Calling glActiveTexture(GL_TEXTURE1) before update() on a QOpenGLWidget causes textures to appear black

Question

When inheriting from QOpenGLWidget and QOpenGLFunctions_4_5_Core, if I call glActiveTexture(GL_TEXTURE1) (or any other non-zero number) before calling update() textures appear completely black on the screen.

Relevant code snippets:

void OpenGLWidget::paintGL() {
    if (fp) {
        // First paint seems to always happen before the first update
        qDebug() << "First Paint";
        fp = false;
    }
    glClear(GL_COLOR_BUFFER_BIT);

    unsigned int tex;
    glGenTextures(1, &tex);

    glActiveTexture(GL_TEXTURE0);
    glBindTexture(GL_TEXTURE_2D, tex);

    QImage img = QImage("textures/awesomeface.png").convertToFormat(QImage::Format_RGBA8888).mirrored();
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, img.width(), img.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, img.bits());

    glTextureParameteri(tex, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTextureParameteri(tex, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTextureParameteri(tex, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTextureParameteri(tex, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

    glUseProgram(frame_shader.get_id());

    glBindVertexArray(frame_vao);
    glDrawArrays(GL_TRIANGLES, 0, 6);

    // Clean up
    glDeleteTextures(1, &tex);
    glBindTexture(GL_TEXTURE_2D, 0);
    glBindVertexArray(0);

    // This doesn't cause the textures to be black
    glActiveTexture(GL_TEXTURE1);
}

And the mainloop function:

void OpenGLWidget::main_loop() {
    if (fu) {
        qDebug() << "First Update";
        fu = false;
    }

    // This causes the textures to completely black
    // Change GL_TEXTURE1 to GL_TEXTURE0 (or get rid of it glActiveTexture
    // completely) and textures will work once again
    glActiveTexture(GL_TEXTURE1);
    update();
}

The shaders can still draw to the screen; if I set a solid color in the fragment shader, the color will properly display to the screen. It seems only user-made textures are turning completely black.

I'm not getting any OpenGL Errors.

The full MRE can be found at: https://github.com/Luminic/MRE_QOpenGLWidget_glActiveTexture_GL_TEXTURE1_before_update

Does anyone know what is causing this odd behavior?

Answer 1

Why is this supposed to be weird behaviour? It is completely reasonable that telling OpenGL to use the Texture Unit 1 of your hardware only draws black content, because in your code you do not have a texture attached to that unit. The glActiveTexture(GL_TEXTURE1) command sets the active texture unit to 1. Before you created the texture, you bound unit 0, so the texture lives there. Not setting this parameter at all is the same as setting it to 0, as this is the default unit. Calling glBindTexture after the glActiveTexture would resolve your problem too!