Using of a custom QPaintEngine

[Jonas_]

Hi there,
first I have to say sorry for my english.

I' ve got this problem:
I want to change the "drawPixmap(..)" methode. I've red in the Qt-Assistant there is the class QPaintEngine, wich is pure virtual. So I must reimplement this class. I've done it. Here is the code:
the header.

class MyPaintEngine : public QPaintEngine
{
	//Q_OBJECT
 
private:
	HDC mHDC;
 
public:
	MyPaintEngine(PaintEngineFeatures caps = 0);
	~MyPaintEngine();
 
	bool begin(QPaintDevice* pdev);
	void setHDC(const HDC& hDC); 
	void drawPixmap(const QRectF& r,const QPixmap& pm, const QRectF& sr );
	bool end();
	QPaintEngine::Type type() const;	
	void updateState(const QPaintEngineState& state );
	void drawPolygon ( const QPoint * points, int pointCount, PolygonDrawMode mode );
};

class MyPaintEngine : public QPaintEngine
{
	//Q_OBJECT

private:
	HDC mHDC;

public:
	MyPaintEngine(PaintEngineFeatures caps = 0);
	~MyPaintEngine();

	bool begin(QPaintDevice* pdev);
	void setHDC(const HDC& hDC); 
	void drawPixmap(const QRectF& r,const QPixmap& pm, const QRectF& sr );
	bool end();
	QPaintEngine::Type type() const;	
	void updateState(const QPaintEngineState& state );
	void drawPolygon ( const QPoint * points, int pointCount, PolygonDrawMode mode );
};

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the cpp.

void MyPaintEngine::drawPixmap(const QRectF& r,const QPixmap& pm, const QRectF& sr )
{
	const unsigned int width(300),height(300);
	unsigned char* pixelBuffer = new unsigned char[height*width*4];
	unsigned int ii(0);
	for(int xx(0); xx<width; ++xx)
	{
		for(int yy(0); yy<height; ++yy)
		{
			pixelBuffer[ii]   = 255;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
		}
	}
	BITMAP bmp; 
	HDC hDCBmp(CreateCompatibleDC(mHDC));   // mHDC is a member which I set before i call this methode
	DWORD error(GetLastError());
	HBITMAP hBitmap(CreateCompatibleBitmap(mHDC,width,height));
	SelectObject (hDCBmp, hBitmap) ;
	SetBitmapBits(hBitmap,width*height*4,pixelBuffer);
 
 
	BitBlt(mHDC,(int)(100),(int)(100), 300,300, hDCBmp, 0,0, SRCCOPY);
}

void MyPaintEngine::drawPixmap(const QRectF& r,const QPixmap& pm, const QRectF& sr )
{
	const unsigned int width(300),height(300);
	unsigned char* pixelBuffer = new unsigned char[height*width*4];
	unsigned int ii(0);
	for(int xx(0); xx<width; ++xx)
	{
		for(int yy(0); yy<height; ++yy)
		{
			pixelBuffer[ii]   = 255;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
			pixelBuffer[ii] = 0;
			++ii;
		}
	}
	BITMAP bmp; 
	HDC hDCBmp(CreateCompatibleDC(mHDC));   // mHDC is a member which I set before i call this methode
	DWORD error(GetLastError());
	HBITMAP hBitmap(CreateCompatibleBitmap(mHDC,width,height));
	SelectObject (hDCBmp, hBitmap) ;
	SetBitmapBits(hBitmap,width*height*4,pixelBuffer);
	
	
	BitBlt(mHDC,(int)(100),(int)(100), 300,300, hDCBmp, 0,0, SRCCOPY);
}

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But this doesn't work. I only get my window in the default color.
I choose this as an example, only drawing a blue rect.

Why it doesn't work?
What I have to do?
Can anybody help my please?

[bear101]

Looks fine but how do you call your MyPaintEngine?

[Jonas_]

I use this class as a pointer in MyChildWindow-class.
There is a virtual function

QPaintEngine* MyChildWindow::paintEngine() const
{
	return mPaintEngine; // MyPaintEngine* mPaintEngine=new MyPaintEngine();
}

QPaintEngine* MyChildWindow::paintEngine() const
{
	return mPaintEngine; // MyPaintEngine* mPaintEngine=new MyPaintEngine();
}

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When i debug, VC++ steps in my "drawPixmap" method.
But nothing happens......

[bear101]

I think the problem lies in the way Windows draws. You should probably use the HDC which Qt provides. Get it by calling MyPaintEngine::getDC(). Remember to release it again.

-- Bjoern

[Jonas_]

Okay, I've done this. I used the "getDC()" of my MyPainterEngine-Class, but nothing happens.
The result of the "getDC()" is NULL (?!?!?!) in my "drawPixmap"-function.

*AAARRRGGGHHH*