How to display image with QPixmap from QVector? - c++

Question

Could someone help me with displaying video from a camera. The values from camera are stored in QVector. Is it possible to display it this way?

Answer 1

Here is the code:
void MainWindow::slotDataUpdate(QVector<uint16_t> vec)
{
ui->setupUi(this);
double min = *std::min_element(vec.begin(), vec.end());
double max = *std::max_element(vec.begin(), vec.end());
std::transform(vec.begin(), vec.end(), vec.begin(), std::bind2nd(std::plus<double>(), (-1)*min));
int co = 65536/(max - min);
std::transform(vec.begin(), vec.end(), vec.begin(), std::bind2nd(std::multiplies<double>(), co));
const QImage tmpImage((const uchar*)vec.data(), 160, 120, QImage::Format_RGB16);
QByteArray data;
QDataStream stream(&data, QIODevice::WriteOnly);
stream << vec; //conversion from Q vector to QByteArray
// QByteArray data = QByteArray::fromRawData(reinterpret_cast<const char*>(vec.constData()),sizeof(double)* vec.size()); //conversion from Q vector to QByteArray
for (int i = 0; i < data.size(); i++)
{
std::cout << data[i] << " " << std::endl;
}
QImage tmpImage(160,120, QImage::Format_RGB32); //prazna slika pravih dimenzij
QRgb *pixels=reinterpret_cast<QRgb*>(tmpImage.bits());
for (size_t i=0;2*i<data.size();++i)
{
uchar pixel_msb=data[2*i+];
pixels[i]=qRgb(pixel_msb, pixel_msb, pixel_msb);
}
QPixmap pixmap(QPixmap::fromImage(tmpImage));
ui->label_img->setPixmap(pixmap.scaled(480,360,Qt::KeepAspectRatio));
}

Answer 2

RGB source image
Direct conversion is not possible, unless the QVector is a memory-mapped file. If it contains the raw bytes instead (as it seems to), I suggest constructing a QImage from the raw data as an intermediate object and then converting it to QPixmap:
// assuming 'v' is the vector, and 'w' and 'h' the size of the image
const QImage tmpImage((const char*)v.data(), w, h, QImage::Format_RGB16);
QPixmap pixmap(QPixmap::convertFromQImage(tmpImage));
Given the original element size is 16 bits, you may need QImage::Format_RGB16 (for 5-6-5) or QImage::Format_RGB555 (for 5-5-5). Check available formats for more options. Assuming, of course, an RGB image.
Grayscale images
If you image only has one color channel, then you'll have to make a manual conversion, since Qt is not able to manipulate 16-bits grayscale images.
Easy code, but may not produce the results you want:
const QImage tmpImage((const char*)v.data(), w, h, QImage::Format_RGB16);
QPixmap pixmap(QPixmap::convertFromQImage(
image.convertToFormat(QImage::Format::Format_Grayscale8)));
Otherwise, you have to reduce the depth of your image. Check this answer for a more detailed implementation. Nevertheless, this is a slow operation. If your Qt version allows it, use QImage::Format_Grayscale8 for destination image instead of the QImage::Format_RGB32 proposed in the linked answer, you'd write 3x less elements.
// 'image' is a QImage(w, h, QImage::Format_Grayscale)
// if possible, re-use it on each frame to reduce construction overhead
// 'ptr' is the pointer to your 16-bits data (const short* ptr)
auto ptr = v.constData();
#pragma omp parallel for // if can use OpenMP you can gain some speed here
for (int ii = 0; ii < image.height(); ++ii) {
auto scanLine = image.scanLine(ii);
for (int jj = 0; jj < image.width(); ++jj, ++ptr, ++scanLine) {
*scanLine = (unsigned char)(*ptr >> 8); // high byte, most significative bits
}
}