commit	d41467f3c3bd8f29493db056382e071c6c62564b	[log] [tgz]
author	John McDole <john@mcdole.org>	Mon Jul 24 23:52:43 2017
committer	GitHub <noreply@github.com>	Mon Jul 24 23:52:43 2017
tree	276cf05eadd1cb578d60deb19f3db83575cfc40d
parent	44cc5c7fab742d6483d8c586ba3a1daf34390523 [diff]

tree: 276cf05eadd1cb578d60deb19f3db83575cfc40d

README.md

Dart native extension for GLES2

NOTE: It is unlikely that programs written with these bindings will work on OSX due to platform requirements that the main thread be the one handling events.

Steps to generate the bindings

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

Steps to compile the bindings

Linux with GNU Make:

If your dart_api.h, GLES2/gl2.h, or libGL.so files are not in the standard locations, set the enviroment variables DART_INCLUDE, GL_INCLUDE, and/or GL_LIB and GL_LIB_NAME and run the following commands.

cd lib/
dart ../tools/gl_compile.dart

Note that if you set the GL_LIB variable when compiling, you must also set LD_LIBRARY_PATH to include the same directory when running your program or the Dart VM will not be able to find libGL.so/lib$(GL_LIB_NAME).so.

Whitelisting

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 whitelisted and use the --whitelist flag in tools/gl_generator.dart`

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

Other Platforms

TODO

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 and anytime control returns to the event loop, and reacquire it when ready to make OpenGL calls again.