constructing image using img.setpixel(x,y,qRgb())

Prakash08

I'm constructing the image from the pixel values stored in a integer vector. i have three vectors for storing r,g,b values of each pixel and later i will retreive the same and construct the image. while retrieving from the vectors the values of 1st pixel are 36,155,92. but after constructing the image the values of the same are 187,202,39. why both are not the same ?

```

for (int i = 0; i < 1; ++i) {
                    for(int j=0;j<2;j++){
                        int x = i % 2;
                        int y = i / 2;
                        int rVal = currentFrameRed[i];
                        int gVal = currentFrameGreen[i];
                        int bVal = currentFrameBlue[i];
                        qDebug()<<rVal<< gVal<< bVal;
                        img.setPixel(x, y, qRgb(rVal, gVal, bVal));
                        QRgb n=qRgb(rVal, gVal, bVal);
                        QString hex=QString::number(n,16);
                        qDebug()<<hex;
                        qDebug()<<qRgb(rVal, gVal, bVal);
                    }
                }

in the above code i am constructing a 2x2 image. currentFrameRed, currentFrameGreen, currentFrameBlue are the integer vectors that store color values of each pixel.  and i'm using qDebug() to verify the pixel values before and after constructing the image.

jsulm

Do you mean qDebug()<<rVal<< gVal<< bVal prints 36,155,92 and qDebug()<<qRgb(rVal, gVal, bVal) prints 187,202,39.
Your loops and vector access are wrong: first of all the outer loop will be iterated only once, but you will also overwrite the 0,0 pixel in the second inner loop iteration (0/2=0 and 1/2=0).

Prakash08

the second debug statement prints a 10 digit decimal representation of hexa value, which is the returned by qRgb(rVal,gVal,bVal).
And the loop and vector access is wrong i agree, but the problem is image pixel value is changing after constructing the image , how to do that ?

Pl45m4

@Prakash08 said in constructing image using img.setpixel(x,y,qRgb()):'

for (int i = 0; i < 1; ++i) {
                    for(int j=0;j<2;j++){
                        int x = i % 2;
                        int y = i / 2;

As @jsulm wrote, are you aware that this loop and these x and y values might not do what you think they do?!
Your X is always 0. (i is always 0, and so is x = i mod 2)

In addition:
See what qRgb documentation states:

QRgb qRgb(int r, int g, int b)

Returns the ARGB quadruplet (255, r, g, b).

So in your hex conversion you have a 255

Prakash08

yeah i have255 in it

Pl45m4

@Prakash08 said in constructing image using img.setpixel(x,y,qRgb()):

yeah i have255 in it

Can you say what's wrong and give an example what you actually get?

Prakash08

my problem is i'm storing red, green and blue value of each pixel in respective color vectors in the form of integer. when i start constructing image with img.setpixel(x,y,qRgb(rVal,gVal,bVal) the image is being constructed with some width and height. Later after construction of image, the RGB values of each pixel is not matching with the values stored inside the vectors .. how is that ?

jsulm

@Prakash08 said in constructing image using img.setpixel(x,y,qRgb()):

my problem

Your problem is that your code is wrong as already stated - are you going to ignore that? Fix the code...

Pl45m4

@Prakash08 said in constructing image using img.setpixel(x,y,qRgb()):

my problem is i'm storing red, green and blue value of each pixel in respective color vectors in the form of integer. when i start constructing image with img.setpixel(x,y,qRgb(rVal,gVal,bVal) the image is being constructed with some width and height. Later after construction of image, the RGB values of each pixel is not matching with the values stored inside the vectors .. how is that ?

You are misunderstanding what qRgb(r, g, b) does. Read that part of the documentation I've linked above.

Besides that your loop and indices are wrong, your code is correct, but you are just interpreting it wrong.

Change the last line of debug from

qDebug() << qRgb(rVal, gVal, bVal);

to

qDebug() << qRed(n) << qGreen(n) << qBlue(n);

where n is your constructed QRgb triplet.

Then you will get the same color values as in your three vectors.
Setting qRgb to your image won't change anything as you thought at first. Everything works as expected.

The issue with your current qDebug() output is that qRgb(r,g,b) will always add the alpha value a (= 255) as the code shows:

inline Q_DECL_CONSTEXPR QRgb qRgb(int r, int g, int b)// set RGB value
{ return (0xffu << 24) | ((r & 0xffu) << 16) | ((g & 0xffu) << 8) | (b & 0xffu); }

That's also why your hex string will start with ff

Prakash08

            int i=0;
            for (int j = 0; j < 2; ++j) {
                for(int k=0;k<2;k++){
                    int x = j;
                    int y = k;
                    int rVal = currentFrameRed[i];
                    int gVal = currentFrameGreen[i];
                    int bVal = currentFrameBlue[i];
                    ++i;

                    qDebug()<<rVal<< gVal<< bVal;
                    img.setPixel(x, y, qRgb(rVal, gVal, bVal));
                    QRgb n=qRgb(rVal, gVal, bVal);
                    QString hex=QString::number(n,16);
                    qDebug()<<hex;
                    qDebug() << qRed(n) << qGreen(n) << qBlue(n);
                }
            }

i have changed the above code to set pixel at correct position. but the problem is :
actual pixel values:
pixel-1: 207,209,1131
pixel-2: 2,185,169
pixel-3: 165,133, 205
pixel-4: 18,180,121

constructed image:
pixel-1: 134,211,135
pixel-2: 111,188,172
pixel-3: 85,162,146
pixel-4: 72, 149, 133

how should i modify the the code in order to have my actual pixel values and constructed pixel values to be same ?

Pl45m4

@jsulm said in constructing image using img.setpixel(x,y,qRgb()):

Your problem is that your code is wrong as already stated - are you going to ignore that? Fix the code...

@Prakash08

Can't believe that I've done this, but enjoy

    QImage img(QSize(2,2), QImage::Format_RGB32);
    QVector<int> vRed   { 20,  40,  80, 160 };
    QVector<int> vGreen { 10,  30,  60, 120 };
    QVector<int> vBlue  {  5,  10,  20,  40 };
    //                   0/0, 0/1, 1/0, 1/1

    int x = 0;
    int y = 0;
    Q_ASSERT( (vRed.size() == vGreen.size()) && (vRed.size() == vBlue.size()) );
    for (int i = 0; i < 4; i++){

   //     if (i >= 2)
   //        x = 1;
   //     else
   //        x = 0;
   //
   //     int y = i % 2;

  // Even better and shorter:
        if (i > 0)
            x = i % (y+i);
        y = i % 2;
  // for 2x2 and all colors same size

        int rVal = vRed.at(i);
        int gVal = vGreen.at(i);
        int bVal = vBlue.at(i);

        img.setPixel(x, y, qRgb(rVal, gVal, bVal));

        qDebug() << "\n";
        qDebug() << "( " << x << " | " << y << " )";
        qDebug() << "Vector RGB: " << rVal << gVal << bVal;
        QRgb n = qRgb(rVal, gVal, bVal);
        QString hex = QString::number( n,16 );
        qDebug() << hex;
        qDebug() << "Pixel RGB: " << qRed(n) << qGreen(n) << qBlue(n);
    }

Impossible to solve, right? :)
Try to understand why your loop was still wrong. You corrected the output now after my comment, but as @jsulm said two times, you had to fix your loop also (i, j, x and y).
My code above should work for you.

Output

(  0  |  0  )
Vector RGB:  20 10 5
"ff140a05"
Pixel RGB:  20 10 5


(  0  |  1  )
Vector RGB:  40 30 10
"ff281e0a"
Pixel RGB:  40 30 10


(  1  |  0  )
Vector RGB:  80 60 20
"ff503c14"
Pixel RGB:  80 60 20


(  1  |  1  )
Vector RGB:  160 120 40
"ffa07828"
Pixel RGB:  160 120 40

Edit:
Improved loop and x/y condition

Prakash08

@Pl45m4 said in constructing image using img.setpixel(x,y,qRgb()):

Q_ASSERT( (vRed.size() == vGreen.size()) && (vRed.size() == vBlue.size()) );

may I know why this line is used ?

Pl45m4

@Prakash08 said in constructing image using img.setpixel(x,y,qRgb()):

Q_ASSERT( (vRed.size() == vGreen.size()) && (vRed.size() == vBlue.size()) );

may I know why this line is used ?

In my updated code I used

for (int i = 0; i < vRed.size(); i++){
}

instead of


for (int i = 0; i < 4; i++){
}

Therefore all three color vectors have to have the same length. Tried to make the code as variable as possible, to make it work for 3x3, 4x4 or other image dimensions. But then you would have to re-think the X, Y coordinate handling anyway.

So this code

Q_ASSERT( (vRed.size() == vGreen.size()) && (vRed.size() == vBlue.size()) );

will stop/interrupt the execution of the program if the condition inside is false. In this case you gonna know directly where and why it stopped instead of crashing somewhere else randomly because the vectors are not the same size.
Not really needed here, but might help in other places and when you work on bigger projects.
Q_ASSERT is just the Qt macro to call assert() from cassert.h