4. Advanced Movement¶

In this lesson we will learn

what a frame is in GameFrame
three different ways to move an object with keys
how to use flowcharts to compare algorithms
how to move an object by changing its coordinates or its speed

Terminology

acceleration – a change in speed over time, such as the ship moving faster the longer a key is held.
frame – each time the screen is redrawn while the game runs.
frame rate – how many frames are drawn each second, which in GameFrame is 30.

Different ways to move¶

In the last lesson we made the spaceship move when a key is pressed. Pressing W starts the ship moving up and pressing S starts it moving down, and it keeps going until we press the other key. That's not the only way movement can work, though. We could have:

always in motion
in motion while a key is pressed
always in motion with acceleration

To understand these, we need to know how GameFrame works with an object's coordinates. The GameFrame API lists six variables to do with an object's position in the Room:

x and y → the object's current coordinates
prev_x and prev_y → the object's coordinates in the last frame
x_speed and y_speed → how far the object moves in the x and y directions every frame

Frames

In GameFrame, a frame is each time the screen is redrawn. How often that happens is set by FRAMES_PER_SECOND in Globals.py, which is 30. So the screen is redrawn every 1/30 of a second (about 33 milliseconds), and x_speed and y_speed are how many pixels the object moves every 33 milliseconds.

Always in motion¶

Below is the flowchart for always in motion. On every loop:

the y position changes by the value of y_speed
y_speed starts at 0
y_speed changes if W or S is pressed
there's no way for y_speed to go back to 0

Always moving flowchart

This is the movement we already have in our key_pressed method:

Objects/Ship.py
    def key_pressed(self, key):
        """
        Respond to keypress up and down
        """

        if key[pygame.K_w]:
            self.y_speed = -10
        elif key[pygame.K_s]:
            self.y_speed = 10

In motion while a key is pressed¶

Below is the flowchart for in motion while a key is pressed. Notice that:

y_speed stays at 0, so y never changes by itself
the key press changes y directly, so while W is held down, y decreases by 10 every frame

Move while key pressed flowchart

To try this movement, change the highlighted lines in the key_pressed method in Objects/Ship.py.

Objects/Ship.py
    def key_pressed(self, key):
        """
        Respond to keypress up and down
        """

        if key[pygame.K_w]:
            self.y -= 10
        elif key[pygame.K_s]:
            self.y += 10

Code explanation

line 28 → while W is held, moves the ship up 10 pixels by decreasing y directly.
line 30 → while S is held, moves the ship down 10 pixels by increasing y directly.

PRIMM

Predict how this movement will feel different from the last one.
Run MainController.py and try it.
Investigate: what happens when you let go of the key? Why?

Always in motion with acceleration¶

Below is the flowchart for always in motion with acceleration. It combines the last two approaches:

y_speed is used to update y every frame
pressing a key increases or decreases y_speed
this gives the ship a sense of acceleration: the longer we hold a key, the faster the ship moves that way

Acceleration flowchart

To try this movement, change the highlighted lines in the key_pressed method.

Objects/Ship.py
    def key_pressed(self, key):
        """
        Respond to keypress up and down
        """

        if key[pygame.K_w]:
            self.y_speed -= 5
        elif key[pygame.K_s]:
            self.y_speed += 5

Code explanation

line 28 → while W is held, reduces y_speed by 5 every frame, so the ship speeds up going up (or slows down if it was going down).
line 30 → while S is held, increases y_speed by 5 every frame, so the ship speeds up going down.

PRIMM

Predict what will happen if you hold W for a long time.
Run MainController.py and try it.
Investigate: how do you stop the ship? Is this harder or easier to control?

Choose your movement¶

Try all three options, then choose the one you want to use in your game.

The lessons use always in motion

The rest of the lessons show the always in motion code. If you choose a different movement, your key_pressed method will look a little different from the code on the pages, and that's fine. Just keep your own version when you add the highlighted lines.

Commit and push¶

In GitHub Desktop, type Chose ship movement in the Summary box.
Click Commit to main.
Click Push origin.