Multithreaded per pixel operations on QImage

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Hi, I'm writing a small program which does per pixel operations on an QImage. Because of its use to handle large images I want several threads to work on the image. Therefore I handle n parts of the image with QtConcurrent, which sadly doesn't work properly.
As a first test I just try to convert the image to grayscale, but instead of gray the Image gets black, just a few Pixels (ordered in a characteristic pattern ) really get gray. I add an result image under the post...
Here is the code I wrote:

// imgOrig is QImage* which holds the original image
// imgDone is QImage* which should hold the result image
 
// function which is called
void Scan::convertToGrayscale() {
   // get best number of threads
   int n = QThreadPool::globalInstance()->maxThreadCount();
   // calculate part width (each thread will handle a part of the image)
   int step = imgOrig->width() / n;
   for (int i=0; i<n; i++) {
      if (i+1 < n) {
         QtConcurrent::run(this, &Scan::convertToGrayscale, i*step, (i+1)*step);
      }
      else {
         // the last part should really reach the end (int a/b*b != a)
         QtConcurrent::run(this, &Scan::convertToGrayscale, i*step, imgOrig->width());
      }
   }
   QThreadPool::globalInstance()->waitForDone();
}
 
// function to handle a part of the image
void Scan::convertToGrayscale(int from, int to) {
   int temp;
   for (int i=from; i<to; i++) {
      for (int j=0; j<imgOrig->height(); j++) {
         temp = qGray(imgOrig->pixel(i, j));
         imgDone->setPixel(i, j, qRgb(temp, temp, temp));
      }
   }
}

// imgOrig is QImage* which holds the original image
// imgDone is QImage* which should hold the result image

// function which is called
void Scan::convertToGrayscale() {
   // get best number of threads
   int n = QThreadPool::globalInstance()->maxThreadCount();
   // calculate part width (each thread will handle a part of the image)
   int step = imgOrig->width() / n;
   for (int i=0; i<n; i++) {
      if (i+1 < n) {
         QtConcurrent::run(this, &Scan::convertToGrayscale, i*step, (i+1)*step);
      }
      else {
         // the last part should really reach the end (int a/b*b != a)
         QtConcurrent::run(this, &Scan::convertToGrayscale, i*step, imgOrig->width());
      }
   }
   QThreadPool::globalInstance()->waitForDone();
}

// function to handle a part of the image
void Scan::convertToGrayscale(int from, int to) {
   int temp;
   for (int i=from; i<to; i++) {
      for (int j=0; j<imgOrig->height(); j++) {
         temp = qGray(imgOrig->pixel(i, j));
         imgDone->setPixel(i, j, qRgb(temp, temp, temp));
      }
   }
}

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The Screenshot was scaled down to 25% in width and heigth, but you can still see the strange paterns. The System I runned the program on has two cores, as you can see the image really is split into two halfes with similar patern. Is this misbehaviour because both threads write into the same QImage or something?

PS: Qt 4.5 under Windows XP SP3, don't know if this matters.