I have created two instances of Squircle(openglunderqml) item

First instance have size 500,500 alignment in center to screen

ex:Mini Player in preview

Second instance have size of screen size alignment to complete screen

ex:Main Player in viewer

Initially Main Player does noting will goto preview there we can see mini player user say ok will launch main player

But during preview I could not see mini player because main player glviewport covering complete screen and its clearing because paint is getting alway its is connected to beforeRendering signal

code:
http://doc.qt.io/qt-5/qtquick-sceneg...l-example.html

I followed above link and made small change glviewport according to instance boundary
Qt Code: 
  1. glViewport(m_viewportSize.x(), m_viewportSize.y(), m_viewportSize.width(), m_viewportSize.height());
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glclear update
Qt Code: 
  1. glClearColor(0, 0, 0, 1);
  2.  
  3.     glScissor(m_ViewportSize.x(),m_ViewportSize.y(), m_ViewportSize.width(), m_ViewportSize.height());
  4.  
  5.     glEnable(GL_SCISSOR_TEST);
  6.     glClear(GL_COLOR_BUFFER_BIT);
  7.     glDisable(GL_SCISSOR_TEST);
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main.qml
Qt Code: 
  1. import QtQuick 2.0
  2. import OpenGLUnderQML 1.0
  3.  
  4. Item {
  5.     width: 1200
  6.     height: 600
  7.     Test{
  8.         width: 500
  9.         height: 500
  10.         anchors.centerIn: parent
  11.         color: "green"
  12.     }
  13.     Test{
  14.         width: parent.width
  15.         height: parent.height
  16.         anchors.top: parent.top
  17.         anchors.left: parent.left
  18.         color: "red"
  19.     }
  20. }
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Test.qml
Qt Code: 
  1. import QtQuick 2.0
  2. import OpenGLUnderQML 1.0
  3.  
  4. Rectangle {
  5.  
  6.     opacity: 0.2
  7.     Squircle {
  8.         SequentialAnimation on t {
  9.             NumberAnimation { to: 1; duration: 2500; easing.type: Easing.InQuad }
  10.             NumberAnimation { to: 0; duration: 2500; easing.type: Easing.OutQuad }
  11.             loops: Animation.Infinite
  12.             running: true
  13.         }
  14.     }
  15.     //! [1] //! [2]
  16.     Rectangle {
  17.         color: Qt.rgba(1, 1, 1, 0.7)
  18.         radius: 10
  19.         border.width: 1
  20.         border.color: "white"
  21.         anchors.fill: label
  22.         anchors.margins: -10
  23.     }
  24.  
  25.     Text {
  26.         id: label
  27.         color: "black"
  28.         wrapMode: Text.WordWrap
  29.         text: "The background here is a squircle rendered with raw OpenGL using the 'beforeRender()' signal in QQuickWindow. This text label and its border is rendered using QML"
  30.         anchors.right: parent.right
  31.         anchors.left: parent.left
  32.         anchors.bottom: parent.bottom
  33.         anchors.margins: 20
  34.     }
  35. }
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