How to solve the problem that multithreaded drawing is not smooth? - c++

Question

I wrote a data acquisition program with Qt. I collect data using the child threads of the dual cache region written by QSemphore.
void QThreadShow::run() {
m_stop=false; // when start thread,m_stop=false
int n=fullBufs.available();
if (n>0)
fullBufs.acquire(n);
while (!m_stop) {
fullBufs.acquire(); // wait fo full buffer
QVector<double> dataPackage(BufferSize);
double seq=bufNo;
if (curBuf==1)
for (int i=0;i<BufferSize;i++){
dataPackage[i]=buffer2[i]; // copy data from full buffer
}
else
for (int i=0;i<BufferSize;i++){
dataPackage[i]=buffer1[i];
}
for (int k=0;k<BufferSize;k++) {
vectorQpointFbufferData[k]=QPointF(x,dataPackage[k]);
}
emptyBufs.release(); // release a buffer
QVariant variantBufferData;
variantBufferData.setValue(vectorQpointFbufferData);
emit newValue(variantBufferData,seq); // send data to main thread
}
quit();
}
When a cache of sub-threads has collected 500 data, the data is input into a QVector and sent to the main thread and is directly assigned to a lineseries in qchartview every 20ms for drawing. I use QtChart to chart the data.
void MainWindow::onthreadB_newValue(QVariant bufferData, double bufNo) {
// Analysis of QVariant data
CH1.hardSoftDataPointPackage = bufferData.value<QVector<QPointF>>();
if (ui->CH1_Source->currentIndex()==0) {
for (int p = 0;p<CH1.hardSoftDataPointPackage.size();p++) {
series_CH3->append(CH1.hardSoftDataPointPackage[p]);
}
}
}
There is a timer in the main thread.The interval is 20ms and there is a double time (time = time +1), which controls the X-axis.
void MainWindow::drawAxis(double time) {
// dynamic draw x axis
if (time<100) {
axisX->setRange(0, TimeBase/(1000/FrameRate) * 10);
// FrameRate=50
} else {
axisX->setRange(time-TimeBase/(1000/FrameRate) * 10, time);
}
}
But when I run my program, there is a problem that every time the subthread sends data to the main thread, the main thread gets stuck for a few seconds and the plot also gets stuck for a few seconds. I added a curve in the main thread getting data from the main thread, and found that both two curves will be stuck at the same time. I don't know how to solve this problem.
Besides, I want the main thread to draw the data from the child thread evenly within 20ms, instead of drawing all the points at once.

Answer 1

Your main thread stucks because you copy (add to series) a lot of data at one time. Instead this you can collect all your data inside your thread instance without emitting a signal. And from main thread just take little pieces of collected data every 20 ms.
Something like this:
while(!m_stop)
{
...
//QVariant variantBufferData;
//variantBufferData.setValue(vectorQpointFbufferData);
//emit newValue(variantBufferData,seq);//send data to main thread
//instead this just store in internal buffer
m_mutex.lock();
m_internalBuffer.append(vectorQpointFbufferData);
m_mutex.unlock();
}
Read method
QVector<QPointF> QThreadShow::takeDataPiece()
{
int n = 4;
QVector<QPointF> piece;
piece.reserve(n);
m_mutex.lock();
for (int i = 0; i < n; i++)
{
QPointF point = m_internalBuffer.takeFirst();
piece.append(point);
}
m_mutex.unlock();
return piece;
}
And in Main thread read in timeout slot
void MainWindow::OnDrawTimer()
{
QVector<QPointF> piece = m_childThread.takeDataPiece();
//add to series
...
//drawAxis
...
}