Moving A Cube

In this short tutorial we will introduce two essential elements for logic in UPBGE: logic triggering and game properties. These elements allows the interactivity with the set up objects.

A game property (also known as variable) is a value that is kept inside a object, allowing multiple uses. A common use of game properties is the ammount life of the player, the current song playing, or anything else that can be useful to track for later use.

The logic triggering is the act of triggering some kind of event in the game. There’s lots of ways to trigger events in UPBGE, from detecting if a key is pressed to detecting an object colliding with another, or even triggering events continuously without detecting anything. After the detection of an trigger, an event can be happen, like a object to move, a property be changed, etc.

Setup Scene

First, we must add objects to compose our scene. We need three basic objects:

Camera

A camera will allow us to see our scene from a point of view.

Lamp

A lamp will illuminate the scene objects, allowing us to see them. In this example we’ll use a Hemi lamp, which is a directional lamp that illuminates all objects in the scene.

Cube

A Cube object will be our visual feedback of our logic. As we can’t see a camera or lamp, we’ll move the Cube.

Once all objects were added, place them somewhat like the picture below:

../../../_images/tutorials-move_object-initial_scene.png

The Camera is showing the Cube, the Cube is at the center of the scene and the Hemi lamp can be anywhere.

Our Cube is not centered in the screen on purpose: we’ll move it in the front direction (-Y), so it’s good to see it moving after certain point.

Adding Logic

After the scene is set up, follow these steps:

Now we must fill some information on the bricks:

  • On the Keyboard Sensor, click on the Key field and press a key to assign a key to it.

  • On the Motion Actuator, insert the value -0.05 in the field Y of Loc.

The Logic Bricks Editor should look like this:

Now, start the game engine (by default, pressing P while focusing the 3D Viewport). If you press the key you assigned to the Keyboard Sensor, the Cube will move in the -Y direction, and if you release the key, the Cube will stop.

This behavior happens for several reasons:

  • The Keyboard Sensor emits a positive signal when the selected key is pressed, and emits a negative signal when the key is released.

  • The AND Controller receives the signals from all connected sensors, and if all signals are positive, the controller emits an activation signal to all connected actuators, or an deactivation signal if one or more incoming signals are negative.

  • The Motion Actuator receives the activation signal from the controller and perform the motion. When it receives a deactivation signal, it stops performing the motion.

This is the basic of visual logic when using UPBGE, pretty straightforward. However, according to what you want to achieve, it can get a lot more complex.

Logic Depending On Properties

In games, the logic depends on statuses most of the time. An enemy dies when its life reaches 0, the player can shoot while its ammo is greater than 0, and so on. In UPBGE, you can do these conditions through the use of properties.

To continue, perform the following steps:

../../../_images/tutorials-move_object-logic_editor_2.png

The property fuel added and the Property Sensor properly filled.

This makes our Cube move only if the value of fuel is greater than 0. You can set the property fuel to 0 and play the game, and you will see that the Cube will not move. However, it would be good if we decrease the value of fuel as our Cube moves, until it reaches 0. To do that, do the following steps:

../../../_images/tutorials-move_object-logic_editor_3.png

The sensors connected to AND Controller and the new Property Actuator properly filled.

There’s a new factor involved here: the pulse mode on Keyboard Sensor. By default, a sensor sends a single positive signal to the controller when active, and a single negative signal when inactive. The pulse mode makes the signal be sent each logical frame (default is 60 frames per second). This is useful for us now, because we need our fuel to be decreased while we press the key without the need of releasing and pressing it again.

Go ahead em play the game. The Cube will move and, after some time, it stops. It happens because the Property Actuator has decreased 1 unit of fuel each frame, according to the Keyboard Sensor pulse mode, and when fuel reaches 0, the logic of the Motion Actuator doesn’t respond anymore. It would be good, however, to see the value of fuel be decreased over time. You can do this by enabling the debug flag on the fuel property and the Show Debug Properties on menu Game > Show Debug > Show Debug Properties, or on the Render editor, as shown in the figure below.

../../../_images/tutorials-move_object-debug_properties.png

Conclusion

The goal of this basic tutorial is to show how to work with the visual logic and properties on UPBGE. There’s more to be discovered about visual logic and properties, like other Property Types, the use of States with logic bricks, etc, and those subjects can be better understood on their own pages.