ParallaxScrollingBackground

The ParallaxScrollingBackground allows to create an endlessly scrollable background, usable by SideScroller games for example. More...

Import Statement: import VPlay 2.0
Inherits:

Item

Properties

Methods

Detailed Description

The ParallaxScrollingBackground uses one or two MultiResolutionImage instances next to each other to create an endless scrollable background. This can be used to create a parallax scrolling background effect in a game. Make sure you read our tutorial about How to create mobile games for different screen sizes and resolutions to learn about the recommended size for your background images.

Mirrored Images

The property sourceImage needs to bet set or the item won't work correctly. If sourceImage2 is empty, the image declared in sourceImage is mirrored as second image. It is mirrored vertically when scrolling into y direction and horizontally when scrolling into x direction.

Every image could be mirrored, but only a correct prepared image looks well afterwards. Following picture shows an image which can be used for mirroring. Both sides have the same height at the left and right edge and a very similar appearance as seen on the left side. It will look great when Image1, Image2 and again Image1 are beside each other as seen on the right side.

In contrast to the correct image from above, the follwoing image illustrates an incorrect picture for endless scrolling. The background looks faulty when the edges and appearance do not fit accordingly as seen on the left side. The incorrect result can be seen on the right side.

Furthermore, mirroring images only effects the fluctuation of your image data and does not matter if the images are well prepared. To avoid this problem, you can create one image which is big enough and includes variation. You could also prepare two images which fit together as seen in the picture below. There is only one important criteria: The left edge of Image1 needs to fit together with the right edge of Image2 as seen on the left side. The images will fit perfectly together when Image1, Image2 and again Image1 are beside each other as seen on the right side.

Movement Support

The ParallaxScrollingBackground can only move either horizontally or vertically. Directional changes along one axis (e.g. changing from left to right) is supported. Changing from horizontal to vertical movement or vice versa is not supported.

Examples for supported directions:

Example Usage

Following examples demonstrate the usage of the ParallaxScrollingBackground.

Single Background Horizontal Movement

This example shows a single background image which leaves sourceImage2 empty to mirror sourceImage source file. The background scrolls with constant speed (10px per second) towards the left border with movementVelocity set to Qt.point(10,0). It scrolls with a ratio of 1/1 compared to its parent.

 import VPlay 2.0
 import QtQuick 2.0

 GameWindow {

   Scene {

     ParallaxScrollingBackground {
       movementVelocity: Qt.point(10,0)
       ratio: Qt.point(1.0,1.0)
       sourceImage: "../assets/img/background-mountains.png"
     }
   }
 }

Single Background Vertical Movement

This example shows a single background image which leaves sourceImage2 empty to mirror sourceImage source file. The background scrolls with constant speed (10px per second) towards the bottom border with movementVelocity set to Qt.point(0,10). It scrolls with a ratio of 1/1 compared to its parent.

 import VPlay 2.0
 import QtQuick 2.0

 GameWindow {

   Scene {

     ParallaxScrollingBackground {
       movementVelocity: Qt.point(0,10)
       ratio: Qt.point(1.0,1.0)
       sourceImage: "../assets/img/background-wood.png"
     }
   }
 }

Dual Background

This example shows two different backgrounds using sourceImage and sourceImage2. The background scrolls with constant speed (50px per second) towards the right border with movementVelocity set to Qt.point(-50,0). It scrolls with a ratio of 1/1 compared to its parent.

 import VPlay 2.0
 import QtQuick 2.0

 GameWindow {

   Scene {

     ParallaxScrollingBackground {
       movementVelocity: Qt.point(-50,0)
       ratio: Qt.point(1.0,1.0)
       sourceImage: "../assets/img/background-hills.png"
       sourceImage2: "../assets/img/background-hills2.png"
     }
   }
 }

Several ParallaxScollingBackgrounds

When a game should use different parallax effects it needs to use several items which move with different speeds. This case is demonstrated in this example.

 import VPlay 2.0
 import QtQuick 2.0

 GameWindow {

   Scene {

     ParallaxScrollingBackground {
       movementVelocity: Qt.point(100,0)
       ratio: Qt.point(0.5,1.0)
       sourceImage: "../assets/img/background-mountains.png"
     }

     ParallaxScrollingBackground {
       id: background1
       movementVelocity: Qt.point(-100,0)
       ratio: Qt.point(1.0,1.0)
       sourceImage: "../assets/img/background-hills.png"
     }

     ParallaxScrollingBackground {
       id: background2
       movementVelocity: Qt.point(10,0)
       ratio: Qt.point(1.3,1.0)
       sourceImage: "../assets/img/background-lawn.png"
     }

   } // end of Scene

 } // end of GameWindow

Also see the Parallax Test for more information about the correct usage of the ParallaxScrollingBackground.

Property Documentation

image : alias

Note: The ParallaxScrollingBackground now has the size of the used MultiResolutionImage (sourceImage) by default. You can therefore center the ParallaxScrollingBackground (or other ways of anchoring) in the scene like you would do it with any other MultiResolutionImage.

You can still read the size of the used MultiResolutionImage with the image alias like shown in the example below. In this example it is used to position the ParallaxScrollingBackground in the center of the scene, however, the recommened approach is to just set anchors.centerIn: parent.

 import VPlay 2.0
 import QtQuick 2.0

 GameWindow {

   Scene {
     ParallaxScrollingBackground {
       id: pxBg
       sourceImage: "../assets/img/background-wood.png"
       x: -(pxBg.image.width-scene.width)/2
       y: -(pxBg.image.height-scene.height)/2
       mirrorSecondImage: false
       movementVelocity: Qt.point(-75, 0)
     }
   } // end of Scene

 } // end of GameWindow

mirrorSecondImage : bool

Set this property if you want to mirror the sourceImage2 image. By default, if you only have specified the sourceImage property but no sourceImage2 property, the second image will be mirrored. If you explicitly set the sourceImage2 property, it will not be mirrored.

The mirroring is performed automatically based on the movementVelocity: if you set its x property, the image will be mirrored horizontally, or vertically when the y property is set.


movementVelocity : point

The movementVelocity property is used to set the direction and speed of the movement.

  • Left achieved with movementVelocity: Qt.point(10,0)
  • Right achieved with movementVelocity: Qt.point(-10,0)
  • Up achieved with movementVelocity: Qt.point(0,-10)
  • Down achieved with movementVelocity: Qt.point(0,10)

ratio : point

The ratio property is used to set how much shift should occur between parent and image.

  • Left/Right faster than parent achieved with ratio: Qt.point(1.5,1.0)
  • Left/Right slower than parent achieved with ratio: Qt.point(0.5,1.0)
  • Up/Down faster than parent achieved with ratio: Qt.point(1.0,1.5)
  • Up/Down slower than parent achieved with ratio: Qt.point(1.0,0.5)

running : bool

If set to true, the background images will scroll with the specified movementVelocity. If set to to false, the scrolling will pause. By default, it is set to true.


sourceImage2 : url

The sourceImage2 property is not always used. It can be used to provide an additional second image which will alternate with the sourceImage during scrolling. If the property sourceImage2 is left empty, the first image from the property sourceImage will be used and mirrored automatically.


sourceImage : url

The sourceImage property is used to provide the image data for the background. This needs to be set accordingly or the ParallaxScrollingBackground will not work correctly.


Method Documentation

resetBackgrounds()

Reset the position of the images in the ParallaxScrollingBackground.


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