Render QImage with OpenGL

Question

Related to my other question, I think the more central question would be, how you render a QImage with OpenGL?

I think the code has to look something like t

Answer 1

I managed to draw a QImage with OpenGL from a picture source, but not from the SoOffscreenRenderer as described here.

void init(){
    loadTexture2("kitten.png", backgroundimage);
}

QImage loadTexture2(char *filename, GLuint &textureID){
    glEnable(GL_TEXTURE_2D); // Enable texturing

    glGenTextures(1, &textureID); // Obtain an id for the texture
    glBindTexture(GL_TEXTURE_2D, textureID); // Set as the current texture

    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL);

    QImage im(filename);
    QImage tex = QGLWidget::convertToGLFormat(im);

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, tex.width(), tex.height(), 0, GL_RGBA, GL_UNSIGNED_BYTE, tex.bits());

    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);

    glDisable(GL_TEXTURE_2D);

    return tex;
}

void renderSceneGL2(){
    glClearColor(0.4f, 0.1f, 0.1f, 1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, backgroundimage);

    glBegin(GL_QUADS);
        glTexCoord2f(0,0); glVertex3f(-1, -1, -1);
        glTexCoord2f(1,0); glVertex3f(1, -1, -1);
        glTexCoord2f(1,1); glVertex3f(1, 1, -1);
        glTexCoord2f(0,1); glVertex3f(-1, 1, -1);

    glEnd();

    glDisable(GL_TEXTURE_2D);
}

Also QGLWidget::convertToGLFormat throws random Access violation errors from time to time.

Answer 2

Nowadays, with Qt 5.11, the easiest way is to use QOpenGLWidget and override its paintEvent method.

In the following code OpenGLWidget inherite from QOpenGLWidget, and display(const QImage& img) is a slot triggered whenever I need to update the image.

No matter how many times display() is called between two OpenGL rendering, the update() call doesn't trig rendering directly, it just marks widget dirty for redrawing on next loop.

This method is actually pretty fast, I'm using it to display camera frames at 240fps (although not all frames are displayed on screen).

void OpenGLWidget::display(const QImage& img)
{
  image = img;
  this->update();
}

void OpenGLWidget::paintEvent(QPaintEvent*)
{
  QPainter painter(this);
  painter.drawImage(this->rect(),image);
}