Why QGLFramebufferObject in QGraphicsItem don’t work?

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

This time i used another approach and got an incorrect rendering of the framebufferobject and a message in my output result. (look at the attachment)

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:

Code:

---

#ifndef __YidQt_FBOGraphicsItem_h__
#define __YidQt_FBOGraphicsItem_h__
 
#include <QGraphicsItem>
#include <QtOpenGL>
 
 
class FBOGraphicsItem : public QGraphicsItem
{
 
	//Q_OBJECT
 
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:

Code:

---

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();
 
						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 ------------
 
}

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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 switch 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

Code:

---

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();

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http://www.czepa.net/output2.jpg