Why QGLFramebufferObject in QGraphicsItem don’t work?

[Student3000]

Hi. I have a problem with programming a QGLFramebufferObject in a QGraphicsItem class.

I get an incorrect rendering of the framebufferobject and a message in my output result. (please look at the output image)

QGLContext::chooseContext() : SetPixelFormat failed : The pixel format is invalid.

The following source code init and renders a QGLFramebufferObject in the QGraphicsItem::paint method and after that it binds the FBO Texture to a GL_QUADS in the scene.

header file:

@
#ifndef YidQt_FBOGraphicsItem_h
#define YidQt_FBOGraphicsItem_h

#include <QGraphicsItem>
#include <QtOpenGL>

class FBOGraphicsItem : public QGraphicsItem
{

public:

FBOGraphicsItem();

// pure virtual functions from QGraphicsItem
virtual QRectF boundingRect() const;
virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);

private:

QGLContext *context;
QGLFramebufferObject *fbo;

QRectF myBoundingRect;
float xx;
float yy;

};

#endif // YidQt_FBOGraphicsItem_h
@

source file:

@
void FBOGraphicsItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
{

if (painter->paintEngine()->type() != QPaintEngine::OpenGL &&
painter->paintEngine()->type() != QPaintEngine::OpenGL2)
{
qWarning("TestGLGraphicsItem: drawBackground needs a QGLWidget to be set as viewport on the graphics view");
return;
}

float a1 = 0.5f * 1;
float b1 = 0.5f * 1;

// !!! error with this context !!!
context = new QGLContext(QGLFormat(QGL::SampleBuffers | QGL::DoubleBuffer | QGL::AlphaChannel), widget);
context->create(); // this line create an error: QGLContext::chooseContext() : SetPixelFormat failed
context->makeCurrent();

// ------------ FBO begin ------------

fbo = new QGLFramebufferObject(512, 512);

fbo->bind();
{
glPushAttrib(GL_ALL_ATTRIB_BITS);
{
glViewport(0, 0, fbo->size().width(), fbo->size().height());

glClearColor(0.0f, 1.0f, 0.0f, 1.0f);
glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

glMatrixMode(GL_PROJECTION);
glPushMatrix();
{
 glLoadIdentity();
 glOrtho(-1.0, 1.0, -1.0, 1.0, 99, -99);
 gluLookAt(0.0, 0.0, 1.0, 
     0.0, 0.0, 0.0, 
     0.0, 1.0, 0.0);
 
 glMatrixMode(GL_MODELVIEW);
 glPushMatrix();
 {
  glLoadIdentity();

  // red rectangle
  glColor3f(1.0f, 0.0f, 0.0f);
  glBegin(GL_QUADS);
   glVertex2f(-a1, -b1);
   glVertex2f(-a1,  b1);
   glVertex2f( a1,  b1);
   glVertex2f( a1, -b1);
  glEnd();

  // control points
  glPointSize(5.0f); 
  glColor3f(0.0f, 0.0f, 0.0f);
  glBegin(GL_POINTS);           
   glVertex3f(0.0f, 0.0f, 0.0f);

   glVertex3f(a1, 0.0f, 0.0f);
   glVertex3f(-a1, 0.0f, 0.0f);

   glVertex3f(0.0f, a1, 0.0f);
   glVertex3f(0.0f, -a1, 0.0f);

   glVertex3f(0.0f, 0.0f, a1);
   glVertex3f(0.0f, 0.0f, -a1);
  glEnd();
 }
 glMatrixMode(GL_MODELVIEW);
 glPopMatrix();
}
glMatrixMode(GL_PROJECTION);
glPopMatrix();

}
glPopAttrib();
}

fbo->release();
fbo->toImage().save("d:\fbo.bmp");

// ------------ FBO end ------------

// ------------ SZENE begin ------------

painter->beginNativePainting();

glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

glMatrixMode(GL_PROJECTION);
glPushMatrix();
{
glLoadIdentity();
gluPerspective(70, WIDTH / HEIGHT, 0.01, 1000);

glMatrixMode(GL_MODELVIEW);
glPushMatrix();
{
glLoadIdentity();
gluLookAt(0,0,1,
0,0,0,
0,1,0);

glTranslatef(0, 0, 0);

glEnable(GL_TEXTURE_2D);
{
 glBindTexture(GL_TEXTURE_2D, fbo->texture());
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
 glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
 glTexEnvi(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);

 glBegin(GL_QUADS);
  glTexCoord2f(0.0f, 0.0f); glVertex2f(-xx, -yy);
  glTexCoord2f(0.0f, 1.0f); glVertex2f(-xx,  yy);
  glTexCoord2f(1.0f, 1.0f); glVertex2f( xx,  yy);
  glTexCoord2f(1.0f, 0.0f); glVertex2f( xx, -yy);
 glEnd();
}
glDisable(GL_TEXTURE_2D);     

}
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
}
glMatrixMode(GL_PROJECTION);
glPopMatrix();

painter->endNativePainting();

// ------------ SZENE end ------------

}
@

Ouput:

!http://www.czepa.net/output.jpg(output image)!

[Student3000]

good news, i've found a way to rendering a correct fbo to the quad in the scene.

it was dependend of the order of the painter->beginNativePainting(); line. i changed the order, that the painter->beginNativePainting(); is before the QGLContext and QGLFramebufferobject is init.

the message is still active but the result is correct.

source
@
painter->beginNativePainting();

// ------------ FBO begin ------------

context = new QGLContext(QGLFormat(QGL::SampleBuffers | QGL::DoubleBuffer | QGL::AlphaChannel), widget);
context->create(); // !!!
context->makeCurrent();

fbo = new QGLFramebufferObject(512, 512); // GLWidget::GLWidget (constructor)

fbo->bind();
{
...
}
fbo->release();
@

Output

!http://www.czepa.net/output2.jpg(Output Image)!