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::SmoothTransformatio n)));
else
this->ie->label_11->setPixmap(QPixmap::fromImage(prev_image.scaled(thi s->ie->label_11->size(),Qt::KeepAspectRatio,Qt::SmoothTransformatio n)));

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/kdesupport-apidocs/qimageblitz/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::convertFromPremul t(*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++;
}