QT Rotating object problems - c++

Question

I am currently working on a project that has a spinning object. The object's rotation speed is changed and its direction of rotation is changed (forwards/backward). After doing some research I came across RotationAnimator on the QT documentation, it seems to have everything I need and I can easily reverse direction. The only problem is that is in QML. My program is entirely in Qt's C++.
I tried an attempt at using QVariantAnimation (documentation here)but every way I have tried there is always some problem. Changing speed is relatively easy, just fooAnimation->setLoop(-1) (loop indefinitely) and change the duration of the animation. The problem I run into is looping forwards AND backward. Or in other terms, changing direction. I have attempted to change the direction of the animation using fooAnimation->setDirection(<Direction goes here>), but if I run the animation in reverse long enough it will stop the animation entirely. It will only loop forwards. That's the key part of the problem.
So I tried alternative ways to change the direction. One idea was changing the endValue from 360 degrees to -360 using fooAnimation->setEndValue(<value goes here>) when I want to reverse, but this has an adverse effect. If I were to swap the direction, the current rotation: let's say it's 110, will be inverted. So now it suddenly jumps to -110 and starts from there. This leads to very jittery rotation as every time a direction swap happens it teleports the rotation of the object.
This seems like a pretty simple thing to implement, rotating an object that can have its rotation speed and direction changed, but I just can't wrap my head around how I could implement it with animations.
A side note here is that all this is being done in Qt 3D so I can grab rotation and other properties from the rotating object (or in this case its QTransform)

Answer 1

There are multiple ways to achieve this effect. On this Qt/QML demo, I used an approach based on NumberAnimation to rotate the cube.
These are the key ideas behind it:
The cube has a Transform component that defines the 4x4 matrix that Qt3D uses to place the cube in the world using predefined rotation and scale values. The NumberAnimation simply changes the rotation angle used on this transform to make the cube rotate on the screen;
The rotation speed is defined by the duration property of that NumberAnimation: a smaller value makes the cube rotate faster. The cube takes 5 seconds to rotate from 0 to 360 degrees;
Changing the rotation speed means recalculating the duration of the animation based on how far away from 0 degrees the current rotation angle is. The animation needs to be restarted whenever we change ANY NumberAnimation properties;
When the direction of the rotation is changed, we store the current rotation angle in cubeTransform.startAngle and update the to and from properties of NumberAnimation along with duration so the cube can be rotated in a new direction starting from the current angle.
I added a few buttons to the UI to change the direction and speed in runtime:
main.qml:
import QtQuick 2.12 as QQ2
import QtQuick.Controls 2.12
import QtQuick.Scene3D 2.12
import Qt3D.Core 2.0
import Qt3D.Render 2.0
import Qt3D.Input 2.0
import Qt3D.Extras 2.0
import Qt3D.Logic 2.0
QQ2.Item {
id: windowId
width: 800
height: 800
property real cubeScale: 100.0
property int rotationSpeed: 5000
property real speedFactor: 1.0
property bool clockwiseRotation: true
Scene3D {
id: scene3D
anchors.fill: parent
aspects: ["input", "logic"]
focus: true
cameraAspectRatioMode: Scene3D.AutomaticAspectRatio
Entity {
id: rootEntity
Camera {
id: camera
projectionType: CameraLens.PerspectiveProjection
fieldOfView: 45
nearPlane : 0.1
farPlane : 100000.0
position: Qt.vector3d(0.0, 0.0, 500.0)
viewCenter: Qt.vector3d(0.0, 0.0, 0.0)
upVector: Qt.vector3d( 0.0, 1.0, 0.0 )
}
components: [
RenderSettings {
activeFrameGraph: ForwardRenderer {
camera: camera
clearColor: "silver"
}
},
InputSettings { }
]
Entity {
id: lightEntity
enabled: true
DirectionalLight {
id: infiniteLight
color: "white"
intensity: 1.0
worldDirection: Qt.vector3d(0, 0, 0)
}
Transform {
id: infiniteLightTransform
translation: Qt.vector3d(0.0, 0.0, 500.0)
}
components: [ infiniteLight, infiniteLightTransform ]
}
Entity {
id: cubeEntity
CuboidMesh {
id: cubeMesh
}
PhongMaterial {
id: cubeMaterial
ambient: "red"
diffuse: Qt.rgba(1.0, 1.0, 1.0, 1.0)
}
Transform {
id: cubeTransform
property real angle: 0.0
property real startAngle: 0.0
matrix: {
var m = Qt.matrix4x4();
m.rotate(cubeTransform.angle, Qt.vector3d(0, 1, 0));
m.translate(Qt.vector3d(0, 0, 0));
m.scale(windowId.cubeScale, windowId.cubeScale, windowId.cubeScale);
return m;
}
}
components: [ cubeMesh, cubeMaterial, cubeTransform ]
}
QQ2.NumberAnimation {
id: cubeAnimation
property int startPos: {
if (cubeTransform.startAngle === 0)
{
if (windowId.clockwiseRotation === true)
return 0;
return 360;
}
return cubeTransform.startAngle;
}
property int endPos: (windowId.clockwiseRotation === true) ? 360 : 0
target: cubeTransform
property: "angle"
duration: windowId.rotationSpeed / windowId.speedFactor
from: cubeAnimation.startPos
to: cubeAnimation.endPos
loops: 1
running: true
onStarted: {
console.log("onStarted");
}
onFinished: {
console.log("onFinished");
// reset the starting angle
cubeTransform.startAngle = 0;
// reset the duration of the animation
cubeAnimation.duration = windowId.rotationSpeed / windowId.speedFactor;
// the animation is currently stopped, run it again
cubeAnimation.running = true;
}
}
}
}
QQ2.Row {
anchors.fill: parent
Button {
text: "Change Direction"
onClicked: {
// pause the animation
cubeAnimation.pause();
// invert rotation
windowId.clockwiseRotation = !windowId.clockwiseRotation;
// store current angle as the starting angle for the rotation
cubeTransform.startAngle = cubeTransform.angle;
// update the remaining time to avoid changing the rotation speed
let progressPercentage = cubeTransform.startAngle / 360.0;
if (windowId.clockwiseRotation)
progressPercentage = 1.0 - progressPercentage;
cubeAnimation.duration = (windowId.rotationSpeed / windowId.speedFactor) * progressPercentage;
// restart the animation from this angle using a new direction
cubeAnimation.restart();
}
}
Button {
text: "1x"
highlighted: (windowId.speedFactor === 1.0)
onClicked: changeSpeed(1.0)
}
Button {
text: "2x"
highlighted: (windowId.speedFactor === 2.0)
onClicked: changeSpeed(2.0)
}
}
function changeSpeed(newSpeed) {
windowId.speedFactor = newSpeed;
// pause the animation
cubeAnimation.pause();
// store current angle as the starting angle for the rotation
cubeTransform.startAngle = cubeTransform.angle;
// update the remaining time to avoid changing the rotation speed
let progressPercentage = cubeTransform.startAngle / 360.0;
if (windowId.clockwiseRotation)
progressPercentage = 1.0 - progressPercentage;
cubeAnimation.duration = (windowId.rotationSpeed / windowId.speedFactor) * progressPercentage;
// restart the animation from this angle using a new direction
cubeAnimation.restart();
}
}