How to create a 2d array from a QImage?

[OliveiraNT]

I need to get a 2d array from a QImage to use in prediction.
Is there any way to do this using only Qt classes (or maybe using openCV)?

Thanks!!

[Todd Morehouse]

Are you trying to convert from QImage to cv::mat?
If so you may be able to skip the process with a function from this helpful guy over here. He helped me with all my opencv issues with qt.

QImage frmMain::convertOpenCVMatToQtQImage(cv::Mat mat) {
    if(mat.channels() == 1) {                   // if grayscale image
        return QImage((uchar*)mat.data, mat.cols, mat.rows, mat.step, QImage::Format_Indexed8);     // declare and return a QImage
    } else if(mat.channels() == 3) {            // if 3 channel color image
        cv::cvtColor(mat, mat, CV_BGR2RGB);     // invert BGR to RGB
        return QImage((uchar*)mat.data, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);       // declare and return a QImage
    } else {
        qDebug() << "in convertOpenCVMatToQtQImage, image was not 1 channel or 3 channel, should never get here";
    }
    return QImage();        // return a blank QImage if the above did not work
}

[OliveiraNT]

@Todd-Morehouse I'm trying to do the opposite of this, I already have the QImage and now I need to create an array from this QImage.
something like that:

[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.01176471
  0.07058824  0.07058824  0.07058824  0.49411765  0.53333336  0.68627453
  0.10196079  0.65098041  1.          0.96862745  0.49803922  0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.11764706  0.14117648  0.36862746  0.60392159  0.66666669
  0.99215686  0.99215686  0.99215686  0.99215686  0.99215686  0.88235295
  0.67450982  0.99215686  0.94901961  0.7647059   0.25098041  0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.19215687  0.93333334  0.99215686  0.99215686  0.99215686  0.99215686
  0.99215686  0.99215686  0.99215686  0.99215686  0.98431373  0.36470589
  0.32156864  0.32156864  0.21960784  0.15294118  0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.07058824  0.85882354  0.99215686  0.99215686  0.99215686  0.99215686
  0.99215686  0.7764706   0.71372551  0.96862745  0.94509804  0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.3137255   0.61176473  0.41960785  0.99215686  0.99215686
  0.80392158  0.04313726  0.          0.16862746  0.60392159  0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.05490196  0.00392157  0.60392159  0.99215686
  0.35294119  0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.54509807  0.99215686
  0.74509805  0.00784314  0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.04313726  0.74509805
  0.99215686  0.27450982  0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.13725491
  0.94509804  0.88235295  0.627451    0.42352942  0.00392157  0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.
  0.31764707  0.94117647  0.99215686  0.99215686  0.46666667  0.09803922
  0.          0.          0.          0.          0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.17647059  0.72941178  0.99215686  0.99215686  0.58823532  0.10588235
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.          0.0627451   0.36470589  0.98823529  0.99215686  0.73333335
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.97647059  0.99215686  0.97647059
  0.25098041  0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.18039216  0.50980395  0.71764708  0.99215686  0.99215686  0.81176472
  0.00784314  0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.15294118
  0.58039218  0.89803922  0.99215686  0.99215686  0.99215686  0.98039216
  0.71372551  0.          0.          0.          0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.09411765  0.44705883  0.86666667
  0.99215686  0.99215686  0.99215686  0.99215686  0.78823531  0.30588236
  0.          0.          0.          0.          0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.          0.          0.
  0.          0.09019608  0.25882354  0.83529413  0.99215686  0.99215686
  0.99215686  0.99215686  0.7764706   0.31764707  0.00784314  0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.          0.          0.          0.          0.          0.
  0.07058824  0.67058825  0.85882354  0.99215686  0.99215686  0.99215686
  0.99215686  0.7647059   0.3137255   0.03529412  0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.21568628  0.67450982
  0.88627452  0.99215686  0.99215686  0.99215686  0.99215686  0.95686275
  0.52156866  0.04313726  0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.          0.        ]
[ 0.          0.          0.          0.          0.53333336  0.99215686
  0.99215686  0.99215686  0.83137256  0.52941179  0.51764709  0.0627451   0.
  0.          0.          0.          0.          0.          0.          0.
  0.          0.          0.          0.          0.          0.          0.
  0.        ]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]
[ 0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.
  0.  0.  0.  0.  0.  0.  0.  0.  0.  0.]

That is a numpy array created from MNIST dataset image but using pyton and I need to recreate this in c++.

[kshegunov]

@OliveiraNT
An image is a compound object, what exactly you want to extract from the image, a specific color channel?

PS.
Do you want to get the array(s) in pyton?

[OliveiraNT]

@kshegunov
This array was created using this code in python

(X_train, y_train), (X_test, y_test) = mnist.load_data()
X_train = X_train.reshape(X_train.shape[0], 1, img_rows, img_cols)
X_train = X_train.astype('float32')
X_train /= 255

and I need reproduce this in c++ but now using a QImage as a source.

[kshegunov]

Something along those lines should be just enough (haven't tested it though):

QImage image; //< This has to be 8 bit monochrome image

qint32 width = image.width(), height = image.height();
quintptr scanLineSize = static_cast<quintptr>(width & 0x3 ? width + 1 : width) << 2;

const quint8 * data = reinterpret_cast<const quint8 *>(image.constBits());

QVector<QVector<float>> matrix;

matrix.resize(height);
for (qint32 row = 0; row < height; row++)  {
    matrix[row].resize(width);
    for (qint32 column = 0; column < width; column++)
        matrix[row][column] = data[column] / 255.0f;
    data += scanLineSize;
}

[OliveiraNT]

@kshegunov I'll test this soonest as possible and post the result here.
Thanks.

[OliveiraNT]

@kshegunov
I test your code and its seems just like what I need.
Really thanks for your help.
Now I using a QVectorIterator to output matrix on console with qDebug but what I need is create a texte file with this, I tryed to do this using QFile but it fail because QFile is not overloaded to read a vector . Do you know some way to do this?

[kshegunov]

That's a good place to use QTextStream. It doesn't have an overloaded operator for vectors, but you can always provide one by yourself, e.g:

template <class T>
QTextStream & operator << (QTextStream & out, const QVector<QVector<T>> & data)
{
    qint32 rows = data.size(), cols = rows > 0 ? data.first().size() : -1;
    if (cols <= 0)  {  //< We passed invalid data
        out.setStatus(QTextStream::WriteFailed); //< Indicate an error
        return out;
    }

    // Write the dimensions first (we can do without, but it's better to have them)
    out << rows << ' ' << cols << endl;
    // Write the data row by row, element by element
    for (qint32 i = 0; i < rows; i++)  {
        if (i > 0)
            out << endl;
        for (qint32 j = 0; j < columns; j++)  {
            if (j > 0)
                out << ' ';
            out << data[i][j];
        }
    }

    return out;
}

You can then use this with a QTextStream object:

QVector<QVector<float>> matrix;

QFile file("myfile.txt");
if (!file.open(QFile::WriteOnly | QFile::Text | QFile::Truncate))
    ; //< Errors should be handled appropriately

QTextStream out(&file);
out << matrix;

[OliveiraNT]

@kshegunov Thank you so much for your help I really learned a lot about Qt and now I'm close to finish my project. Again, thank you for spend your time to help me.