altered image data.
This will lower the image quality because even recompressing an unaltered image does usually not yield a bit-identical JPEG file due to the nature of lossy JPEG compressions.
There are some operations that can be performed on JPEG files without decompressing and recompressing it - for example rotating by 90° - put manipulating single pixels does not fit in this category.

JPEG is not a processing format, it's a storage format.
As such, you don't actually use a JPEG image in memory, you just have an image. It's only when you store it that you pick the format, like PNG or JPEG.
As such, I believe you're looking for the Bitmap class in .NET.

How to read and modify the colorspace of an image in c#

I'm loading a Bitmap from a jpg file. If the image is not 24bit RGB, I'd like to convert it. The conversion should be fairly fast. The images I'm loading are up to huge (9000*9000 pixel with a compressed size of 40-50MB). How can this be done?
Btw: I don't want to use any external libraries if possible. But if you know of an open source utility class performing the most common imaging tasks, I'd be happy to hear about it. Thanks in advance.

The jpeg should start with 0xFF 0xD8. After that you will find various fields in the format:
Field identifier 2 bytes
Field length, excluding field identifier. 2 bytes.
Variable data.
Parse through the fields. The identifier you will be looking for is 0xFF 0xC0. This is called SOF0, and contains height, width, bit depth, etc. 0xFF 0xC0 will be followed by two bytes for the field length. Immediately following that will be a single byte showing the bit depth, which will usually be 8. Then there will be two bytes for height, two for width, and a single byte for the number of components; this will usually be 1 (for greyscale) or 3. (for color)

This isn't something I've tried myself, but I think you might need to acccess the picture's EXIF information as a start.
Check out Scott Hanselman's blog-entry on accessing EXIF information from pictures.

Standard .NET System.Drawing namespace should have all that you need,
but it probably won't be very efficient. It'll load the whole thing into RAM, uncompress it, convert it (probably by making a copy) and then re-compress and save it. If you aim for high performance, I'm afraid you might need to look into C/C++ libraries and make .NET wrappers for them.

As far as I know jpg is always 24 bpp. The only thing that could change would be that it's CMY(K?) rather then RGB. That information would be stored in the header. Unfortunately I don't have any means of creating a CMYK image to test whether loading into a Bitmap will convert it automatically.
The following line will read the file into memory:
Bitmap image = Image.FromFile(fileName);
image.PixelFormat will tell you the image format. However, I can't test what the file load does with files other than 24bpp RGB jpgs. I can only recommend that you try it out.