Question

I am rendering a QPixmap inside of a QThread. the code to paint is inside a function. If I declare the painter inside the drawChart function everything seems ok but if I declare the painter inside the run function the image is wrong in the sense that at the edge of a black and white area, the pixels at the interface are overlapped to give a grey. Does anyone know why this is so? Could it be because of the nature of the run function itself?

//This is ok  
void RenderThread::run()  
{  
  QImage image(resultSize, QImage::Format_RGB32);  
  drawChart(&image);  
  emit renderedImage(image, scaleFactor);  
}

drawChart(&image)  
{  
  QPainter painter(image);
  painter.doStuff()(;  
  ...  
}  

//This gives a image that seems to have artifacts  
void RenderThread::run()  
{  
  QImage image(resultSize, QImage::Format_RGB32);  
  QPainter painter(image);  
  drawChart(painter);  
  emit renderedImage(image, scaleFactor);  
}  

drawChart(&painter)
{   
  painter.doStuff();    
  ...  
}

//bad
.
//good
.

Answer 1

From C++ GUI Programming with Qt 4 by Jasmin Blanchette and Mark Summerfield:

One important thing to understand is
that the center of a pixel lies on
“half-pixel” coordinates. For example,
the top-left pixel covers the area
between points (0, 0) and (1, 1), and
its center is located at (0.5, 0.5).
If we ask QPainter to draw a pixel at,
say, (100, 100), it will approximate
the result by shifting the coordinate
by +0.5 in both directions, resulting
in the pixel centered at (100.5,
100.5) being drawn.

This distinction may seem rather
academic at first, but it has
important consequences in practice.
First, the shifting by +0.5 only
occurs if antialiasing is disabled
(the default); if antialiasing is
enabled and we try to draw a pixel at
(100, 100) in black, QPainter will
actually color the four pixels (99.5,
99.5), (99.5, 100.5), (100.5, 99.5), and (100.5, 100.5) light gray, to give
the impression of a pixel lying
exactly at the meeting point of the
four pixels. If this effect is
undesirable, we can avoid it by
specifying half-pixel coordinates, for
example, (100.5, 100.5).