plot pictures from char buffer

Julian

Hello All. I hope you all are doing well.

I have a raspberry pi and with openCV and a webcam it captures pictures, processes it and sends it using socket. In the socket server side, there is an application created by Qt. I got the data but Im having a problem to plot the image.
Please, remember that as I using socket in armbian system, it sends the data as a char data type flux.

This is what I tried (first sending a grayscale images)

char buffer [307200]; //its big but the data that server gets takes packets with 1000 bytes size (308 packets to form an image of 640*480
 QImage *img;

    img = new QImage ((uchar *)buffer, 640, 480, QImage::Format_Grayscale8);


    ui->label->setPixmap(QPixmap::fromImage(*img));

But it always shown a black pictures.

mranger90

The first thing to check is that the data really isn't all zeros.
Also, I've had better luck displaying Format_Indexed8 with a grayscale color table for 8 bit data.

VRonin

from http://doc.qt.io/qt-5/qimage.html#QImage-3:

data must be 32-bit aligned, and each scanline of data in the image must also be 32-bit aligned.

You are basically missing an argument to use http://doc.qt.io/qt-5/qimage.html#QImage-5

QImage img(reinterpret_cast<uchar*>(buffer), 640, 480, 640,QImage::Format_Grayscale8);

The below is an example showing white noise:

int main(int argc, char *argv[])
{
    QApplication a(argc,argv);
    uchar buffer [648*480];
    std::default_random_engine generator;
    std::uniform_int_distribution<quint16> distribution(0,0xff);
    for(auto i=std::begin(buffer);i!=std::end(buffer);++i)
        *i=distribution(generator);
    QImage testImg(buffer,640,480,640,QImage::Format_Grayscale8);
    QLabel result;
    result.setPixmap(QPixmap::fromImage(testImg));
    result.show();
    return a.exec();
}

Julian

Thanks a lot! problem solved!