- 1. GL Paint Engine
- 2. Problems
- 3. Progress
- 4. Last missing piece
- 5. When?
As many people know, QPainter has a multi-backend architecture and has two main rendering implementations under the hood of Qt 5: The raster rendering engine and an OpenGL2 engine that targets OpenGL ES 2.0.
GL Paint Engine
While the bitmap rendering engine is one of Qt's gems, let's talk about the other half: the GL rendering engine, which is used when QPainter is opened on:
- QOpenGLWindow ,
- QOpenGLWidget ,
- or QOpenGLPaintDevice , which can transfer the current context to the target being drawn, and is mainly used when targeting FBOs .
Problems
What about modern OpenGL?
What problems started showing up early: Around Qt 5.0, for those who needed a main profile context while creating a custom OpenGL rendering that ended up working quite often. Components like Qt Quick couldn't work with such contexts in the beginning due to the need to rely on deprecated/removed functionality (e.g. client-side pointers), missing vertex array object, GLSL version of shaders which support is not required for such contexts.
In some cases, choosing a compatibility profile has been a viable workaround. However, Mac OS X / macOS notoriously lacks support for this: there is a choice between OpenGL 2.1 or the main profile context 3.2+. Attempting to solve this problem by, for example, rendering a texture in one context and then using the texture in another context through resource sharing is often futile, as some platforms tend to reject resource sharing across different version/profile contexts.
Progress
Fortunately, during the Qt 5 life cycle, these things were improved: Firstly Qt Quick, and then other, less custom GL-based components, were patched to function with both core and compatibility contexts.
As of Qt 5.8, there is one big exception: the GL rendering engine for QPainter.
Last missing piece
The good news is, this restriction will be lifted soon. Thanks to the contribution of Krita , see here for reference , QPainter is now able to function in the main contexts profile. Functionally, this will not bring any changes, rendering is still done using the same methods as before.
In addition to fixing the original patch, this has also been integrated with a regression testing system called Lancelot. This means that in addition to bitmap (with different QImage formats) and OpenGL 2, it will also run with the main profile context from now on to ensure that QPainter exits without regression between Qt releases.
In general, this means that the code snippet below will function as expected.
class Window : public QOpenGLWindow {
void initializeGL() override {
QSurfaceFormat fmt;
fmt.setVersion(4, 5); // or anything >= 3.2
fmt.setProfile(QSurfaceFormat::CoreProfile);
setFormat(fmt);
}
void paintGL() override {
QPainter p(this);
p.fillRect(10, 10, 50, 50, Qt::red);
...
}
...
};
When?
Qt 5.9.
The patch is now merged into qtbase in the "dev" branch. Will be added to Qt 5.9 soon.
Источник Qt blog
