Dart native extension for GLES2

Supports Linux and Windows. OSX is not supported due to inherent incompatibilites in the OSX UI threading model, as well as Apple announcing the discontinuing of support of OpenGL on OSX (https://goo.gl/qQdeQ5).

Steps to generate the bindings

mkdir -p lib/src/generated
pub run tools/gl_generator.dart
clang-format -i --style=Google generated/*.{cc,h}
dartfmt -w generated/*.dart
mv generated/* lib/src/generated/

Limiting bindings to actual symbols

Some GL libraries come with headers that list functions not implemented. This will fail at link time. To avoid this, you can dump the symbols in libGLESv2.so to be used with --limit-api flag intools/gl_generator.dart

nm -D /lib/libGLESv2.so | grep " T " | awk '{print $3}' > limit.txt

Steps to compile the bindings

The previous method for compiling the bindings is no longer available. We are working on a new solution for both Linux and Windows.

In the meantime, the Makefile in the lib/ directory is a good starting point for compiling on Linux.

Functions with interfaces with APIs different from GLES2

If a function's only difference is moving pointer parameters to the return value, it is not listed. See lib/src/manual_bindings.dart for a full list.

• glGetActiveAttrib doesn't take a bufSize parameter, and returns an ActiveAttrib instance.
• glGetActiveUniform doesn't take a bufSize parameter, and returns an ActiveAttrib instance.
• glGetAttachedShaders doesn't take a maxCount parameter, returns a List<int>.
• glGetProgramInfoLog doesn't take a maxSize parameter, and returns a String.
• glGetShaderPrecisionFormat returns a ShaderPrecisionFormat instance.
• glGetShaderSource doesn't take a bufSize parameter and just returns a String.
• glReadPixels takes a TypedData pixels parameter and the data is put into it, like in the WebGL API. Your program will most likely crash if the TypedData object you pass in is not big enough, or possibly if it's in the wrong format.
• glShaderSource just takes the parameters int shader and String string. The original API is not really appropriate for Dart.

A note about OpenGL, the Dart VM, and native threads

OpenGL is a thread-bound API. That is, an OpenGL context must be bound (or “made current”) on a thread before any other OpenGL functions may be called.

The Dart VM uses a pool of native threads under the hood to carry out tasks on the event queue (overview of the Dart event loop).

This leads to an unfortunate restriction on the use of asynchronous code while making OpenGL calls from Dart. This is bad:

glfwMakeContextCurrent(context);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// other OpenGL calls ...

// Wait for an async task to finish.
await someFuture;

// Draw a triangle.
glDrawArrays(GL_TRIANGLES, 0, 3);
// etc...

The issue is that the context is made current, then there is a call to await, which causes the current task to return to the event loop until someFuture completes. When control returns to the next line, it may be running on a completely different native thread, where the context is not bound.

To avoid this issue, the code must be changed to something resembling the following:

glfwMakeContextCurrent(context);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// other OpenGL calls ...

// Release the context before waiting.
glfwMakeContextCurrent(NULL);

// Wait for an async task to finish.
await someFuture;

// We're back!  Reacquire the context.
glfwMakeContextCurrent(context);

// Draw a triangle.
glDrawArrays(GL_TRIANGLES, 0, 3);
// etc...

This way, the context is released from the thread before control returns to the event loop and then reacquired when it comes back.

Note that this applies to any asynchronous code (not just an await). Here's another bad example:

glfwMakeContextCurrent(context);

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// other OpenGL calls ...

// Load an image, then create a texture.
var f = new File("image.rgb");
f.readFileAsBytes().then((bytes) {
  var tex = glGenTextures(1);
  glBindTexture(GL_TEXTURE_2D, tex);
  glTexImage2D(GL_TEXTURE_2D, 1, GL_RGBA, width, height, 0, GL_UNSIGNED_BYTE,
      GL_RGBA, bytes);
});

// etc...

In this case, there's no guarantee that the callback to the Future returned by readFileAsBytes would be running on the same thread as the original task.

In practice, most OpenGL code is written synchronously, as it‘s generally not advisable to be waiting for other tasks to complete while in the middle of rendering a frame. However, it’s important to be aware of this restriction. When making OpenGL calls, either avoid awaiting asynchronous methods and making OpenGL calls in async callbacks, or properly release the context anytime control returns to the event loop, and reacquire it when ready to make OpenGL calls again.