[Qt-interest] Blocking Flip Problem

[naiqiao liang]

HI,

The problem is that we will sometimes need to update the screen
every vertical retrace, so we can't put in a delay.

The code below is some try to block the flip, but it failed. Could anyone
give me some tips on that. Thanks ahead.

    def swapBuffers(self):

        # first call the swap on the QGLWidget
        self.glw.swapBuffers()

        # The following is taken from the PsychToolbox
        # Draw a single pixel in left-top area of back-buffer. This
will wait/stall the rendering pipeline
        # until the buffer flip has happened, aka immediately after
the VBL has started.
        # We need the pixel as "synchronization token", so the
following glFinish() really
        # waits for VBL instead of just "falling through" due to the
asynchronous nature of
        # OpenGL:
        glDrawBuffer(GL_BACK)
        # We draw our single pixel with an alpha-value of zero - so
effectively it doesn't
        # change the color buffer - just the z-buffer if z-writes are
enabled...
        glColor4f(0,0,0,0)
        glBegin(GL_POINTS)
        glVertex2i(10,10)
        glEnd()
        # This glFinish() will wait until point drawing is finished,
ergo backbuffer was ready
        # for drawing, ergo buffer swap in sync with start of VBL has
happened.
        glFinish()

I don't know why the glFinish is not blocking...
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