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Custom Movement behavior

Published 22 Aug, 2017
1,201 words
~5-8 mins

The Custom Movement behavior does not directly implement any movement for an object. Instead, it provides features that make it easier to implement your own "custom" (event-based) movement.

The way different movements are made is out of the scope of this manual section. Instead, it will outline the basics of the Custom Movement behavior and what its features do. For an example of Asteroids style movement using the Custom Movement behavior, open the Custom movement ('Asteroids' style) example.

For many games, the built-in behaviors like Platform and 8 Direction are perfectly sufficient. Recreating existing behaviors with the Custom Movement should be avoided, since movements are difficult and time consuming to implement correctly. The built-in behaviours have been thoroughly tested, probably have more features than you imagine (like slope detection in Platform), and are much quicker and easier to use than making your own movement.

Overview of custom movements

Most movements in Construct work by manipulating two values: the speed on the X axis (dx) and the speed on the Y axis (dy). These are also known as VectorX and VectorY in some other behaviors. For example, if an object is moving left at 100 pixels per second, dx is -100 and dy is 0. The object can then be accelerated to the right by adding to dx. This is also how most of the other movement behaviors that come with Construct work internally (like Platform and 8 Direction).

The Custom Movement behavior stores the dx and dy values for you, and provides features that help easily implement the math and algorithms necessary to make a movement.

Every tick the Custom Movement adjusts the object's position according to the dx and dy values. This is called a step. The Custom Movement can also use multiple steps per tick, which can help detect collisions more accurately if the object is moving very quickly. Each step will trigger On step, On horizontal step or On vertical step depending on the Stepping mode property.

Custom Movement properties

Stepping mode

How to step the movement each tick. The number of steps taken (if not None) depends on the Pixels per step property. The different modes are:

  • None simply steps the object once per tick according to its velocity.

  • Linear will step the object in a straight line towards its destination position, triggering On step.

  • Horizontal then vertical will step the object to its destination first on the X axis (triggering On horizontal step), then on the Y axis (triggering On vertical step).

  • Vertical then horizontal will step the object to its destination first on the Y axis (triggering On vertical step), then on the X axis (triggering On horizontal step).

Pixels per step

If Stepping mode is not None, this is the distance in pixels of each step towards the destination position each tick. The default is 5, which means if the object is moving 20 pixels in a tick, it will move in four five-pixel steps.

Enabled

Whether the behavior is initially enabled or disabled. If disabled, it can be enabled at runtime using the Set enabled action.

Custom Movement conditions

Compare speed

Compare the current speed of the movement, in pixels per second. Horizontal and Vertical compares to the dx and dy speeds respectively, and Overall compares to the magnitude of the vector (dx, dy) (the overall movement speed).

Is moving

True if either dx or dy are not zero. Invert to test if stopped.

On horizontal step

On vertical step

Triggered for each step along an axis when Stepping mode is either Horizontal then vertical or Vertical then horizontal. This can be used to accurately detect collisions with Is overlapping.

On step

Triggered for each step when Stepping mode is Linear. This can be used to accurately detect collisions with Is overlapping.

Custom Movement actions

Set enabled

Enable or disable the behavior. If disabled, the behavior will not modify the object's position.

Rotate clockwise

Rotate counter-clockwise

Set angle of motion

Adjust the angle of motion. This will calculate new values for dx and dy reflecting a new angle of motion with the same overall speed. Note: if the overall speed is 0, then setting the angle of motion has no effect, because there is no motion. A common mistake is to set the angle of motion then the speed, and find that the angle is not used. Instead simply set the speed first then the angle of motion and it will work as expected.

Accelerate

Accelerate either the overall movement, or movement on a specific axis.

Accelerate toward angle

Accelerate toward position

Accelerate the movement towards an angle or position.

Push out solid

Only valid when the behavior is currently overlapping an object with the solid behavior. Automatically move the object until it is no longer overlapping the solid. This has no effect if the object is not currently overlapping a solid. The following techniques can be used:

  • Opposite angle reverses (or 'backtracks') the object from its current angle of motion until it is no longer overlapping.

  • Nearest moves the object in an eight-direction spiral out one pixel at a time until it is no longer overlapping. The aim is for the object to end up in the nearest free space, but since only eight directions are used it will be an approximation.

  • Up, down, left and right moves the object along a specific axis until it is no longer overlapping.

Push out solid at angle

Only valid when the behavior is currently overlapping an object with the solid behavior. Move the object from its current position at a given angle until it is no longer overlapping the solid. This has no effect if the object is not currently overlapping a solid.

Reverse

Inverts the movement by flipping the signs of dx and dy.

Set speed

Set the current speed in pixels per second either for the horizontal or vertical axes, or the overall movement speed. Setting horizontal or vertical speeds assigns dx and dy directly. Setting the overall speed calculates new values for dx and dy such that they reflect the new overall speed while keeping the same angle of motion.

Stop

A shortcut for setting both dx and dy to 0, stopping the movement.

Stop stepping

Only valid in On step, On horizontal step and On vertical step. Stop the current stepping for this tick. The object can either go back to its old position (where it was at the start of the tick) or stay at its current position (possibly half way between its start and end positions). Note that in Horizontal then vertical or Vertical then horizontal modes, only the current axis is stopped. The next axis will still continue stepping, unless you also use Stop stepping for that axis as well.

Custom Movement expressions

dx

dy

Return the movement's dx and dy values, which are the speed in pixels per second on each axis.

MovingAngle

Return the current angle of motion, in degrees, calculated as the angle of the vector (dx, dy).

Speed

Return the current overall speed in pixels per second, calculated as the magnitude of the vector (dx, dy).