Get raw data from QImage

Question

I have a QImage that I built from a pixmap something like the following:

QPixmap fullPmap = topItem->pixmap();
fullPmap = fullPmap.copy(isec.toRect());
QImage chip = fullPmap.toImage();

This is basically intersecting with a rectangle on screen to crop the image to a chipped size.

I now need to get the character array representing that data back from chip.

How can I do this?

I tried something like this:

  unsigned char * data = chip.bits();

And when I display "data" I get a completely distorted image, nothing like my actual chip.

fullPmap is an RGB image if that matters. I have some code that I am using to convert it to grayscale:

QRgb col;
int gray;
for (int i = 0; i < chip.width(); ++i)
{
    for (int j = 0; j < chip.height(); ++j)
    {
        col = chip.pixel(i, j);
        gray = qGray(col);
        chip.setPixel(i, j, qRgb(gray, gray, gray));
    }
}

Which I don't really like, but it seemed like the easiest way to do such a thing.

Displaying the data that is returned from bits() looks like this:

imwidth = chip.width();
imheight = chip.height();
QImage *qi = new QImage(imwidth, imheight, QImage::Format_RGB32);
//  #pragma omp parallel for
for (int i = 0 ; i < imheight ; i++)
    for (int j = 0 ; j < imwidth ; j++)
    {
        qi->setPixel(j,i,qRgb(data[i*imwidth + j],data[i*imwidth + j],data[i*imwidth + j]));
    }

Answer 1

When dealing with raw image data, you need to be aware of some basic issues:

• What is the format or layout of pixel data. The QImage converted from QPixmap is not necessarily always in RGB888 or RGB32. Call QImage::format() and deal with different layout differently. Especially with RGB32 family, you need to make sure you access them in right byte order (endian).

• The bytes are not always layout exactly that many bytes to hold the pixels of the line. Typically a line will have more bytes than it's needed. Use QImage::bytesPerLine() to find out and advance the pointer by that many bytes when going to the next line. This is probably your issue in the building of the display image.

• QImage::pixel() and setPixel() are really slow. Get/set pixel values from raw pixel buffer if possible.