Auto threshold value of monochrome conversion

Hi All,

It's easy to convert any image to monochrome format by using QImage::convertToFormat like this:

Code:

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pImage.convertToFormat(QImage::Format_Mono, Qt::MonoOnly|Qt::ThresholdDither|Qt::AvoidDither);

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but convertToFormat always guessing that the value of conversion's threshold is 255 where in some cases we need to use another threshold's value. I checked out GIMP and I noticed that it uses an automatic method for guessing threshold's value (see the example below) so I'm wondering:

How I can convert images to monochrome format by using auto threshold's value just like GIMP conversion method?

Original
http://img134.imageshack.us/img134/9741/77363363.th.jpg

QImage conversion
http://img145.imageshack.us/img145/5563/43162085.th.jpg

Automatic GIMP conversion
http://img402.imageshack.us/img402/7148/20634127.th.jpg

P.S

I've wrote a stupid function for converting images to monochrome by using a specified threshold's value as following but I need to know how I can automatically specify threshold's value:

Code:

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QImage getThresholdBW(QImage pImage, int threshold)
{
    QImage result = pImage;
    result.fill(255);
    for(int x=0; x<pImage.width(); x++)
        for(int y=0; y<pImage.height(); y++)
            result.setPixel(x, y, qGray(pImage.pixel(x, y))>threshold?qRgb(255, 255, 255):qRgb(0, 0, 0));
 
    return result;
}

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It either uses 127 as the threshold or if it is smarter, it first calculates an average or median of lightness (which is an equivalent of colors->levels->auto) (aka value in HSV) and uses that. Try doing colors->levels->auto on your image and then convert it to monochrome using 127 as the threshold. If it works you can repeat the same process in your application.

Quote:

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Try doing colors->levels->auto on your image and then convert it to monochrome using 127 as the threshold. If it works

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It works.

Quote:

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you can repeat the same process in your application.

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I tried to calculate the median but the resultant image isn't correct because the median value is big (about 203)... see my snippet plz:

Code:

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QImage result = pImage;
    QList<int> pixelsValues;
    for(int x=0; x<pImage.width();x++)
    {
        for(int y=0; y<pImage.height();y++)
        {
            QRgb rgb = pImage.pixel(x,y);
            QColor color(rgb);
            pixelsValues<<color.lightness();
        }
    }
    qSort(pixelsValues);
    int median;
    int count = pixelsValues.count();
    if(count%2==0)
        median = (pixelsValues[count/2]+pixelsValues[(count/2)+1]) / 2;
    else
        median = pixelsValues[count/2];
    threshold = median;
    for(int x=0; x<pImage.width(); x++)
        for(int y=0; y<pImage.height(); y++)
            result.setPixel(x, y, qGray(pImage.pixel(x, y))>threshold?qRgb(255, 255, 255):qRgb(0, 0, 0));
    qDebug("threshold = %d", threshold);
    return result;

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P.S
I'm using Qt 4.6.0 which has lightness function

You can preview the resultant image down here:

http://img80.imageshack.us/img80/6408/reso.th.png

Try with an average. Median will tend to make 50% of your pixels white and the other 50% black. If you have one purple spot in a completely yellow picture that's probably not what you would want. Average should do better. If that doesn't help, there is a third choice: (minimum+maximum)/2 - this is exactly what colors->levels->auto should be doing.

Quote:

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Average should do better

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It gave me nearly same result.

Quote:

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there is a third choice: (minimum+maximum)/2 - this is exactly what colors->levels->auto should be doing.

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It's working, thanks wysota.

Code:

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QImage result = pImage;
    QList<int> pixelsValues;
    for(int x=0; x<pImage.width();x++)
    {
        for(int y=0; y<pImage.height();y++)
        {
            QRgb rgb = pImage.pixel(x,y);
            QColor color(rgb);
            pixelsValues<<color.lightness();
        }
    }
    qSort(pixelsValues);
//    int median;
//    int count = pixelsValues.count();
//    if(count%2==0)
//        median = (pixelsValues[count/2]+pixelsValues[(count/2)+1]) / 2;
//    else
//        median = pixelsValues[count/2];
    int median2 = (pixelsValues.first() + pixelsValues.last())/2;
    int threshold = median2;
    for(int x=0; x<pImage.width(); x++)
        for(int y=0; y<pImage.height(); y++)
            result.setPixel(x, y, qGray(pImage.pixel(x, y))>threshold?qRgb(255, 255, 255):qRgb(0, 0, 0));
    qDebug("threshold = %d", threshold);
//    result.save("c:/res.jpg");
    return result;

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