Using QScript to manipulate and pass large arrays

rkintada

Hi,

I am using javascript to manipulate image data and pass it back to C++ code. The performance is very slow. This is required so that the end-user can write java-script to create images based on their needs. The example code snippet is attached below. The processing time for each call is around 50 ms( on windows 7 machine ). Also the code is not returning the image array to the C++. when I did the measurement of getting the "imgData" array to the c++ code(using QScriptValue, not shown in the code below), the time is around 130ms. Just wanted to know if this is the expected performance or is there something that i may be missing.

@
#include <QApplication>
#include <QScriptEngine>
#include <QScriptProgram>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QTextStream out( stdout );
QByteArray program;
QBuffer pBuffer( &program );
pBuffer.open( QBuffer::WriteOnly );
QTextStream stream( &pBuffer );

stream << "var count = 0;\n";
stream << "var imgData = new Array(240*240*3);\n";
stream << "function GetImage()\n";
stream << "{\n";
stream << "   for( var x = 0; x < 240;++x )\n";
stream << "   {\n";
stream << "      for( var y = 0; y < 240; ++y)\n";
stream << "      {\n";
stream << "          imgData[x*240+y*3 +0] = 255;              // r\n";
stream << "          imgData[x*240+y*3 +1] = ((10*count)%5); // b\n";
stream << "          imgData[x*240+y*3 +2] = 255;              // b\n";
stream << "      }\n";
stream << "   }\n";
stream << "   count++;\n";
stream << "   return count;\n";
stream << "}";

pBuffer.close();

out << "Code: " << program << endl;
out.flush();

QString pString( program );
QScriptProgram scriptProgram( pString );
QScriptEngine engine;
engine.evaluate( scriptProgram );
if( engine.hasUncaughtException() )
{
   qDebug() << "Error: loading script program." << engine.uncaughtException().toString();
   qDebug() << engine.uncaughtExceptionBacktrace();
   return -1;
}

//success fully loaded the program. now run the code.
int loopCount = 10;
Stopwatch sw; // this is our custom class. remove this if you are compiling the code.
for( int i = 0; i < loopCount; ++i )
{
   sw.Start();
   engine.evaluate( "GetImage()" );
   sw.Stop();
   if( engine.hasUncaughtException() )
   {
      qDebug() << "Error: " << engine.uncaughtException().toString();
      qDebug() << "Error: " << engine.uncaughtExceptionBacktrace();
   }
}
qDebug() << "Avg. ProcessingTime: " << (int)((double)(sw.GetMicroseconds()/(double)(loopCount)));

return a.exec&#40;&#41;;

}
@

Thanks in advance.

frankcyblogic.de

Hey, cool application!

I think performance of QtScript evalution is quite bad. You can try to improve it by using "QScriptProgram":http://doc.qt.nokia.com/4.7/qscriptprogram.html, which provides a sort of precompiled representation.

rkintada

Hi Thanks for the reply. I am using QScriptProgram to load the program(line 37 above). The example on the doc page is a simple one. I am not sure how to do the same when calling a function in a ScriptProgram. Also is there a way to see whether the program was pre-compiled using JIT?

Thanks

frankcyblogic.de

Jitting is enabled by default. Would like to know how it impacts performance in your example! Never tried QScriptProgram myself;)