From Matrix to QImage and QPixmap

AlvaroS

Hello everyone.
First of all thanks a lot for reading this post and being able to help.

I would like to get an image from a matrix.

This is my code to convert from QImage to matrix:

            QPixmap img_scene(path_name_image); //Pixmap creation to add the image
            img = img_scene.toImage();

             int **matrix;

            //Create matrix where each element will be a pixel value.
            matrix = new int *[img.width()];
            for(int i=0;i<img.width();++i)
                matrix[i] = new int [img.height()];

            for(int i=0;i<img.width();++i){
                for(int j=0;j<img.height();++j){
                    matrix[i][j]=qGray(img.pixel(i,j)) > 127 ? 0:1; //Less than 127 = 1.
                }
            }

Now i would like to convert "matrix" to a QImage. My code looks like this:

                QImage imagen(1000,938, QImage::Format_Grayscale8);

                for(int i=0;i<1000;++i){
                    for(int j=0;j<938;++j)
                        imagen.setPixel(i,j, matrix_binary[i][j]);
                }

It compile but it crashed in the last "for"..

how can I do that?
thanks!

SGaist

Hi,

Did you check that your matrix is pointing to something valid ?

Out of curiosity, why do you need such a matrix ?

AlvaroS

@SGaist Yes, matrix values is 0 = black or 1 = white.

I need a matrix because user can load a binary file where matrix is inside and then image has to be in a scene.

SGaist

I meant the matrix itself, not what its content is representing.

Did you allocate that matrix properly ?

On a side note, you can also use a QVector<QVector<int>> for a two dimensional storage.

jsulm

@AlvaroS I think @SGaist wanted to know whether matrix_binary is pointing to a matrix and whether it has same dimensions as the picture (1000x938). How did you initialise matrix_binary?

AlvaroS

@SGaist said:

I meant the matrix itself, not what its content is representing.

Did you allocate that matrix properly ?

On a side note, you can also use a QVector<QVector<int>> for a two dimensional storage.

@jsulm said:

@AlvaroS I think @SGaist wanted to know whether matrix_binary is pointing to a matrix and whether it has same dimensions as the picture (1000x938). How did you initialise matrix_binary?

Matriz_binary values are 0 or 1 (white or black), now I am trying to represent that matrix in a scene with QPixmap.

My code looks like this right now:

                QImage imagen(1000,938, QImage::Format_Grayscale8);

                for(int i=0;i<938;++i){
                    for(int j=0;j<1000;++j)
                        if (matrix_binary[i][j] == 1)
                            imagen.setPixel(i,j, qRgb(1,1,1));
                        else
                            imagen.setPixel(i,j, qRgb(0,0,0));
                }

                QPixmap pixmap_scene;
                pixmap_scene.fromImage(imagen);
                QGraphicsPixmapItem *item = scene->addPixmap(pixmap_scene);

but in some part before "for" it crashes....
how can I do what I want? Maybe the format image?

Thanks a lot

Devopia53

@AlvaroS

Your matrix_binary allocated incorrectly. You should exchange width and height.

like this:

matrix = new int *[img.height()];
for(int i=0;i<img.height();++i)
       matrix[i] = new int [img.width()];

You can also use a QVector.

like this:

QVector<QVector<int>> matrix(img.height(), QVector<int>(img.width(), 0)); // 0 is initial value

jsulm

Where exactly does it crash? Did you try to debug to see what is happening?
And still you did not answer the question regarding matrix_binary: how did you initialize it? It is clear that it contains 0/1 but that was not the question.
And I think you should swap i and j:

QImage imagen(1000,938, QImage::Format_Grayscale8);

width=1000, height=938
but then you do:

for(int i=0;i<938;++i){
    for(int j=0;j<1000;++j)
        ...
        imagen.setPixel(i,j, qRgb(1,1,1)); // i is x and j is y

AlvaroS

@Devopia53 said:

@AlvaroS

Your matrix_binary allocated incorrectly. You should exchange width and height.

like this:
matrix = new int *[img.height()];
for(int i=0;i<img.height();++i)
       matrix[i] = new int [img.width()];
You can also use a QVector.

like this:
QVector<QVector<int>> matrix(img.height(), QVector<int>(img.width(), 0)); // 0 is initial value

If I do what you say it does not crash, but know it says:

.
.
.
QImage::setPixel: coordinate (937,991) out of range
QImage::setPixel: coordinate (937,992) out of range
QImage::setPixel: coordinate (937,993) out of range
QImage::setPixel: coordinate (937,994) out of range
QImage::setPixel: coordinate (937,995) out of range
.
.
.

The code now look like this:

 int vector_binary[1000*938];

/*matrix binary creation*/
                int **matrix_binary;
                matrix_binary = new int *[938];
                for(int i=0;i<938;++i)
                    matrix_binary[i] = new int [1000];

/*read vecotr from a binary file*/
                infile.read((char*)(vector_binary), sizeof(int)*1000*938);

/*Initialitation of matrix_binary*/
                for(int i=0;i<938;++i){
                    for(int j=0;j<1000;++j){
                        matrix_binary[i][j] = 0;
                    }
                }

/*Store the data into matrix_binary*/
                for(int i=0;i<938;++i){
                    for(int j=0;j<1000;++j){
                        matrix_binary[i][j] = vector_binary[i*1000+j];
                        }
                }



                //From matrix to QImage to visualize
                //Create matrix where each element will be a pixel value.
                QImage imagen(1000,938, QImage::Format_RGB666);
                for(int w=0;w<938;w++){
                    for(int z=0;z<1000;z++)
                        if (matrix_binary[w][z] == 1)
                            imagen.setPixel(w,z, qRgb(250,250,250));
                        else
                            imagen.setPixel(w,z, qRgb(0,0,0));
                }
                //QPixmap pixmap_scene;
                QPixmap pixmap_scene = QPixmap::fromImage(imagen);
            }
        }

I think that the problem is in setPixel...

AlvaroS

@jsulm said:

Where exactly does it crash? Did you try to debug to see what is happening?
And still you did not answer the question regarding matrix_binary: how did you initialize it? It is clear that it contains 0/1 but that was not the question.
And I think you should swap i and j:
QImage imagen(1000,938, QImage::Format_Grayscale8);
width=1000, height=938
but then you do:
for(int i=0;i<938;++i){
    for(int j=0;j<1000;++j)
        ...
        imagen.setPixel(i,j, qRgb(1,1,1)); // i is x and j is y

Yes sorry! See my last post :)

AlvaroS

However if i save pixemap_scene in a .jpeg it save me fine with the correct image...

jsulm

@AlvaroS

QImage::setPixel: coordinate (937,991) out of range
QImage::setPixel: coordinate (937,992) out of range
QImage::setPixel: coordinate (937,993) out of range
QImage::setPixel: coordinate (937,994) out of range
QImage::setPixel: coordinate (937,995) out of range

Now take a look how you initialized QImage:

QImage imagen(1000,938, QImage::Format_Grayscale8);

So, max value for y is 937 but you pass values like 991!

? Offline

Hi! The following is still unnecessarily slow but at least it works and doesn't waste so much memory as before:

    const QPixmap pixmap("/home/user/myimage.jpg");
    ui->label->setPixmap(pixmap);

    const QImage image = pixmap.toImage();
    const int width = image.width();
    const int height = image.height();
    QBitArray bitArray(width*height);

    for (int h=0; h<height; ++h) {
        for (int w=0; w<width; ++w) {
            bitArray[h*width+w] = qGray(image.pixel(w,h)) > 127 ? 0 : 1;
        }
    }

    /* Do something with the bit array here */

    QImage monoImage(width, height, QImage::Format_Mono);
    for (int h=0; h<height; ++h) {
        for (int w=0; w<width; ++w) {
            monoImage.setPixel(w, h, bitArray.at(h*width+w));
        }
    }

    ui->label_2->setPixmap( QPixmap::fromImage(monoImage) );

wangg

@AlvaroS you set the parameters incorrectly.The setPixel of the first is the image's cloumn and the second is row.so you should write like this :
for(int i=0;i<image.height();i++)
for(int j=0;j<image.width();j++)
{
imageset.setPixel(i,j,matrix_binary[i][j] );
}