Picture

[Payx]

Hello guys,

I got a project to do :

i want display a picture (i have already do that) and i want to reduce the resolution of the picture by 4 and work with all this square, like know the main color of these square.

so i work on it with :

int step=4;
	for (int i=0;i<pix.width();i=i+step){                                  // the name of my picture is "pix"
		for (int j=0;j<pix.height();j=j+step){
                                                                                                    // its here where im blocked
}
}

i know what i have to do but i dont know how i can traduce it in C
like with "QImage copy(const QRect & rectangle = QRect()) const" i suppose

can you help me ?

[jsulm]

@Payx said in Picture:

i want to reduce the resolution of the picture by 4 and work with all this square

Sorry I don't understand your use case:
"i want to reduce the resolution of the picture by 4 and work with all this square"
Do you want to reduce the resolution of the picture or divide it into 4x4 sub-pictures?

[Payx]

@jsulm Yes i want to cut the picture in square 4x4

sorry

[Payx]

cut the picture in square 4x4 but with this 4x4 square i will just determinate the most contained color, and by the way do a simple pixel with it

[mrjj]

@Payx said in Picture:

and by the way do a simple pixel with it

Hi can you explain more what you do with it?

[Payx]

@mrjj i cut my picture in square 4x4, and with this square i just calculate the most contained color, i replace all this square by the color and it give me a simple square with just one color

[SGaist]

Hi,

Why not use QImage::scaled ?

[Payx]

@SGaist I don't know if it will work (im a beginner) but the function return only one square ? or i can use this function to cut in 4x4 square my picture with a double "for" ?

[SGaist]

It was just for the resizing part.

For the processing itself, it might be better to use a library like OpenCV which likely already implement what you need.

[Payx]

@SGaist ok i will install OpenCV on Qt and come if i need more help

[Payx]

@SGaist there is a lot of tutorial for install OpenCV, and they are all different... can you have a good tutorial to show me ?

[mrjj]

@Payx
Hi
can i ask what effect you are after ?
It sounds like you want to make the image more pixelated?

[SGaist]

What OS are you on ?

[Payx]

@mrjj No,

I got an image, i just want to cut the image in square 4x4 (pixels) and with this square, i want to detect the most contained color. When i have this color i replace entirely the square by the color.

Exemple : if i got an image, if the first square 4x4 i have on the image is contained by 80% of red, that will replace the entirely square by red only.

This project is to have a "simple" picture.

with this simple picture i can reproduce it with candies (for exemple). Because if i got 800800 pixels i can't buy 800800 candies so i have to "reduce" it

Im on windows 10

[SGaist]

Then you can use the OpenCV installer, just ensure that you have an OpenCV version built with the same compiler you are using with Qt.

[Payx]

@SGaist I spoke with my teacher, he told me we dont need OpenCV

[SGaist]

I never said you needed it just that it might be simpler through it.

In any case, you can use QImage::pixelColor to get the color of one pixel of your image and then do the calculation with the surrounding pixels.

[Payx]

@SGaist said in Picture:

QImage::pixelColor

Ok, i will try to use it tomorrow, but i already have a question, i can't use a function in the "void MainWindow::on_push_clicked()" no ?

[kshegunov]

@Payx said in Picture:

I got an image, i just want to cut the image in square 4x4 (pixels) and with this square, i want to detect the most contained color. When i have this color i replace entirely the square by the color.
Exemple : if i got an image, if the first square 4x4 i have on the image is contained by 80% of red, that will replace the entirely square by red only.
This project is to have a "simple" picture.
with this simple picture i can reproduce it with candies (for exemple). Because if i got 800800 pixels i can't buy 800800 candies so i have to "reduce" it

It sounds to me you're after color space reduction after scaling. Just resample the image to the desired resolution with QImage::scaled as @SGaist suggested. Then the color space reduction boils down to calculating distances in 3 dimensional rectangular coordinate system (colors are vectors in RGB space). Suppose you have a table of 32 colors and an image:

QImage image;

const qint32 colorTableSize = 32;
QColor colorTable[colorTableSize] = { Qt::Red, Qt::Blue, ... };

Then for each pixel you choose the color from the table that's closest in the color space:

// Just go around the image
for (qint32 y = 0, height = image.height(); y < height; y++)  {
    for (qint32 x = 0, width = image.width(); x < width; x++)  {
        QColor pixel = image.pixelColor(x, y);

        // Find the closest match from the table
        qint32 colorIndex = 0;
        qreal minDistance = std::sqrt(3); // Initial value is the longest possible distance
        for (qint32 i = 0; i < colorTableSize; i++)  {
            qreal distance = QVector3D(pixel.redF() - colorTable[i].redF(), pixel.greenF() - colorTable[i].greenF(), pixel.blueF() - colorTable[i].blueF()).length();
            if (distance < minDistance)  {  // Found a closer color than the currently selected
                minDistance = distance;
                colorIndex = i;
            }
        }

        // Now just replace the pixel color with the one from the table
        image.setPixelColor(x, y, colorTable[colorIndex]);
    }
}

This doesn't take into account the disparity of color sensitivity of the human eye. If you need to account for this then you need to dig deeper to find more accurate color distances, e.g. by implementing some sort of a chromaticity diagram.

Kind regards.

[jsulm]

@Payx said in Picture:

i can't use a function in the "void MainWindow::on_push_clicked()" no ?

Sure you can, why not?