Choppy scrolling of QPixmap using Qt Animation Fra

问题:

I created QPropertyAnimation and connected it to my SonogramWidget that scroll a long picture vertically on animation events. The 'long picture' is composed of 100 pre-calculated QPixmap objects 1024x128 placed one after another vertically. They displayed in SonogramWidget::paintEvent() with QPainter. Drawing procedure paint not all QPixmap at once, but only visible of them, considering widget height and current vertical offset. CPU is almost free, because QPixmap is a fastest way to display a picture. There is no big calculations during scrolling, because all the 100 QPixmaps are pre-calculated and stored in memory.

I see strange effect: pulsating movement: 2 times a second the entire image slightly speed-up and moves up by 1..2 pixels faster than usual motion. The same effect when i replace Qt Animation Framework with single 60 fps QTimer and scroll the image in its SLOT.

Video: http://www.youtube.com/watch?v=KRk_LNd7EBg#t=8 (watch from 00:08; My firefox adds more chopping to video playing itself, google chrome plays the video much better).

I see the same effect for my Linux and Windows build.

SOLUTION i figured out the issue: the "chopping" was not a bug, it was a feature! It is a feature of integer-number calculations, so sometimes we had to have different numbers for animations, like: 16,16,16,16,16,16,17,16,16,16,16,16,17,....

回答1:

In the paintEvent add the following assert:

Q_ASSERT(m_animation->currentValue() == m_animatedPropertyValue);

If it triggers, then you know you must use currentValue() instead of the property value. This might be the case. Let me know.