Qt-interest Archive, June 2007
Speed, transparency, and drop issues with QGraphicsView
Message 1 in thread
I've been running into speed issues with QGraphicsView attempting to
do various operations. I was under the impression that all in-window
drawing should be extremely fast, but that doesn't seem to be the
case. What I want to do seems rather simple, but always ends up slow.
I'm putting the genesis of my code below with appropriate questions
sprinkled throughout.
Original idea:
I have a main window that currently has a QGraphicsView located in the
middle. When a certain event happens, I want to have another
QGraphicsView pop up with icons on it, semi-transparent over the
original (so the original seems to "darken"). These icons animate as
they pop up, by resizing.
So originally, I set the background:
setBackgroundRole( QPalette::Base );
QPalette p = palette();
p.setColor( QPalette::Base, QColor(0, 0, 0, 0) );
setPalette( p );
setAutoFillBackground( true );
And I then had a QTimeLine that would cause the alpha value to change.
So when the timer fired:
QPalette p = palette();
p.setColor( QPalette::Base, QColor(0, 0, 0, value*4) );
setPalette( p );
But this was extremely slow. It also caused a "flash" when the QGV
was shown or hidden (using show() or hide()) where the entire thing
would flash white before drawing. So I tried simply setting it to a
final transparency value from the beginning, without using the
QTimeLine to change it. This was better but still very slow.
Question #1: Was I doing something wrong here? Can I expect to get
decent partial-transparency speed somehow?
So at this point I decided to just use full transparency, which worked
much more quickly. But still the "flash" persisted. Eventually I
found out that I could start the "flash" on QGV's show by removing
setAutoFillBackground( true );
from the initial code segment above. Now, the items just seem to "pop
up" on top of the QGV underneath. But when I hide() the QGV, it still
does this "flash" and I've not found a way to get around it.
Question #2: What causes this "flash" and how can I stop it from
happening on hide() (and on show() in case I want to have it fill the
background)?
One thing I noticed at this point is that the QGraphicsView won't
accept drops, despite
setAcceptDrops( true );
in my constructor. But the QGraphicsItems I add to my scene, which
also have this set in their constructors, receive drops just fine.
Question #3: Why isn't the QGV accepting drops?
So at this point I have a transparent QGV with some items derived from
QGraphicsSvgItem. They all share a QSvgRenderer so that I only have
to read the SVG file once. At the moment, they are all the same item,
so they all render the same graphic. I set up a QTimeLine to have
these items resize so that they appear to grow in from nothing. But
as my QGV gets larger, say, if my main window becomes bigger, this
becomes extremely slow, and I'm not sure why (unless the answer is
"because they're SVGs"). I want to use SVGs because I want the size
of the item to change (and look decent) depending on the size of the
QGV.
Question #4: Why is this resizing so slow, why is it affected by the
QGV size, and can I do anything about it?
I think that's it for now. Thanks in advance.
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