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Thread: Plotting of QProcess output taking about 5 minutes to plot single value.

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  1. #1
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    Default Plotting of QProcess output taking about 5 minutes to plot single value.

    Hello all ,
    I am new to Qt , i am designing a system which will receive values from sensors and Plot in GUI. I have complete C code for serial port setting and data Processing . By using Q Process i am running this C program and Plotting the Processed values .
    My GUI contain a text Edit for displaying Proceeded output readings and widget for plotting processed values with respect to time . Output values and Plot are as expected . Device sending with 4800 Baud rate and with odd parity , which is set in C program .But its taking more than 5 Minutes to read and plot one value and taking more than 30 minutes to read and plot about 6 values . why it taking this much time to read ?

    My Code is attached ...

    Qt Code:
    1. ui->setupUi(this);
    2. QVector<double> data_x(101), data_y(101);
    3. timer.start();
    4. init_port();
    5. init_line_plot();
    6. x_position = 0;
    7. }
    8.  
    9. graphwidget::~graphwidget()
    10. {
    11. delete ui;
    12. }
    13. void graphwidget::init_port()
    14. {
    15. process = new QProcess(this);
    16. connect(process, SIGNAL(readyReadStandardOutput()), this, SLOT(receive()));
    17. process->start("./test2");
    18.  
    19. process->waitForStarted();
    20. qDebug()<<"process error code:" <<process->error();
    21.  
    22.  
    23. }
    24. void graphwidget::init_line_plot()
    25. {
    26.  
    27. ui->customPlot->addGraph();
    28. // ui->customPlot->setMinimumSize(500,500);
    29. ui->customPlot->xAxis->setLabel("t");
    30. ui->customPlot->yAxis->setLabel("V");
    31. ui->customPlot->axisRect()->setAutoMargins(QCP::msNone);
    32. ui->customPlot->axisRect()->setMargins(QMargins(0,0,0,0));
    33. }
    34.  
    35.  
    36. void graphwidget::receive()
    37. {
    38. // recieves data as ASCII string
    39. int datalength = 1000;
    40. char data [1000];
    41. int bytesRead =process->readLine(data, datalength);
    42. data[bytesRead]='\0';
    43.  
    44. ui->textEdit->append(QString(data));
    45.  
    46. QTextStream out(stdout);
    47. out << data << endl;
    48. addDataPoint(atof(data));
    49. }
    50.  
    51. void graphwidget::addDataPoint(double datapoint)
    52. {
    53. if (x_position>60)data_x.pop_front();
    54. double ms = timer.elapsed();
    55. data_x.push_back((double)ms/1000);
    56. x_position++;
    57. if (x_position>60) data_y.pop_front();
    58. data_y.push_back(datapoint);
    59.  
    60. ui->customPlot->graph(0)->setData(data_x,data_y);
    61. ui->customPlot->xAxis->grid()->setSubGridVisible(false);
    62. ui->customPlot->yAxis->grid()->setSubGridVisible(false);
    63. ui->customPlot->xAxis->grid()->setVisible(false);
    64. ui->customPlot->yAxis->grid()->setVisible(false);
    65. ui->customPlot->graph(0)->setPen(QPen(QColor(0,200,0)));
    66. ui->customPlot->setBackground(Qt::black);
    67.  
    68. ui->customPlot->graph(0)->rescaleAxes();
    69. ui->customPlot->replot();
    70. }
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    please suggest necessary changes ........
    Last edited by anda_skoa; 20th February 2017 at 11:46. Reason: missing [code] tags

  2. #2
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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    data_x and data_y in line 2 are local stack variables (I assume this is your class constructor) and have no relationship to the variables of the same name you use later.

    In line 56, you increment x_position. I don't see anywhere where you reset its value, so once it reaches 60, it will continue to increase.

    Otherwise, there doesn't appear to be anything in the code you have posted which would cause such delays. I would write a test program that strips out everything except the communication between your sensors and your monitoring program and see if that is where the trouble starts. No GUI, no writing to files, no plotting, just qDebug() statements to write what you receive to the console. And run it in the debugger, of course.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

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    suhairkp (21st February 2017)

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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    k. But i tested same qt program with simple C program . it will just print 1,2,3,4,5,4,3,2,1... and no serial port setting in C program , it worked fine and giving continues output without delay .

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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    Quote Originally Posted by suhairkp View Post
    k. But i tested same qt program with simple C program . it will just print 1,2,3,4,5,4,3,2,1... and no serial port setting in C program , it worked fine and giving continues output without delay .
    So the problem is with the serial port. Why You don't use QSerialPort ?

  6. #5
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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    Serial port is set in C program , so calling QProcess will set serial port . thats y not using Qserialport in Qt .Also , C program process the received signal from serial port and print output . this output is storing in byte array and using for plot .
    I compiled C program from terminal and getting correct output in terminal . but Qt taking more time call QProcess and plot , don't getting idea where this delay occurring ...

  7. #6
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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    I still think you need to break the problem down into the smallest piece possible, than add more features. I would start by writing a Qt program that simply starts the listener process, reads the output from your sensor and prints it using qDebug(). No printing it to a text edit, no logging to files, no plotting. At this point, you have no idea which step in this chain is going wrong, so eliminate everything except the first one: getting data successfully from the QProcess into your program. Once that seems to be working in the same way as your C program, then you know the communication works, so add the graphics and other features.

    You might also try using QProcess:readAllStandardOutput() to get all of the available data at once instead of using readLine(). Use QByteArray::data() or QByteArray::constData() to get the NULL-terminated C string it contains. This way you do not have to worry about the buffer size or whether you have read it all.
    <=== The Great Pumpkin says ===>
    Please use CODE tags when posting source code so it is more readable. Click "Go Advanced" and then the "#" icon to insert the tags. Paste your code between them.

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    suhairkp (23rd February 2017)

  9. #7
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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    k . sir. I am testing it without plot . in my above code , it displaying "process error code : 2 " in Application output area of Qt Creator , when output gui display open and saying " Qprocess : Destroyed while process is still running ". when i changed process->waitForStarted from process->waitForFinished , it show that "process error code : 5 "
    Last edited by suhairkp; 23rd February 2017 at 11:10.

  10. #8
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    Default Re: Plotting of QProcess output taking about 5 minutes to plot single value.

    Once again : why You use external program to read data from serial port ? Why You don't do it in Qt program ?

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