How can I decrease the brightness by coming to original form of image in C++ without using opencv?

Question

How can I decrease the brightness by reading the original form of image in C++, if I don´t want to use opencv or something else? I tried many things and this is what I got. I deeply appreciate your help.

void MyArea::on_brightness_increase()
{
//  m_refpixbuf1 = m_refpiximage->copy();
    nPixelvalue = m_refpixbuf1->get_pixels();

    wincenter = wincenter + 10;
    pixmax2 = wincenter + (winwidth / 2);
    pixmin2 = wincenter - (winwidth / 2);

    nimage_height = m_refpiximage->get_height();
    nimage_width = m_refpiximage->get_width();
    nimage_size = nimage_height*nimage_width*3;


    for(int i=0;i<nimage_size;i++)
    {
        if(nPixelvalue[i]<=250)
            nPixelvalue[i] +=5;
        else
            nPixelvalue[i] = 255;

        if(nPixelvalue[i+1] <= 250)
            nPixelvalue[i+1] +=5;
        else
            nPixelvalue[i+1] = 255;

        if(nPixelvalue[i+2] <= 250)
            nPixelvalue[i+2] +=5;
        else
            nPixelvalue[i+2] = 255;
    }
    get_window()->invalidate(false);
}

void MyArea::on_brightness_decrease()
{
//  m_refpixbuf1 = m_refpiximage->copy();
    nPixelvalue2 = m_refpixbuf3->get_pixels();

    wincenter = wincenter - 10;

    pixmax2 = wincenter + (winwidth / 2);
    pixmin2 = wincenter - (winwidth / 2);

    nimage_height = m_refpiximage->get_height();
    nimage_width = m_refpiximage->get_width();
    nimage_size = nimage_height*nimage_width*3;

    for (int i=0;i<nimage_size;i++)
    {
        if(nPixelvalue2[i] >=5)
            nPixelvalue2[i] -=5;
        else
            nPixelvalue2[i] = 0;

        if(nPixelvalue2[i+1] >=5)
            nPixelvalue2[i+1] -=5;
        else
            nPixelvalue2[i] = 0;
        if(nPixelvalue2[i+2] >=5)
            nPixelvalue2[i+2] -=5;
        else
            nPixelvalue2[i+2] = 0;
    }
    get_window()->invalidate(false);
}

Answer 1

First, what you need to do is create a struct for your pixels for your image class

struct pixel {
    public float red; // 0 = 0, 1 = 255
    public float blue; // 0 = 0, 1 = 255
    public float green; // 0 = 0, 1 = 255
    // appropriate methods, constructors
    // if you don't want hdr pixels, cap your members at 1.0
}

You then want to create a method that increases brightness. see here for the formula of brightness:

https://en.wikipedia.org/wiki/Brightness

Think of a method so that you use to increase or decrease brightness. Add it to the pixel struct, and call it for each pixel in your image.