Basics & Requirements

What is OpenTK?

OpenTK is a library that provides high-speed access to native OpenGL, OpenCL, and OpenAL for .NET programs. OpenTK aims to make working with OpenGL, OpenCL, and OpenAL in .NET-supported languages like C# similar in both performance and feel to the equivalent C code, while still running in a managed (and safe!) environment.

Note that OpenTK is not a high-level library: It’s not a game engine, or a framework, or a full renderer, or a complete audio system by itself. Instead, it’s the low-level foundation on which you can build those kinds of things. If you want a framework or a renderer or a game engine that’s already built, there are lots of those elsewhere on the Internet.

OpenTK also includes a convenient math library for common graphics types like vectors and matrices, so that you don’t have to implement those yourself or find another library to provide them. These types are designed to be very fast and flexible, and they include considerable functionality out-of-the-box too.

What is OpenGL?

OpenGL is a cross-platform graphics-rendering library, originally developed by Silicon Graphics (SGI), and now maintained by the Khronos group. OpenGL is used for everything from video games to CAD tools to web browsers to mobile phones.

What is OpenCL?

OpenCL is a cross-platform parallel-computation specification, originally developed by Apple, and now maintained by the Khronos group. It’s primarily used for performing computation on GPUs that would otherwise be computed less efficiently on CPUs, but it can also be used to control digital signal processors (DSPs), field-programmable gate arrays (FPGAs), and other kinds of hardware compute accelerators.

What is OpenAL?

OpenAL is a cross-platform 3D audio library, originally developed by Loki Software, and released as an open specification by Creative Labs, with multiple implementations in both hardware and software. OpenAL is used in lots of games to provide both sound effects and music and is the successor to EAX and A3D.

What operating systems are supported by OpenTK?

OpenTK runs on Windows, Linux, and MacOS X.

(Older versions of OpenTK supported Android and iOS as well; current OpenTK might work on these, but it’s untested.)

What hardware is supported?

OpenTK supports any hardware that is supported by your OS’s native OpenGL, OpenCL, and OpenAL bindings: Since OpenTK is just a .NET layer around the native libraries, it does whatever they do.

Which .NET versions are supported?

OpenTK 4.x+ runs on .NET Core 3.x and .NET 5+.
OpenTK 3.x runs on .NET Framework 4.x.

Which version of OpenTK should I use?

If you’re able to use .NET Core or .NET 5+, you should use .NET Core or .NET 5+, and then use the latest version of OpenTK 4 or higher with it.

If you’re stuck on the older .NET Framework 4.x, you will need to use OpenTK 3.x with it. OpenTK 4.x+ is not compatible with .NET Framework, and there are no plans to support newer releases of OpenTK on .NET Framework.

What’s the status of OpenTK 5.0?

OpenTK 5.0 is currently in development, and is not yet ready for release. We have a TODO list tracks its current status. We’d welcome any help, if you’d like to contribute!

Which release is the official release?

https://github.com/opentk/opentk/ is the official source-code repository for OpenTK. There are also several related projects managed by the OpenTK team, including the GLControl and GLWpfControl and ObjectTK.
https://www.nuget.org/packages/OpenTK/ is the official Nuget package for OpenTK. Similarly, there are other Nuget packages for the related projects.

All other distributions of OpenTK are not officially supported by the OpenTK team. In particular:

The Xamarin/Microsoft OpenTK package is a fork of a very old version of OpenTK 1.x. The OpenTK team does not support the Xamarin code.
The OpenTK.NETCore package (now unlisted) is a fork of OpenTK 3. .NET Core projects should use OpenTK 4+ instead.

Which versions of OpenGL are supported?

OpenTK primarily supports OpenGL 3+ and OpenGL ES 2+ — in other words, modern OpenGL. The fixed-function pipeline (FFP) — a.k.a. “classic OpenGL” — can be made to work, but it is not directly supported.

What about UI frameworks?

OpenTK can be used without a UI framework: It is capable of spawning a window and taking input directly, which is often sufficient for many programs like games.

But if you need to integrate OpenTK with an existing UI, it includes some packages to do so:

OpenTK 4.x has —
- A WinForms-compatible GLControl Nuget package.
- A WPF-compatible GLWpfControl Nuget package.
OpenTK 3.x contains its own WinForms-compatible GLControl component.

How do I use Dear ImGui with OpenTK?

To get started with Dear ImGui checkout the sample repo created by NogginBops. Copy the source of ImGuiController.cs into your project to get going.

Warning

There is a nuget package called OpenTK.ImGui that should NOT be used. It's not up to date and is not created by NogginBops. It will not work.

How fast is OpenTK?

For a .NET library, OpenTK is very fast. OpenTK 3 and 4 use hand-optimized IL assembly to minimize overhead when calling OpenGL functions, and OpenTK 5 is planned to use the new C# function pointers to accomplish the same thing.

We take great pains to make it as efficient as possible. However, keep in mind that the underlying runtimes (.NET/Mono) use both JIT compilation and garbage collection, which can complicate the definition of “fast.” Still, we place significant emphasis on performance, so if you believe something could run faster, please open an issue on our GitHub!

Does OpenTK support...

Vulkan or Metal?

OpenTK does not currently support Vulkan or Metal. OpenTK 5 may not either, but the changes for OpenTK 5 may make it easier to support Vulkan in the future. Feel free to submit a pull request and help out!

DirectX or Direct3D?

OpenTK is focused on cross-platform, portable, open standards. There are other libraries that support DirectX if you want to exclusively target Microsoft platforms.

OpenVG? OpenVX? OpenXR?

OpenTK does not currently support the Khronos group’s other open standards. Feel free to submit a pull request and help out!

Installing and Using OpenTK

How do I get OpenTK?

OpenTK is distributed as a Nuget package. The release Nuget package can be found on nuget.org, and it can be installed via Nuget using your IDE, or via the nuget command-line tool.

The source code is in our GitHub repository.

In Visual Studio, installing OpenTK is as simple as installing any other Nuget package:

Right-click on your C# project in the Solution Explorer.
Choose “Manage Nuget packages.”
Type opentk into the Search box and press Enter.
Click on “OpenTK by Team OpenTK.”
Click on the “Install” button.

Which `using` directives do I need?

OpenTK works just like the underlying OpenGL/CL/AL calls, so if you know how to program one of those libraries, you already know most of what you’ll need to know.

First, add a using directive for the underlying subsystem you want to use. For example, you probably want to use one or more of these:

using OpenTK.Graphics.OpenGL;
using OpenTK.Graphics.OpenGL4;
using OpenTK.Graphics.ES30;
using OpenTK.Audio.OpenAL;
using OpenTK.Compute.OpenCL;

Which ones to use depends on which flavors of OpenGL/CL/AL you’re targeting. We generally recommend with OpenGL that you should use OpenGL4 unless you need the legacy fixed-function pipeline (FFP); the OpenGL4 namespace works for all OpenGL versions, but it excludes the FFP functions and their overhead. (In OpenTK 5, we plan to rename these namespaces to make this distinction more obvious.)

How do I create a window using OpenTK?

There are a lot of older, outdated explanations for this across the Internet. For OpenTK 4+, you’ll want to inherit a class from NativeWindow or GameWindow, instantiate your custom window, and then poll for events. Here’s a simple, complete, working example Program.cs that uses GameWindow:

using OpenTK.Mathematics;
using OpenTK.Windowing.Desktop;

namespace MyOpenTKExample
{
    public static class Program
    {
        public static void Main()
        {
            var nativeWindowSettings = new NativeWindowSettings
            {
                Size = new Vector2i(800, 600),
                Title = "My OpenTK Example Program"
            };
            
            using (var window = new MyExampleWindow(GameWindowSettings.Default,
                                                    nativeWindowSettings))
            {
                window.Run();
            }
        }
    }
    
    public class MyExampleWindow : GameWindow
    {
        public MyExampleWindow(GameWindowSettings gameWindowSettings,
                              NativeWindowSettings nativeWindowSettings)
            : base(gameWindowSettings, nativeWindowSettings)
        {
        }
        
        protected override void OnUpdateFrame(FrameEventArgs e)
        {
            base.OnUpdateFrame(e);

			// This gets called every 1/60 of a second.
            if (KeyboardState.IsKeyDown(Keys.Escape))
                Close();
        }
        
        protected override void OnRenderFrame(FrameEventArgs e)
        {
            // Show that we can use OpenGL: Clear the window to cornflower blue.
            GL.ClearColor(0.39f, 0.58f, 0.93f, 1.0f);
            GL.Clear(ClearBufferMask.ColorBufferBit);

            // Show in the window the results of the rendering calls.
            SwapBuffers();
        }
    }
}

More sophisticated uses will load textures and shaders and models during the OnLoad() handler, clean up during the OnUnload() handler, and do a lot more than just wait for the user to press Escape to close the window — but the code above is enough to get you started.

(Creating a window in OpenTK 3 requires a similar amount of code, but it looks a bit different from this example. New projects should use OpenTK 4+.)

How do I call OpenGL/CL/AL functions?

In your code, you can invoke the OpenGL/CL/AL functions directly using the static classes that OpenTK exposes. OpenTK provides all of the same functions that the underlying native libraries provide, just with the initial gl* or cl* or al* prefix turned into a static class name. So, for example:

In C you’d call int error = glGetError();. In C# you’d write ErrorCode error = GL.GetError();.
In C you’d call glBindVertexArray(handle);. In C# you’d write GL.BindVertexArray(handle);.
In C you’d write glCreateShader(GL_FRAGMENT_SHADER);. In C# you’d write GL.CreateShader(ShaderType.FragmentShader).

As you can see, the programming models are nearly identical, by design: If you know how to use OpenGL/OpenCL/OpenAL, you know how to use OpenTK.

Arrays and pointers

Passing arrays from .NET to OpenGL/CL/AL (or to any C library) is a complex topic, but OpenTK has built-in support for it: See the section below on “How do I pass pointers and arrays?”.

Constant symbols

Note in the third example that the constants change syntax slightly: They aren’t global #define directives as in C, but instead have some minor alterations to make them more .NET-friendly:

The constants are grouped into strong enum types like ErrorCode and ShaderType — they’re not just a big pile of unrelated values like they are in C.
The names are changed from SHOUT_CASE to PascalCase to match C# style conventions.
The constants' GL_ prefixes have been removed for readability.

There is also a special enum type, All, that contains all of the GL constants; this can simplify porting code from other languages, and it ensures that all of the OpenGL constants are available. All should be avoided in favor of more specialized enum types where possible (but it isn't always possible, so don't forget you can cast from All to int or to the enum type you need!).

Where do I start with OpenGL?

Check out our tutorial here, which will teach you how to build a basic working program that can draw simple objects. You can learn more about OpenGL in depth over at Learn OpenGL; our tutorials are based on theirs.

What’s the difference between OpenGL and OpenGL ES?

OpenGL ES is “OpenGL for Embedded Systems.” It’s a subset of OpenGL, designed for low-power and mobile devices. It has many similarities with modern OpenGL, especially OpenGL ES 2.0 and later, but it typically strips out lesser-used functionality. However, OpenGL ES is not a proper subset of OpenGL, so an exact comparison between them is difficult.

But typically, if you’re coding for desktop PCs or Macs, you’ll want to use OpenGL 4.x; while if you’re targeting a mobile phone or a gaming console, you’ll want to use the newest version of OpenGL ES that’s supported on it.

How do I load textures?

That’s mostly outside the scope of this FAQ, but there are a variety of techniques for loading images from files in .NET, and once the image is loaded into main memory, you then use one of the various GL texture functions to load it into the GPU, like GL.TexImage2D(). For more details, read through the section on loading and creating textures in our tutorial, derived from the tutorial on textures at Learn OpenGL.

How do I create shaders?

That’s outside the scope of this FAQ, but there’s a tutorial on shaders and GLSL as part of our lessons here, derived from the tutorial on shaders and GLSL at Learn OpenGL.

How do I handle errors?

The same way as in OpenGL/CL/AL! OpenTK exposes the same glError() functions, so you can test for errors in C# the same way you would in C. Here’s a simple example:

Example code in C:

int shader = glCreateShader(GL_FRAGMENT_SHADER);

int errorCode = glGetError();
if (errorCode) {
    printf("Uh oh.");
    exit(-1);
}

C# equivalent:

int shader = GL.CreateShader(ShaderType.FragmentShader);

ErrorCode errorCode = GL.GetError();
if (errorCode != ErrorCode.NoError)
{
    throw new UhOhException(...);
}

Arguably, throwing exceptions is a better way to handle errors than aborting.

Gl.GetError() in OpenTK reads errors from a queue, just like glGetError() does — it’s just a passthrough to the same underlying error state.

That seems like a lot of if-statements; is there a better way?

OpenGL 4.3+ offers a new technique, called a debug message callback. (Many older OpenGL drivers also support this via the KHR_debug extension.) The general idea behind this is to “register” a debug callback function with OpenGL; any time OpenGL encounters an error, it will call your function and pass useful things to it like an error message.

Read more about how to hook this up in our learn article.

How do I pass pointers and arrays?

OpenTK includes several method overloads for every OpenGL/CL/AL function that requires a pointer: These overloads are designed to make working with the functions as natural as possible in .NET while still providing high performance.

So, for example, consider glBufferData(). This OpenGL function takes four parameters:

void glBufferData(GLenum target, GLsizeiptr size, const void *data, GLenum usage);

The C form of this requires you to pass an array of data that could be composed from a variety of different data structures; so how can we invoke this function in .NET?

OpenTK exposes this single function with a dozen different overloads on the GL class!

1. void BufferData<T>(BufferTarget target, int size, T[] data, BufferUsageHint usage)
2. void BufferData<T>(BufferTarget target, int size, T[,] data, BufferUsageHint usage)
3. void BufferData<T>(BufferTarget target, int size, T[,,] data, BufferUsageHint usage)
4. void BufferData<T>(BufferTarget target, int size, ref T data, BufferUsageHint usage)
5. void BufferData   (BufferTarget target, int size, IntPtr data, BufferUsageHint usage)

6. void BufferData<T>(BufferTarget target, IntPtr sz, T[] data, BufferUsageHint usage)
7. void BufferData<T>(BufferTarget target, IntPtr sz, T[,] data, BufferUsageHint usage)
8. void BufferData<T>(BufferTarget target, IntPtr sz, T[,,] data, BufferUsageHint usage)
9. void BufferData<T>(BufferTarget target, IntPtr sz, ref T data, BufferUsageHint usage)
10. void BufferData  (BufferTarget target, IntPtr sz, IntPtr data, BufferUsageHint usage)

(Each of the generic forms has a where T: struct constraint, but it’s been omitted for brevity.)

Overloads #1-5 differ from #6-10 only by using IntPtr instead of int to represent the size of the data, so let’s look at what techniques overloads #1-5 support:

You can pass managed arrays of struct objects via the T[], T[,] and T[,,] forms: Forms #1-3 support one-dimensional, two-dimensional, and three-dimensional arrays of managed structs, respectively.
You can pass a ref to a single struct using form #4.
You can pass a raw pointer from an unsafe/fixed block of memory via the IntPtr form (#5).

(And in OpenTK 5, we plan to support ReadOnlySpan<T> as well, adding yet another way to do it!)

Passing managed arrays (forms #1-3)

The first three overloads let you pass arrays of any struct type, so that every major atomic type is supported — int, long, short, byte, float, double, custom structs like a Color or DateTime, and more. This means that simple, straightforward OpenGL code is very similar between C and C#, as in the example below.

Example code in C:

unsigned char *data = (unsigned char *)malloc(bufferSize);
...
glBufferData(GL_TEXTURE_BUFFER, bufferSize, data, GL_STATIC_DRAW);

C# equivalent:

byte[] data = new byte[bufferSize];
...
GL.BufferData(BufferTarget.TextureBuffer, bufferSize, data, BufferUsageHint.StaticDraw);

Passing refs (form #4)

The fourth overload takes a ref to a struct type, which you can use to pass simple structs directly. Here’s an example where both C and C# allocate a struct on the stack and then pass it to OpenGL:

Example code in C:

MyStruct data;
...
glBufferData(GL_TEXTURE_BUFFER, bufferSize, data, GL_STATIC_DRAW);

C# equivalent:

MyStruct data;
...
GL.BufferData(BufferTarget.TextureBuffer, bufferSize, data, BufferUsageHint.StaticDraw);

Passing raw pointers (form #6)

OpenTK also lets you pass raw pointers to unmanaged or pinned memory directly, if you need maximum power and speed. The two forms below are equivalent:

Example code in C:

unsigned char *data = (unsigned char *)malloc(bufferSize);
...
glBufferData(GL_TEXTURE_BUFFER, bufferSize, data, GL_STATIC_DRAW);

C# equivalent:

byte[] data = new byte[bufferSize];
...
unsafe
{
    fixed (byte* ptr = data)
    {
        GL.BufferData(BufferTarget.TextureBuffer, bufferSize, (IntPtr)ptr,
                      BufferUsageHint.StaticDraw);
    }
}

On OpenTK 4 and earlier, this is also how you pass a ReadOnlySpan<T> to OpenTK; you need to pin the data yourself with the fixed keyword and then pass it as a pointer, as in the example below:

ReadOnlySpan<float> data = stackalloc float[1024];
...
unsafe
{
    fixed (float* ptr = data)
    {
        GL.BufferData(BufferTarget.TextureBuffer, bufferSize, (IntPtr)ptr,
                      BufferUsageHint.StaticDraw);        
    }
}

How do I control OpenGL contexts?

On many OSes, there is the notion of an OpenGL context, a container that holds all of the allocated OpenGL resources like textures and shaders and buffers. There is also typically the concept of a current context — there’s usually only one context “active” at a time, and you have to switch between them if you have more than one.

OpenTK exposes this through NativeWindow in the form of its Context property and its MakeCurrent() method. Context is an IGraphicsContext, and includes low-level OpenGL operations like MakeCurrent() and MakeNoneCurrent() and SwapBuffers().

The graphics context is created automatically for you when a NativeWindow is created, so the only thing you may need to do is occasionally invoke MakeCurrent() or MakeNoneCurrent().

(If you only have one OpenGL context, you can ignore all of this.)

Is OpenTK thread-safe?

Yes, but OpenGL/CL/AL often aren’t!

As a general rule, you should only ever invoke OpenGL/CL/AL from a single thread, because they often aren’t thread-safe or have serious threading limitations, depending on your underlying OS or implementation. OpenTK doesn’t change the OS’s behavior: It just provides access to whatever the OS already does.

A more complete answer is far outside the scope of this FAQ.

Beginner Questions

How do I render my first triangle?

The same way you would with OpenGL in C! The semantics are the same — only the syntax is different, since you’ll likely be doing it in C#, not C.

Rendering your first triangle can be challenging just because there are many new ideas and new concepts to learn. It’ll take some code, and some learning, and some patience, but you can do it!

We have some introductory tutorials that should help you get your first project off the ground. There are a few major parts to drawing your first triangle:

You’ll need to first create a window (there’s a quick example above).
You’ll need to fill some buffers with the data for your triangle — you’ll need at least a VBO and vertex attributes.
You’ll need to create and load some shaders — at least a simple vertex shader and a simple fragment shader.
Finally, in your window’s render phase, you’ll need to tell OpenGL to use your shaders to draw the triangle data in your buffers.

How do I draw things more complex than a single triangle?

Lots of triangles!

OpenGL has always supported things like triangle strips and triangle fans (and their modern equivalent, the EBO) to make it easy to render “more complex polygons”, but even the most complex graphical scene is really broken down into just lots and lots of triangles. A rectangle, for example, is just two triangles that share an edge. A pentagon can be made from three triangles, and a hexagon can be made from four. A cube is made up of six rectangles — or twelve triangles.

So once you can render a single triangle, you just use the same techniques to render more of them, with different colors, textures, and positions, to make anything you can imagine.

How do shaders/buffers/vertex arrays/etc. work?

This is a common question when you’re new to modern OpenGL — there’s a lot of new terminology, and it’s very confusing! We recommend reading some of the introductory articles at Learn OpenGL (and some of our similar articles converted to C#) to understand what all these strange concepts are and how they fit together.

But the short short short explanations for the mort important terms are:

Triangles are what your GPU actually knows how to draw.
Textures are the parts of the GPU’s memory that you upload your image data into.
Buffers are parts of the GPU’s memory that you upload your triangle data into.
- Vertex buffers (VBOs) store information about the corners (vertices) of your triangles, like where they are in space, and what parts of a texture to use.
- Vertex attributes describe how the vertex buffer is laid out: They assign meaning to each value in the vertex buffer.
  - Vertex attributes and VBOs together describe an “array of structs” containing all of your data. The VBO is the “array” part, and the attributes are the definition of each “struct.”
- Vertex array objects (VAOs) store vertex attribute data in the GPU’s memory, so that you don’t have to send it to the GPU for every triangle.
- Element buffers (EBOs) describe which vertices should be used to make each triangle, and let you reuse vertices between multiple triangles.
Shaders are little programs that tell your GPU how to draw things like triangles.
- Vertex shaders calculate where the corners of the triangles are, using the vertex data.
- Fragment shaders calculate the color of the pixels inside the triangles, using the coordinate data from the vertex shader, and image data from various textures.
- Compute shaders are specialty programs that use the GPU for doing general-purpose math that the CPU would normally do (only much slower!).

How do I use OpenTK with C++?

OpenTK is an OpenGL/CL/AL library for managed .NET languages like C# and F#. You can use OpenTK with managed C++ (i.e., C++/CLI), but you neither need it nor likely want it for plain C++.

How do I use the fixed-function pipeline (FFP)?

Modern OpenGL doesn’t use things like glBegin() and glDrawTriangle() — even though those kinds of functions were easy to use, they didn’t scale well. Modern OpenGL uses shaders and buffers instead, which can handle much more complex rendering than the older OpenGL fixed-function pipeline did.

If you’re trying to use the old FFP functions, they’re exposed by OpenTK, but they won’t work out-of-the-box, because you need to turn on compatibility mode to enable them.

When creating a NativeWindow, you pass a NativeWindowSettings object to describe what kind of OpenGL context you need. To enable compatibility mode, you change the ContextProfile of the NativeWindowSettings:

var nativeWindowSettings = new NativeWindowSettings
{
    ...
    ContextProfile = ContextProfile.Compatibility
};

This will provide access to the older FFP functions, at a small cost in memory overhead and performance. (But if you’re concerned about performance, you shouldn’t be using the FFP anyway!)

How do I center my window on the screen?

OpenTK 4.4 adds support for centering a NativeWindow or GameWindow directly:

myWindow.CenterWindow();

You can also center-and-resize the window as a single operation, which is a common need when an application starts:

myWindow.CenterWindow(new Vector2i(800, 600));

This overload avoids flicker if you need to both move and resize the window.

For OpenTK 4.x prior to OpenTK 4.4, you need to query the window’s Monitor information and reposition the window manually, as described here. For OpenTK 3.x, you will need to manually position the window relative to its given DisplayDevice.

How can I control the initial appearance of my window?

The NativeWindowSettings class, an instance of which you must pass to your window’s constructor, contains many properties for controlling how your window initially appears (or doesn’t appear). Here are the major properties you might want to use to control how your window looks and behaves:

public class NativeWindowSettings
{
    public string Title { get; set; }              // Text for the title bar
    public WindowIcon Icon { get; set; }           // Array of images of various sizes
    
    public bool StartFocused { get; set; }         // True by default
    public bool StartVisible { get; set; }         // True by default
    
    public Vector2i? Location { get; set; }        // Top-left corner position
    public Vector2i? Size { get; set; }            // Client/content size, in pixels
    
    public WindowBorder WindowBorder { get; set; } // Fixed, Resizable, Hidden (no bord.)
    public WindowState WindowState { get; set; }   // Normal, Minim., Maxim., Hidden
    public bool IsFullscreen { get; set; }         // Like Maximized, only bigger :-)
}

A number of these, like Location and Size and WindowState and IsFullscreen, are re-exposed after the window is constructed as properties on the window itself, so you can use the equivalent window properties to update your window’s appearance and state as well.

If you want to delay your window’s position/layout until after it has been constructed and important data for it has been loaded (like its position), a common pattern is something like this:

var nativeSettings = new NativeWindowSettings { StartVisible = false };
using (var myWindow = new MyWindow(nativeSettings, gameSettings))
{
    myWindow.Run();
}

...
    
public class MyWindow : GameWindow
{
    ...
        
    public override OnLoad()
    {
        // Do setup...
        ...
            
        // Now show it.
        CenterWindow();
        WindowState = WindowState.Normal;
    }
}

(Note that OpenTK 3 used a very different technique for setting up its initial window state: Window parameters are passed as part of the GameWindow or NativeWindow constructor; there is no such thing as NativeWindowSettings; and configuration options are fairly limited.)

What’s up with OpenTK’s matrices?

For historical reasons, OpenTK’s built-in Matrix structs store their data in row-major order, not column-major order. This provides consistent backward-compatibility with existing software that uses OpenTK, but it can be confusing, for two reasons:

First, if you copy other code found online for OpenGL, you may need to transpose the matrices their code uses to get the expected behavior.
Second, matrix-matrix multiplications typically need to be done in reverse order of those of most other libraries.

This is a common source of confusion when using OpenTK, since it often prevents “code found on the Internet” from working directly. We’re sorry if it causes you trouble, but it’s easy to work around it, and because so much existing software depends on it, it’s not something we plan to change any time soon.

What’s the difference between NativeWindow and GameWindow?

OpenTK includes two classes, NativeWindow and GameWindow, for opening and displaying OpenGL windows. What’s the difference between these two classes?

NativeWindow is the lowest-level class. It includes the bare minimum of required mechanics to open and display a window, and not much else. It’s small, light, and fast, and good for situations where you don’t need an update/render loop or you intend to write most of the update/render logic yourself. (Note that NativeWindow is not the lowest level, merely the lowest level in managed .NET: NativeWindow wraps native functionality provided by GLFW.)
GameWindow is a higher-level class that inherits from NativeWindow and adds functionality often needed by video games. It provides timing logic, separated update and render methods, and a variety of ways to synchronize and control the game’s performance.

If you’re making a game, you probably want to use GameWindow as your base class. Otherwise, you probably want to use NativeWindow.

How do I add a “main loop” or “Run” method to NativeWindow?

NativeWindow contains minimal functionality, and unlike GameWindow, it doesn’t include a Run() method. The bare minimum of a Run() method is simply this much:

public void Run(MyWindow myWindow)
{
    while (true)
    {
        // Read input devices, and let the OS update the window.
        myWindow.ProcessEvents();
        
        // Draw the next frame!
        myWindow. ...update and draw something ... ;
    }
}

Which is to say: A Run() method, at its core, consists of a loop that alternates between invoking ProcessEvents() to handle keyboard and mouse input and window management; and then updating state and drawing a frame. A good implementation of a Run() method would yield the CPU after each frame, respond to window-close events, and the loop should exit when the program is done — but the example above is enough to be usable for very simple programs.

For comparison, the Run() method in GameWindow looks something like this (with many parts omitted for brevity):

public virtual unsafe void Run()
{
    Context.MakeCurrent();
    OnLoad();

    ...setup logic...
        
    while (GLFW.WindowShouldClose(WindowPtr) == false)
    {
        ProcessEvents();
        
        ...timing logic...
        OnUpdateFrame();
        ...more timing logic...
        OnRenderFrame();
        ...yet more timing logic...
    }
}

Implementing the logic to update and render frames with consistent timing of, say, 60 frames per second can be hard; but writing the core of a simple Run() loop is easy.

What’s the relationship between OpenTK and GLFW?

OpenTK 4.x+ uses the GLFW C++ library to provide cross-platform backend support: GLFW is responsible for opening windows, handling input, and handling as much as possible of the OS dependencies in a portable way. There’s no reason for us to implement a portability layer when a good, well-supported one already exists.

As an example, consider the NativeWindow.Title property, which lets you read and update the window’s title. Here’s its entire implementation, verbatim:

public string Title
{
    get => _title;
    set
    {
        unsafe
        {
            GLFW.SetWindowTitle(WindowPtr, value);
            _title = value;
        }
    }
}

As you can see, the set accessor does little more than ask GLFW to update the window title with the new string. GLFW is responsible for actually invoking SetWindowTextW() on Windows, or XSetWindowTitle() on Linux, or [window setTitle:title] on MacOS X — and for dealing with all of the marshalling and proxying and allocation and character-set conversions and quirks and compatibility issues.

The OpenTK Nuget package includes a copy of a compiled build of GLFW inside it. It’s worth knowing that it’s there and that it provides a lot of the low-level magic, but most of the time, you can safely ignore that GLFW exists, and you can work exclusively with OpenTK’s managed .NET classes and interfaces.

Help & Support

Where can I get help?

We recommend you visit our Discord server and ask a question in the #support channel. Our server is highly-active, so if you’re stuck, someone will be happy to help you.

I think I found a bug; how do I report it?

Please read through the open tickets in the OpenTK Issues on GitHub, and see if any of them match your bug. If not, please open a new ticket, and be sure to include all of the information requested in the template.

Who maintains OpenTK?

The OpenTK Team! The current maintainers are @frederikja163, @jvbsl, @NogginBops, @PJB3005, and @varon. Previous maintainers include @Nihlus and @Perksey, and the original author of OpenTK was @thefiddler. And we have lots of other contributors, too, who have helped add features and fix bugs.

How can I help?

Check the open tickets in the OpenTK Issues on GitHub and see if one of them sounds interesting to work on. You can also drop by our Discord server and offer to help in the #development channel; we’d be happy for any contribution you’re willing to make!

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Basics & Requirements

What is OpenTK?

What is OpenGL?

What is OpenCL?

What is OpenAL?

What operating systems are supported by OpenTK?

What hardware is supported?

Which .NET versions are supported?

Which version of OpenTK should I use?

What’s the status of OpenTK 5.0?

Which release is the official release?

Which versions of OpenGL are supported?

What about UI frameworks?

How do I use Dear ImGui with OpenTK?

How fast is OpenTK?

Does OpenTK support...

Vulkan or Metal?

DirectX or Direct3D?

OpenVG? OpenVX? OpenXR?

Installing and Using OpenTK

How do I get OpenTK?

Which using directives do I need?

How do I create a window using OpenTK?

How do I call OpenGL/CL/AL functions?

Where do I start with OpenGL?

What’s the difference between OpenGL and OpenGL ES?

How do I load textures?

How do I create shaders?

How do I handle errors?

That seems like a lot of if-statements; is there a better way?

How do I pass pointers and arrays?

How do I control OpenGL contexts?

Is OpenTK thread-safe?

Beginner Questions

How do I render my first triangle?

How do I draw things more complex than a single triangle?

How do shaders/buffers/vertex arrays/etc. work?

How do I use OpenTK with C++?

How do I use the fixed-function pipeline (FFP)?

How do I center my window on the screen?

How can I control the initial appearance of my window?

What’s up with OpenTK’s matrices?

What’s the difference between NativeWindow and GameWindow?

How do I add a “main loop” or “Run” method to NativeWindow?

What’s the relationship between OpenTK and GLFW?

Help & Support

Where can I get help?

I think I found a bug; how do I report it?

Who maintains OpenTK?

How can I help?

Which `using` directives do I need?