Sorry for the formatting. Rich formatting ruined it. Here we go again :
Hello,
We have a Quick scene where we draw a lot of semi-transparent rectangles and
those rectangles are rounded in one side. As a representative :
Rectangle {
id: clipper
width: 100
height: 100
opacity: 0.5
clip: true
Rectangle {
id: clipped
radius: 20.0
width: parent.width + radius
height: parent.height
color: 'red'
}
}
As it can be seen from the snippet above we use clipping to achieve rounding in
one side however that comes with a significant cost in batching as
the number of those rectangles are quite high. I've looked at what we can do to
get rid of clipping while preserving the existing and UI what I've found is as
follows:
Using Canvas API in QML
This will probably be slower than QQuickRectangle with clipping.
Using QQuickPaintedItem with QPainter API
This will be faster than canvas API but still slower than QQuickRectangle with
clipping.
Custom QQuickItem
This seems like the only way we can outperform QQuickRectangle with clipping
however the amount of implementation needed for a simple rounded rectangle
makes me think twice
about this approach. TBH I'm also a bit scared about some potential issues like
aliasing.
Using OpacityMask from QtGraphicalEffects
I am not sure about this approach. Can you shed some light on how this works
behind the scenes in scene graph renderer if I have, let's say, a hundred
instances of the following :
Rectangle {
// some properties
OpacityMask {
// some properties
}
}
As far as I understand each shader is a unique state in graphics API which
results in a seperate draw call but is it also the case if we use the same
shader for repeated items like above ?
I mean this should be fine if the shader is set for once because I assume items
can be batched afterwards. But if each item requires a different batch then
this has no gain over clipping.
Am I correct about the assumptions I make above regarding the performance
characteristics ? What is the best way to deal with this ?
Thank you.
Murat Seker
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