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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