CODEWITHSUNDEEP
c++qtopenglqml
Krustenkaese
Krustenkaese

Reputation: 155

QQuickFramebufferObject redraw only shows clear color

I'm using a QQuickFramebufferObject object to render a red triangle to a framebuffer, which itself gets drawn to the QML scene.
To do that i overwrote the render function of the associated QQuickFramebufferObject::Renderer class.
This render function looks like following:

void GLRenderEngine::render()
{
    glClearColor(0,0,0,1);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

    glColor3d(1,0,0);
    glBegin(GL_TRIANGLES);
        glVertex2d(0,0);
        glVertex2d(1,0);
        glVertex2d(0,1);
    glEnd();

    glFlush();

    //QQuickWindow context of encapsuling QQuickFramebufferObject
    //is set in overwritten synchronize call
    if(m_pWindow)
    {
        m_pWindow->resetOpenGLState();
        update();
    }
}

The problem i experence is that the first frame gets drawn correctly, while all other frames only show the clear color. I've analyzed the opengl api calls with vogl and posted the result in pastebin:

Frame0 (correct Frame): https://pastebin.com/aWu4ee6m

Frame1: https://pastebin.com/4EmWmnMv

The only differences i noticed were the initializing calls, where Qt querys the statemachines states, so i'm curious what else i did wrong.
Thanks for help in advance.

Small update:

If i remove glClear(...) The frames show the correct image, though i doubt this is correct behaviour.

The framebuffer bound when I use glClear is the one Qt created for me to use. It is bound with flag GL_FRAMEBUFFER, which also enables drawing.

After i returned from the function the default framebuffer (0) is bound and cleared. This procedure can be seen in Frame 1 pretty well.

What I've been wondering about is whether glBlitFrameBuffer is being called. Vogl doesn't seem to catch that call, also in the preview of the individual framebuffers, provided by Vogl, i couldn't see my red triangle in Frame1, while it is visible in Frame0.

Upvotes: 2

Views: 591

Answers (1)

Krustenkaese
Krustenkaese

Reputation: 155

I solved the problem when i compared the statemachines states and saw, that the Shaderprogram switched from 0 to 1. Changing it back to 0, and thus disabling shaderprograms, at every start of the render function resulted in the expected behaviour.

Upvotes: 1

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