How can I use concurrency to generate an QImage

Hi,

I need to generate an QImage repeatedly from data using following code. How can I use QtConcurrent to take advantage of multi-cores computer?

Thanks!

Code:

---

QImage toImage( QVector<const float*> data, int width, int height )
{
  QImage image( width, height, QImage::Format_ARGB32 );
 
  for ( int iy = 0; iy < image.height(); iy++ ) {
    for ( int ix = 0; ix < image.width(); ix++ ) {
       QRgb rgb = calcRbgFromSomewhere( data[ ix ][ iy ] );
       image.setPixel( ix, iy, pixel );
    }
  }
  return image;
}

---

Hi!

Yeah.. the docs on the QtConcurrent stuff are really poor!

What you could do is store your float data together with the width and height value together in a small class, like so:

Code:

---

class FloatImage
{
public:
    FloatImage(QVector<const float*> data, int width, int height) : data(data),width(width),height(height) {}
    QVector<const float*> data;
    int width;
    int height;
    QImage toImage()
    {
       // Your code..
    }
};

---

You can call this with QtConcurrent::run

Code:

---

FloatImage fi(yourData,200,100);
QFutureSynchronizer<QImage> synchronizer;
synchronizer.addFuture( QtConcurrent::run(fi,&FloatImage::toImage));
...
synchronizer.waitForFinished();

---

This shows how to wait for multiple operations with QFutureSynchronizer!

HIH

Johannes

But this way you will only use one core. It's better to use the "map" semantics and let every pixel or scanline be processed by a different thread.
For instance:

Code:

---

QImage img(640,480);
struct Elem {
  uchar *scanLine;
  int lineIndex;
  int lineSize;
};
 
void func(Elem &e) {
  for(int i=0;i<e.lineSize;++i){
    QRgb rgb = calc(i, e.lineIndex);
    e.scanLine[i] = rgb; // or something like this
  }
}
 
QList<Elem> image;
for(int i=0;i<img.height();++i){
  Elem e;
  e.scanLine = img.scanLine(i);
  e.lineIndex = i;
  e.lineSize = img.width();
}
QtConcurrent::blockingMap(image, func);

---

Hello wysota!

I understood lni as if he had multiple images to process. For that he only would need to call QtConcurrent:run multiple times with his different images. I tried to indicate that with "..."

But now he has examples for both granularities!

I think "image.append(e);" is missing after line 20 in your code.

Cheers,

Joh

Yes, of course, there is a missing line in my code.

Thank you all!! I will try that.