Increasing contrast of QImage

[freely]

Hello,

I develop GUI and algorithm for editing images , ideally it should work like Photoshop ( hhh ).
I already made brightness control algorithm and UI and now working on contrast increase/decrease of QImage,

next code ( part of a class method ) computes histogram of QImage ( imageToProcess and prev_image are class variables ), finds histogram center ( middle ) and tries to change image contrast by input value factor ( from 0 to 1 ) :

QMap<QRgb,long> histo; // histogram map
     QMap<QRgb,long>::iterator j;
     QRgb c = 0;
     int l, k;
     histo.clear(); 
     for(k = 0; k < prev_image.width(); ++k)
        for( l = 0; l < prev_image.height(); ++l) {
              c = prev_image.pixel(k,l);
 
            if(!histo.contains(c))
                histo.insert(c,0);
        }
     //computation of occurences
     for( k = 0; k < prev_image.width(); ++k)
        for( l = 0; l < prev_image.height(); ++l) {
            c = prev_image.pixel(k,l);
            histo[c] = histo[c]+1;
        }
                 //compute average value
                 long sum_1 = 0;
                 long sum_2 = 0;
                 for(j = histo.begin();j!=histo.end();j++)
                 {
                     sum_1+=j.value()*j.key();
                 }
                 for(j = histo.begin();j!=histo.end();j++)
                 {
                     sum_2+=j.value();
                 }
                 long av = sum_1/sum_2;
                 av_r = qRed(av);
                 av_g = qGreen(av);
                 av_b = qBlue(av);
                //changing contrast of an image by factor getted from horizontal slider ui:
                double factor = (double)( (double)ie->horizontalSlider_2->value() )/100 ;    // to change also   //got to be a value between 0 - 1
                if(factor!=0.99)
                for (int y = 0; y < prev_image.height(); ++y) {
                for (int x = 0; x < prev_image.width(); ++x) {
                c = QRgb(prev_image.pixel(x,y));
                QColor col(c);
                col.getRgb(&r,&g,&b);
                r = (int)(r*(1-factor)+av_r*factor);
                g = (int)(g*(1-factor)+av_g*factor);
                b = (int)(b*(1-factor)+av_b*factor);
                if(r>255)
                    r=255;
                if(g>255)
                    g=255;
                if(b>255)
                    b=255;
                imageToProcess->setPixel(x,y,qRgb(r,g,b));
             }
          }
            if(factor!=0.99)
                    this->ie->label_11->setPixmap(QPixmap::fromImage(imageToProcess->scaled(this->ie->label_11->size(),Qt::KeepAspectRatio,Qt::SmoothTransformation)));
            else
                    this->ie->label_11->setPixmap(QPixmap::fromImage(prev_image.scaled(this->ie->label_11->size(),Qt::KeepAspectRatio,Qt::SmoothTransformation)));

QMap<QRgb,long> histo; // histogram map
     QMap<QRgb,long>::iterator j;
     QRgb c = 0;
     int l, k;
     histo.clear(); 
     for(k = 0; k < prev_image.width(); ++k)
        for( l = 0; l < prev_image.height(); ++l) {
              c = prev_image.pixel(k,l);

            if(!histo.contains(c))
                histo.insert(c,0);
        }
     //computation of occurences
     for( k = 0; k < prev_image.width(); ++k)
        for( l = 0; l < prev_image.height(); ++l) {
            c = prev_image.pixel(k,l);
            histo[c] = histo[c]+1;
        }
                 //compute average value
                 long sum_1 = 0;
                 long sum_2 = 0;
                 for(j = histo.begin();j!=histo.end();j++)
                 {
                     sum_1+=j.value()*j.key();
                 }
                 for(j = histo.begin();j!=histo.end();j++)
                 {
                     sum_2+=j.value();
                 }
                 long av = sum_1/sum_2;
                 av_r = qRed(av);
                 av_g = qGreen(av);
                 av_b = qBlue(av);
                //changing contrast of an image by factor getted from horizontal slider ui:
                double factor = (double)( (double)ie->horizontalSlider_2->value() )/100 ;    // to change also   //got to be a value between 0 - 1
                if(factor!=0.99)
                for (int y = 0; y < prev_image.height(); ++y) {
                for (int x = 0; x < prev_image.width(); ++x) {
                c = QRgb(prev_image.pixel(x,y));
                QColor col(c);
                col.getRgb(&r,&g,&b);
                r = (int)(r*(1-factor)+av_r*factor);
                g = (int)(g*(1-factor)+av_g*factor);
                b = (int)(b*(1-factor)+av_b*factor);
                if(r>255)
                    r=255;
                if(g>255)
                    g=255;
                if(b>255)
                    b=255;
                imageToProcess->setPixel(x,y,qRgb(r,g,b));
             }
          }
            if(factor!=0.99)
                    this->ie->label_11->setPixmap(QPixmap::fromImage(imageToProcess->scaled(this->ie->label_11->size(),Qt::KeepAspectRatio,Qt::SmoothTransformation)));
            else
                    this->ie->label_11->setPixmap(QPixmap::fromImage(prev_image.scaled(this->ie->label_11->size(),Qt::KeepAspectRatio,Qt::SmoothTransformation)));

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But actually what happens is that a contrast of an image changes ( only decreases in such a way ) and a background gets green when initial background is black or red when initial background is white.

( screenshot of part of UI with the result is in the attachment )

How to increase contrast also ?

What to fix so that a background stays the same after contrast changing manipulations and does not get green/red ?

Sorry if the code is not formatted / eased to read.

Thanks in advance.

[mvuori]

The problem is in changing the RGB values so that the white balance stays the same, as the system is not linear. I think this is best done in HSV space. A code snipped for doing pixel value conversion using qimageblitz (http://api.kde.org/kdesupport-api/kd...ageblitz/html/)

unsigned int *data, *end;
    data = (unsigned int *)qimage->bits();
    end = data + (qim->width() * qim->height());
    InlineHSV hsv;
    while(data < end){
        // convert pixel to HSV
        hsv.convertRGB2HSV(BlitzPrivate::convertFromPremult(*data));
        // change H, S or V as needed
        hsv.setSaturation(somethingBasedOn(hsv.saturation());
        // and back to RGB
        hsv.convertHSV2RGB();
        (*data) = BlitzPrivate::convertToPremult(qRgba(hsv.red(), hsv.green(),
                                                     hsv.blue(), qAlpha(*data)));
        data++;
    }

unsigned int *data, *end;
    data = (unsigned int *)qimage->bits();
    end = data + (qim->width() * qim->height());
    InlineHSV hsv;
    while(data < end){
        // convert pixel to HSV
        hsv.convertRGB2HSV(BlitzPrivate::convertFromPremult(*data));
        // change H, S or V as needed
        hsv.setSaturation(somethingBasedOn(hsv.saturation());
        // and back to RGB
        hsv.convertHSV2RGB();
        (*data) = BlitzPrivate::convertToPremult(qRgba(hsv.red(), hsv.green(),
                                                     hsv.blue(), qAlpha(*data)));
        data++;
    }

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