flextGL

flextGL is an OpenGL and Vulkan header and loader generator.

It is a bit different than other comparable systems:

• Gives complete control over exposed version and extensions
• Bindings directly generated from the OpenGL / Vulkan registry gl.xml / vk.xml files
• For OpenGL, optionally exports only core-profile functions
• For Vulkan, it's possible to adapt the templates to generate separate instance-specific and device-specific loader for faster runtime performance, or switch from globally visible symbols to local ones (blog post with details)
• Only requested extensions are loaded
• Flexible python template system for source generation
• Source templates easy to adapt to project requirements
• (Optionally) specify only particular functions to be loaded

You will need the following dependencies:

• Python 3.x
• Wheezy Template

What's new?

• October 2019: Breaking change -- the GL <KHR/khrplatform.h> include is now required to be specified by the template instead of being added implicitly from the gl.xml definition. This is done in order to allow more flexibility and is consistent with how vk_platform.h is handled. See commit bef1e12c98562fa87b698751a75d9da84e11c98d for details.
• May 2018: Vulkan support, 100% test coverage, @mosra took over the maintainership from @ginkgo
• January 30 2015: Adding function whitelist and lite template
• August 7 2014: Adding support for generating OpenGL ES loading code
• July 10 2014: flextGL now parses gl.xml instead of the deprecated .spec files.

Source tree

• flextGLgen.py

  The generator script.

• flext.py

  The parsing code

• templates/

  The sub-directories in here contain the different template sets. You can add your own template by simply creating a new folder in there.

• profiles/

  Some example profile files to give you an idea on how to write your own. profiles/exampleProfile.txt contains a lot of comments to get you up to speed.

• spec/ (generated)

  This directory is automatically created by the script to store the downloaded OpenGL and Vulkan XML spec files.

Usage

You create your loader code by writing a profile file and passing it to the script.

Here is what a typical profile might look like:

version 3.3 core
extension EXT_texture_filter_anisotropic optional
extension ARB_tesselation_shader optional

This requests an OpenGL core profile and the extensions for anisotropic filtering and tesselation shaders. Those extensions were requested as optional. This means that a missing extension won't cause an error. Instead, the programmer will have to check before using it. This can be easily done by testing a generated global variable. For OpenGL ES a typical profile might look like:

version 3.0 es
extension OES_standard_derivatives optional
extension OES_vertex_array_object optional

Because the world is far from ideal, certain vendors don't bother upstreaming their extensions. To handle that, one can supply additional spec URLs to get merged to the main one:

version 2.0 es

extraspec https://raw.githubusercontent.com/google/angle/master/scripts/gl_angle_ext.xml

# This one is from the vanilla gl.xml
extension OES_vertex_array_object optional
# This one ... thanks for the unnecessary extra pain, Google
extension ANGLE_multi_draw optional

A Vulkan profile, whitelisting only some particular functions, can look like:

version 1.1 vulkan

extension KHR_swapchain optional

begin functions
    CreateInstance
    CreateBuffer
    CmdDraw
    CreateSwapchainKHR
end functions

In case of Vulkan it's also possible to generate the output from a particular release by saying e.g. version 1.1.73 vulkan. That is useful in case you need to match exactly the release version required by vulkan.hpp, for example.

The profile file is then passed to the script like this:

$ python flextGLgen.py -D generated -t <template-of-your-choice> profile.txt

This will create the requested source code and put it in the generated/ directory.

The best way to work with flextGL is to integrate it in your build-system. The example project demonstrates this for Make and CMake here.

Generated API for OpenGL

The builtin OpenGL templates all generate the following API:

• int flextInit()

  Initializes the OpenGL functions after context creation.

• FLEXT_MAJOR_VERSION

  The OpenGL major version defined in the profile file.

• FLEXT_MINOR_VERSION

  The OpenGL minor version defined in the profile file.

• FLEXT_CORE_PROFILE

  Boolean variable. Is GL_TRUE, if the profile file defined a core profile.

• FLEXT_<extension-name>

  Generated global variables for checking if a specific extension is supported.

Take a look at the example program to get an idea on how it's used.

Note that it's possible to supply your own template with your own API that might or might not resemble the above.

Generated API for Vulkan

For simplicity, the default template generates all function pointers globally and loads them as instance-specific. See this blog post for other options. Apart from Vulkan APIs, two functions are defined:

• void flextVkInit()

  Initializes all global function pointers (pointers not depending on a particular VkInstance). Call this before doing anything else.

• void flextVkInitInstance(VkInstance instance)

  Initializes all instance-specific function pointers using given instance pointer. Call this once you have created a VkInstance.

Note that it's possible to supply your own template with your own API that might or might not resemble the above.

Templates

At the moment, there are three template sets available:

• compatible

  This loads the extensions using a framework-agnostic method with WGL AGL or GLX. This is probably a sensible default for most people.

• glfw3, glfw3-es

  These use GLFW 3 functions for loading and testing for OpenGL extensions either for desktop OpenGL or OpenGL ES. In these templates, the flextInit() function takes a pointer to GLFWwindow.

• lite

  A variant of the compatible template that generates more compact code. It has not been thoroughly tested yet, though.

• vulkan

  A basic loader for Vulkan.

• vulkan-dynamic

  A variant of the vulkan template that loads the Vulkan library dynamically. No static linking to Vulkan is necessary and the signature of flextVkInit is changed to bool flextVkInit(). This lets you additionally check for the availability of Vulkan itself and display an error message or fallback to another API if not available.

Installing Wheezy Template on Windows

If you have Python 3.4+ installed you should be able to install Wheezy using pip.

$ pip install --user wheezy.template

The --user does a local install in your home-folder. You can omit it if you want to do a system-wide installation.

If you have an older version of Python or don't have pip for some reason, then you need to install it first.

Credits

• Thomas Weber (@ginkgo) is the original author.
• The compatible template uses a few code snippets from Slavomir Kaslev's gl3w OpenGL core profile loader for portable loading of procedures and checking minor/major version in OpenGL < 3.0.
• Mykhailo Parfeniuk (@sopyer) provided most of the gl.xml parsing code.
• Vladimír Vondruš (@mosra) added support for OpenGL ES and Vulkan loader generation.
• Leander Beernaert (@LeanderBB) did various fixes on the compatible template.
• @eternalrain added support for function white-lists and created the lite template.
• The Khronos Group Inc. (https://www.khronos.org/vulkan/) for Vulkan itself and the dynamic Vulkan library loader code.
• Tolga Mizrak (@to-miz) added the dynamic Vulkan loader template variant.

License

flextGL is licensed under the MIT/Expat license, see the COPYING file for details.