Use glBlendFunc in QOpenGLWidget

Question

I'm trying to use glBlendFunc<\/code> in QOpenGLWidget<\/code> (in paintGL), but objects do not mix (alpha is works).

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
glEnable(GL_BLEND);

Answer 1

If you want to achieve a blending effect, the you have to disable the depth test:

glDisable(GL_DEPTH_TEST);

Note, the default depth test function is GL_LESS . If a fragment is draw on a place of a previous fragment, then it is discarded by the depth test, because this condition is not full filled.
If the depth test is disabled, then the fragments are "blended" by the blending function ( glBlendFunc ) and equation ( glBlendEquation ).

I recommend to use the following blending function:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Answer 2

In my case (Qt 5.15.2) I found that using a color call with no alpha component (eg. glColor3f(1,0,0) ) causes the blending to be disabled for any subsequent rendering. To my surprise I could not even recover it by re-issuing these commands:

glEnable(GL_BLEND); // wtf has no effect
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Blending simply remains disabled until the next paint begins. This did not happen with the original QGLWidget class. It only happens with QOpenGLWidget and only on Windows (Mac + Linux are fine).

The good-enough solution for me was to replace any non-alpha color calls with alpha equivalents, at least for cases where you need to use blending later in the render. Eg.

glColor3f(1,0,0);    // before
glColor4f(1,0,0,1);  // after

Another issue that might come up is if you use QPainter along with direct rendering, because the QPainter will trash your OpenGL state. See the mention of 'beginNativePainting' in the docs:

https://doc.qt.io/qt-5/qopenglwidget.html#painting-techniques