8bit greyscale raw picture find painted area

I want to find and cut out the painted area (painted means: not transparent.). Do you got any Ideas how to do that?
Data Informations:
unsigned char array* imageData; // (each entry = grey value of pixel)
int imageHeight;
int imageWidth;
int imageSize;
Example Code:

Code:

QPixmap img(imageWidth , imageHeight);
	img.fill(Qt::transparent);
	QPainter paint(&img);
	paint.setCompositionMode(QPainter::CompositionMode_Source);
 
	int currentPos;
 
 
	while(currentpos < imageDataSize)
	{
		int x = imageDataSize % imageWidth;
		int y = imageDataSize / imageWidth+1;
		int greyVal =  imageData[currentPos];
 
		paint.setPen(QColor(greyVal,greyVal,greyVal);
		if(hmm < 160)
		{
			paint.drawPoint(x,y);
		} else {
			paint.save();
			paint.setOpacity(0.0);
			paint.setPen(QColor(255,255,255,255));
			paint.drawPoint(x,y);
			paint.restore();
 
		}
		currentPos++;
	}