[Development] Charts and DataVis Questions

[Sean Harmer]

On 23/01/2016 12:45, Uwe Rathmann wrote:
> Hi,
>
>> The OpenGL acceleration in Charts module is really impressive ...
> Unfortunately part of the truth is, that the performance of the software
> renderer does not necessarily be that far behind.

Now try it against OpenGL with 100k points rendering to a 4k screen. The 
difference between software and hardware will increase with those 
parameters (up to some fill rate or vertex rate that the hardware can 
handle).

Cheers,

Sean

>
> An example: in a test program I'm creating a polygon of 10000 points in
> an area of 1000x1000 using (qAbs(random) % 1000) and then I'm drawing it
> this way:
>
> void draw( const QPolygonF &points, QPaintDevice &paintDevice )
> {
>      QPainter painter( &paintDevice );
>      painter.setPen( QPen( Qt::black, 2 ) );
>      painter.drawPolyline( points );
>      painter.end();
> }
>
>
> As I want to compare hardware vs. software renderer I'm using Qt 4.8 with
> X11 ( "native" ) as backend. Here drawing to a QImage uses the software
> renderer, while drawing to a pixmap is usually hardware accelerated.
>
> To make it comparable I'm converting the pixmap to an image, so that I
> have the same input and the same output.  ( in case you are interested I
> added the code at the end of this posting. )
>
> O.k. this is what I see with Qt 4.8:
>
> - Raster 735ms
> - X11 120ms
>
> When doing the same with a Qt-5.6 beta:
>
> - Raster 775ms
> - X11 775ms ( no surprise )
>
>
> Next I modified the draw method a bit:
>
> void draw( const QPolygonF &points, QPaintDevice &paintDevice )
> {
>      QPainter painter( &paintDevice );
>      painter.setPen( QPen( Qt::black, 2 ) );
>
>      const int numPoints = 100;
>      const int numChunks = points.size() / numPoints;
>      for ( int i = 0; i < numChunks; i++ )
>      {
>          painter.drawPolyline( points.constData() + i * numPoints,
> numPoints );
>      }
>      painter.end();
> }
>
> ( of course this is not exactly the same as the pixels to join the chunks
> at there ends are missing ).
>
> Now the result is:
>
> Raster ( Qt 4.8 ): 382ms
> Raster ( Qt 5.6 ): 403ms
>
> When using a chunk size ( = numPoints ) of 10 I have:
>
> Raster ( Qt 4.8 ): 192
> Raster ( Qt 5.6 ): 181
> X11    ( Qt 4.8 ):  93
>
> In the end the implementation of a chart package is full of finding and
> implementing workarounds and optimizations like this one - at least this
> is my experience with being the maintainer of a chart package ( Qwt )
> over the years.
>
> Uwe
>
> --
>
>
> QImage renderRaster( const QPolygonF &points )
> {
>      QImage img( 1000, 1000, QImage::Format_RGB32 );
>      img.fill( Qt::white );
>
>      QElapsedTimer timer;
>      timer.start();
>
>      draw( points, img );
>
>      qDebug() << "Raster" << timer.elapsed();
>
>      return img;
> }
>
> QImage renderX11( const QPolygonF &points )
> {
>      QPixmap pm( 1000, 1000 );
>      pm.fill( Qt::white );
>
>      QElapsedTimer timer;
>      timer.start();
>
>      draw( points, pm );
>
>      const QImage img = pm.toImage();
>      qDebug() << "X11" << timer.elapsed();
>
>      return img;
> }
>
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