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    Forum Updated on Feb 6th

    Image processing and sizes

    General and Desktop
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    • A
      Anticross last edited by

      I write some application which process the image finding the color which match with given color. If it not matched it will be replaced by white color. I know that my method far from perfection. But almost all is OK, except bug with large images. If I proceed small images it works but if image is big the program is stacked and CPU shows the 100% load. So here is the code:

      @
      void ImageWidget::proccessImage(QColor color)
      {
      // Search algorithm

      QImage image = m_image;
      int match = 5;

      for(int i = 0; i<image.height(); i++)
      {
      for(int j = 0; j<image.width(); j++)
      {
      QRgb currentPixel = image.pixel(i,j);

      QColor pixColor = QColor::fromRgb(currentPixel);

      if(pixColor.red()-color.red() > match || pixColor.red()-color.red() < - match  && 
         pixColor.green()-color.green() > match || pixColor.green()-color.green() < - match &&
         pixColor.blue()-color.blue() > match || pixColor.blue()-color.blue() < - match)
      image.setPixel(i,j,qRgb(255,255,255));
      

      }
      }
      m_imageLabel->setPixmap(QPixmap::fromImage(image));
      }
      @

      m_imageLabel is QLabel and m_image is given image. Maybe anyone can help me to understand why it happens and give some corrections.

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      • Z
        ZapB last edited by

        Why do you think it odd that the cpu uses 100% for large images? The bigger the image the longer it will take.

        You can reduce the number of checks you are doing by a factor of 2 by having checks like this:

        @
        qAbs( pixColor.red() - color.red() ) < match
        @

        If you wish to keep your GUI alive whilst you are doing these calculations then I suggest that you move them into another thread and emit the modified image in a signal when it is finished.

        Nokia Certified Qt Specialist
        Interested in hearing about Qt related work

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        • A
          Anticross last edited by

          I've got a double core cpu with freq 2.3 ghz and the program freeze more than 10 min till processing image 1024*768 and don't unfreezes. So I don't know how long I need wait, maybe it is because I using different image formats: png, bmp, jpeg or different color dept in image or any else.

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          • Z
            ZapB last edited by

            As an aside the number of cores is irrelevent here as you are only using one thread. Having said that it should not take that long to process such modestly sized image. Have you tried breaking the execution using the debugger when it freezes to see where it is getting stuck?

            Nokia Certified Qt Specialist
            Interested in hearing about Qt related work

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            • A
              Anticross last edited by

              is there a difference between pixel loops in different types of images ?

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              • Z
                ZapB last edited by

                What does QImage::format() return for an image that works and one that doesn't?

                Nokia Certified Qt Specialist
                Interested in hearing about Qt related work

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                • A
                  Anticross last edited by

                  I've got massages in output window of my debbuger
                  QImage::pixel: coordinate (0,2336) out of range
                  QImage::pixel: coordinate (0,2337) out of range
                  QImage::pixel: coordinate (0,2338) out of range
                  QImage::pixel: coordinate (0,2339) out of range
                  QImage::pixel: coordinate (0,2340) out of range
                  QImage::pixel: coordinate (0,2341) out of range
                  ...
                  So it's happens when I processed 3504*2336 image. why it not limited by size of image or I have a mistake in my loops limits?

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                  • Z
                    ZapB last edited by

                    You have you're i and j loop counters back to front in the call to image.pixel(). It should be:

                    @
                    QRgb currentPixel = image.pixel( j, i );
                    @

                    since you are using j to loop over the x-coord and i to iterate over the y coord. See how the debugger helps you ;-)

                    Nokia Certified Qt Specialist
                    Interested in hearing about Qt related work

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                    • Z
                      ZapB last edited by

                      You have made the same mistake in the call to setPixel() too.

                      Nokia Certified Qt Specialist
                      Interested in hearing about Qt related work

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                      • Z
                        ZapB last edited by

                        So with all the above in place your code should look like this:

                        @
                        void ImageWidget::proccessImage(QColor color)
                        {
                        // Search algorithm

                        QImage image = m_image;
                        int match = 5;

                        for(int i = 0; i<image.height(); i++)
                        {
                        for(int j = 0; j<image.width(); j++)
                        {
                        QRgb currentPixel = image.pixel(j,i);

                        QColor pixColor = QColor::fromRgb(currentPixel);

                        if(qAbs(pixColor.red()-color.red()) > match && 
                           qAbs(pixColor.green()-color.green()) > match &&
                           qAbs(pixColor.blue()-color.blue()) > match)
                            image.setPixel(j,i,qRgb(255,255,255));
                        

                        }
                        }
                        m_imageLabel->setPixmap(QPixmap::fromImage(image));
                        }
                        @

                        Nokia Certified Qt Specialist
                        Interested in hearing about Qt related work

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                        • A
                          Anticross last edited by

                          Damn, it's so easy :) I need to be more attentive. Thanks for the help and some code optimization.

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                          • G
                            giesbert last edited by

                            Hi Anticross,

                            One idea to make the code a bit more readable und to easier find such problems (use x and y instead of i and j), make some renaming. You can also use QRgb directly.

                            @
                            void ImageWidget::proccessImage(QColor color)
                            {
                            // Search algorithm

                            QImage image = m_image;
                            int difference = 5;
                            
                            for(int y = 0; y < image.height(); y++)
                            {
                                for(int x = 0; x < image.width(); x++)
                                {
                                    QRgb currentPixel = image.pixel(x,y);
                            
                                    if( qAbs(qRed(currentPixel) - color.red()) > difference &&
                                        qAbs(qGreen(currentPixel) - color.green()) > difference &&
                                        qAbs(qBlue(currentPixel) - color.blue()) > difference)
                                    {
                                        image.setPixel(x,y,qRgb(255,255,255));
                                    }
                                }
                            }
                            m_imageLabel->setPixmap(QPixmap::fromImage(image));
                            

                            }
                            @

                            Nokia Certified Qt Specialist.
                            Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                            • Z
                              ZapB last edited by

                              I usually tend to use i for iterating over x and j for iterating over ysince it reminds me of i and j unit vectors. I also tend to use epsilon or delta for small values such as your match or difference variables. It's just taste though and this is my background in mathematics prejudicing me. ;-)

                              Nokia Certified Qt Specialist
                              Interested in hearing about Qt related work

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                              • G
                                giesbert last edited by

                                As we are talking about coordinates, and height and width, it's usually x and y. That's why I changed that. i is usually (with the developers I work with :-) ) a variable for a normal for loop. For cascading, I normally use row and column or s9imilar things, por x and y if they are coordinated (which they are in this case).

                                But it's a matter of taste.

                                And match was (for me) just nothing that was clear from the name. match for me would be a boolean (matches or not)

                                Nokia Certified Qt Specialist.
                                Programming Is Like Sex: One mistake and you have to support it for the rest of your life. (Michael Sinz)

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                                • Z
                                  ZapB last edited by

                                  Yeah for me it also depends upon the context in which the loop(s) occur. If I am iterating over rows then I'll use "row". Usually I try to use something that makes it obvious to which the counter refers.

                                  Nokia Certified Qt Specialist
                                  Interested in hearing about Qt related work

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