How can I use concurrency to generate an QImage

[lni]

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!

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;
}

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;
}

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[JohannesMunk]

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:

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..
    }
};

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..
    }
};

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You can call this with QtConcurrent::run

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

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

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This shows how to wait for multiple operations with QFutureSynchronizer!

HIH

Johannes

[wysota]

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:

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);

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);

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[JohannesMunk]

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

[wysota]

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

[lni]

Thank you all!! I will try that.