DropShadow is very slow when its radius is getting changed

[Stefan Monov76]

I'll begin with my testcase. It's pretty basic.

• It creates a rectangle rotating in 3D, and a DropShadow element for the rectangle.
• It animates the radius property of the DropShadow.
• It creates a 1x1px Canvas3D repainted constantly, so I can check how often it manages to get repainted with all the other stuff going on (Canvas3D has a built-in fps property). I use that for measuring the app's framerate.

When the value of radius is animated, the app is running at 10FPS. When this value is not animated, the app is running at 60FPS.

The point of the 3D rotation is to have some animation in the rectWrapper element, so the shadow has to get constantly recalculated, even if its radius is not changed. This proves that calculating the shadow is very fast, and what's slowing it down is just the constant changing of the radius.

My code:

main.cpp: (the only unusual thing in it is AA_UseOpenGLES, which is necessary because of my sub par OpenGL drivers)

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication::setAttribute(Qt::AA_UseOpenGLES);
    QGuiApplication app(argc, argv);
    
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    
    return app.exec();
}

main.qml:

import QtQuick 2.5
import QtQuick.Window 2.2
import QtGraphicalEffects 1.0
import QtCanvas3D 1.1

Window {
    visible: true
    width: 640
    height: 480
    
    Text {
        text: canvas3d.fps + " FPS"
        font.pointSize: 18
    }
    // `rectWrapper` is needed as a layer of indirection around
    // `rect` so DropShadow is aware of `rect`'s the rotation
    Item {
        id: rectWrapper
        width: rect.width
        // +100 here to accomodate the extra space needed due to
        // 3D perspective projection of the 3D-rotated inner item
        height: rect.height + 100
        anchors.centerIn: parent
        visible: false
        
        Rectangle {
            id: rect
            color: "red"
            width: 300
            height: 100
            anchors.centerIn: parent
        
            property real yRot: 50
            
            transform: Rotation {
                origin.x: rect.width/2;
                origin.y: rect.height/2;
                axis { x: 0; y: 1; z: 0 }
                angle: rect.yRot
            }
            
            SequentialAnimation on yRot {
                loops: Animation.Infinite
                NumberAnimation {
                    from: -180; to: 180; duration: 5000
                }
            }
        }
    }
    
    DropShadow {
        source: rectWrapper
        anchors.fill: rectWrapper
        samples: 16
        radius: 16

        SequentialAnimation on radius {
            //running: false
            loops: Animation.Infinite
            NumberAnimation {
                from: 2; to: 16; duration: 1000
            }
        }
    }
    
    Canvas3D {
        id: canvas3d
        width: 1; height: 1 // nonzero size so it can be redrawn
        property var gl;

        onInitializeGL: {
            // should get and save context, otherwise FPS isn't measured for some reason
            gl = canvas3d.getContext("canvas3d", {depth:true, antialias:true, alpha:true});
        }
    }
}

Uncomment the running: false to see the huge speedup.

My theory: The blur shader used by DropShadow may be getting recompiled on every change of radius. I've heard of that happening when you upload new values of bool uniforms in GLSL, but in this case it's not a bool, it's a real.

The question: Why does this slowdown occur? Can I work around it?