[Qt-interest] Enhanced monochrome conversion

[M. Bashir Al-Noimi]

Andre Somers wrote:

> M. Bashir Al-Noimi wrote:
>   
>> Hi All,
>>
>> It's easy to convert any image to monochrome format by using 
>> QImage::convertToFormat like this:
>>
>> pImage.convertToFormat(QImage::Format_Mono, Qt::MonoOnly|Qt::ThresholdDither|Qt::AvoidDither);
>>
>> 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?*
>>
>> *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:
>>
>> 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;
>> }
>>     
> Hi, interesting problem, but I'd say there is a relatively straitforward 
> way to get an answer: read the GIMP sourcecode and see how it does this! 
> Of course, I have no idea if the source of the GIMP is actually 
> readable, I never tried.
>
> André
>  (use the Force, read the Source!)
>
>   
This is the hard way for solving the problem!!!!

Any way, I asked same question in the forum and I got a response from 
guy explained how GIMP deal with this problem (read the quote down here):
>  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.
so directly I tried to test an image in GIMP then I discovered that GIMP 
behaves this way exactly, so I coded the following but it didn't give me 
a correct result just like GIMP (threshold is 209 where it must around 165):

    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 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);
//    result.save("c:/res.jpg");
    return result;


The resultant image:


Do you've any idea how I can fix this issue?


-----
Best Regards
Muhammad Bashir Al-Noimi
My Blog: http://mbnoimi.net/
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