Create QImage from an unsigned char array

[Vick]

Hi,

I would like to print a picture from an unsigned char array. I tried to use QImage but I don't think I use it properly, the GUI always shows me a grey picture.

Here is my sample code :

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QLabel l;
    QImage i;

    unsigned char* data = (unsigned char*) malloc (1024*768*sizeof(unsigned char));

    for (int i = 0; i < 1024; i++)
    {
        for (int j = 0; j < 768; j++)
        {
            data[768 * i + j] = 255;
        }
    }

    free(data);

    i = QImage(data, 768, 1024, QImage::Format_Indexed8);
    l.setPixmap(QPixmap::fromImage(i));
    l.show();

    return a.exec();
}

How would you do to get a correct picture ? Thanks for your help !

[raven-worx]

@Vick said:

I tried to use QImage but I don't think I use it properly, the GUI always shows me a grey picture.

You just fill the data with the value 255?! If i would have to guess it seems like it's grey, because you monitor needs some calibration maybe. I would expect it to be white.

Also you mixed the height and width in this line.

i = QImage(data, 768, 1024, QImage::Format_Indexed8);

But it's not noticeable yet, since the data is all filled with the same value.

[mrjj]

hi
You might need a ColorTable?

void MainWindow::paintEvent(QPaintEvent* e) {
  uchar imagio [500][500];
  for (int i = 0; i < 500; i++)
    for (int j = 0; j < 500; j++)
      imagio[i][j] = qrand() % 256;
  QVector<QRgb> colorTable;
  for (int i = 0; i < 256; i++)
    colorTable.push_back(QColor(qrand() % 256, qrand() % 256, qrand() % 256).rgb());
  QImage image((uchar*)imagio, 500, 500, 500, QImage::Format_Indexed8);
  image.setColorTable(colorTable);
  QPainter p(this);
  p.drawImage(QPoint(0, 0), image);
}

[Vick]

Yes, but I tried other values like 0, i and it does not change anything... Always getting this grey picture !

[mrjj]

@Vick
the sample gives this:

[Photatum]

If you just want grayscale you can also use Format_Grayscale8 instead of Format_Indexed8.
...
Just to expand my comment, like the other two said you need a color table for indexed format. Format_Grayscale8 would take care of that in this case and display the respective gray values.

[raven-worx]

@Vick said:

Yes, but I tried other values like 0, i and it does not change anything... Always getting this grey picture !

because @mrjj is correct.
The image format QImage::Format_Indexed8 needs a color map.
The way you want to use it is specifying the color for each pixel, so you need to use for example QImage::Format_ARGB32_Premultiplied.

[jsulm]

@Vick You free the memory with your image data before you pass it to QImage. Why?

unsigned char* data = (unsigned char*) malloc (1024*768*sizeof(unsigned char));

    for (int i = 0; i < 1024; i++)
    {
        for (int j = 0; j < 768; j++)
        {
            data[768 * i + j] = 255;
        }
    }

    // Here you free the memory
    free(data);

    i = QImage(data, 768, 1024, QImage::Format_Indexed8);

In C++ you should not use malloc/free. Use new/delete[] instead. And in this case there is no need to allocate the memory on the heap.

[Vick]

Hi all and many thanks for your replies. I think you pointed me on the fact that I should not use Format_Indexed8 to print only grayscale !

Thanks for the idea of ColorTable, I will use it when I need to display colors.

[Vick]

@jsulm

Thanks, it was a mistake to free the memory at this point ! Sorry for opening a thread for so little interest...