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Bird

A tutorial for Python and Pygame Zero 1.2

Rules

Fly through the spaces between the pipes by flapping.

A point is scored for every pipe passed.

Controls

Any key

Flap

Overview

The bird doesn't actually move forward; it stays still on the X axis while the pipes move to the left.

The bird is drawn as a yellow rectangle, and each pipe is drawn as two green rectangles.

The bird's Y position moves down by a certain speed each frame. Each frame this speed is increased to simulate gravity.

When a key is pressed, the bird's speed is set to a high number in the upward direction.

The pipes share the same width, and the same space height between the segments.

Each pipe has its own X position, and Y position the space starts at.

Since there are only two pipes in the playing area at one time, the information for only two pipes needs to stored. Once a pipe goes out of the playing area, it gets a new space position and its X position is reset to the right edge of the playing area.

To see if the bird has collided with a pipe's two segments, three things need to be checked for each segment.

The bird has collided with the top segment if...

The left edge of the bird is to the left of the right edge of the pipe, and...
The right edge of the bird is to the right of the left edge of the pipe, and...
The top edge of the bird is above the bottom edge of the pipe segment

The bird has collided with the bottom segment if...

The left edge of the bird is to the left of the right edge of the pipe, and...
The right edge of the bird is to the right of the left edge of the pipe, and...
The bottom edge of the bird is below the top edge of the pipe segment

Coding

Drawing the background

A blue rectangle is drawn for the background.

Full code at this point

def draw():
    screen.fill((0, 0, 0))

    screen.draw.filled_rect(
        Rect(
            (0, 0),
            (300, 388)
        ),
        color=(35, 92, 118)
    )

Drawing the bird

A yellow rectangle is drawn for the bird.

Full code at this point

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (62, 200),
            (30, 25)
        ),
        color=(224, 214, 68)
    )

Moving the bird

The bird's Y position is made into a variable, and every frame it increases by 30 multiplied by dt, making the bird move down 30 pixels per second.

Full code at this point

bird_y = 200

def update(dt):
    global bird_y

    bird_y += 30 * dt

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (62, bird_y),
            (30, 25)
        ),
        color=(224, 214, 68)
    )

Gravity

Instead of moving at a constant speed, the number added to the bird's Y position also increases over time.

The bird's speed is made into a variable which starts at 0, and every frame it increases by 516 multiplied by dt.

Full code at this point

bird_y = 200
bird_y_speed = 0

def update(dt):
    global bird_y
    global bird_y_speed

    bird_y_speed += 516 * dt
    bird_y += bird_y_speed * dt

Flapping

Pressing any key sets the bird's speed to a negative number so that it moves upwards.

Full code at this point

def on_key_down():
    global bird_y_speed

    bird_y_speed = -165

Preventing flapping when above the playing area

So that the bird can't fly completely above the playing area, flapping is only possible if the top edge of the bird is not above the playing area.

Full code at this point

def on_key_down():
    global bird_y_speed

    if bird_y > 0:
        bird_y_speed = -165

Drawing a pipe

For now, a solid rectangle is drawn for the pipe, with its left edge touching the playing area's right edge.

The width and height of the playing area are reused from drawing the background, so they are made into variables.

Full code at this point

# etc.

playing_area_width = 300
playing_area_height = 388

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (0, 0),
            (playing_area_width, playing_area_height)
        ),
        color=(35, 92, 118)
    )

    # etc.

    screen.draw.filled_rect(
        Rect(
            (playing_area_width, 0),
            (54, playing_area_height)
        ),
        color=(94, 201, 72)
    )

Drawing two pipe segments

Instead of drawing one rectangle, two rectangles are drawn for the upper and lower segments.

The top rectangle's height is set to where the space between the two segments starts. For now, this is 150.

The bottom rectangle starts at the Y position of the top rectangle's height plus the amount of space between them (which is 100).

The bottom rectangle's height is the playing area's height, minus the height of the top rectangle and the space.

The pipe width is the same for both segments, so it is made into a variable.

Full code at this point

def draw():
    # etc.

    pipe_width = 54
    pipe_space_height = 100
    pipe_space_y = 150

    screen.draw.filled_rect(
        Rect(
            (playing_area_width, 0),
            (pipe_width, pipe_space_y)
        ),
        color=(94, 201, 72)
    )

    screen.draw.filled_rect(
        Rect(
            (playing_area_width, pipe_space_y + pipe_space_height),
            (pipe_width, playing_area_height - pipe_space_y - pipe_space_height)
        ),
        color=(94, 201, 72)
    )

Randomizing the space position

The pipe's space position is set to a random number between 0 (the top of the playing area) and the playing area height minus the space height (so that the bottom of the space would be touching the bottom of the playing area).

The random module is imported so that random.randint can be used.

The pipe's space position should be set only once, so it is moved to be global. Setting the pipe's space position requires the space's height, so it is also moved to be global.

To test this, the code for setting the pipe's space position is copied into on_key_down.

Full code at this point

import random

# etc.

pipe_space_height = 100
pipe_space_y = random.randint(0, playing_area_height - pipe_space_height)

def on_key_down():
    # etc.

    # Temporary
    global pipe_space_y
    pipe_space_y = random.randint(0, playing_area_height - pipe_space_height)

def draw():
    # Moved: pipe_space_y = 150
    # Moved: pipe_space_height = 100

The space's minimum distance from top/bottom

So that there is some distance between the top/bottom of the playing area and the space, a minimum number is added/subtracted from the random minimum/maximum values.

Full code at this point

# etc.

pipe_space_y_min = 54
pipe_space_y = random.randint(
    pipe_space_y_min,
    playing_area_height - pipe_space_height - pipe_space_y_min
)

def on_key_down():
    # etc.

    # Temporary
    global pipe_space_y
    pipe_space_y = random.randint(
        pipe_space_y_min,
        playing_area_height - pipe_space_height - pipe_space_y_min
    )

Moving the pipe

The pipe's X position is made into a variable, and is moved left 60 pixels per second.

Full code at this point

# etc.

pipe_x = playing_area_width

def update(dt):
    # etc.
    global pipe_x

    # etc.

    pipe_x -= 60 * dt

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (pipe_x, 0),
            (pipe_width, pipe_space_y)
        ),
        color=(94, 201, 72)
    )

    screen.draw.filled_rect(
        Rect(
            (pipe_x, pipe_space_y + pipe_space_height),
            (pipe_width, playing_area_height - pipe_space_y - pipe_space_height)
        ),
        color=(94, 201, 72)
    )

Resetting the pipe

When the pipe goes out of the playing area, its X position is reset and it gets a new random space position.

Determining if the pipe has gone out of the playing area requires knowing the pipe's width, which is reused from drawing the pipe, so the pipe's width is moved to be global.

Setting the initial X position of the pipe and its random space position is reused, so a function is made.

Full code at this point

# etc.

bird_y = 200
bird_y_speed = 0

playing_area_width = 300
playing_area_height = 388

pipe_space_height = 100
pipe_width = 54

def reset_pipe():
    global pipe_space_y
    global pipe_x

    pipe_space_y_min = 54
    pipe_space_y = random.randint(
        pipe_space_y_min,
        playing_area_height - pipe_space_height - pipe_space_y_min
    )

    pipe_x = playing_area_width

reset_pipe()

def update(dt):
    # etc.

    if (pipe_x + pipe_width) < 0:
        reset_pipe()

def draw():
    # Moved: pipe_width = 54

Colliding with the top pipe segment

The bird is colliding with the top pipe segment if...

the left edge of the bird is to the left of the right edge of the pipe (i.e. the bird is not completely past the pipe on the right side), and...
the right edge of the bird is to the left of the pipe (i.e. the bird is not completely to the left of the pipe), and...
the top edge of the bird is above the bottom edge of the pipe segment (i.e. the bird is not completely below the pipe segment)

For now, reset_pipe is called when the bird and pipe collide.

The bird's X position and width are reused from drawing the bird, so they are made into variables.

Full code at this point

# etc.

bird_x = 62
bird_width = 30

# etc.

def update(dt):
    # etc.

    if (
        # Left edge of bird is to the left of the right edge of pipe
        bird_x < (pipe_x + pipe_width)
        and
        # Right edge of bird is to the right of the left edge of pipe
        (bird_x + bird_width) > pipe_x
        and
        # Top edge of bird is above the bottom edge of first pipe segment
        bird_y < pipe_space_y
    ):
        # Temporary
        reset_pipe()

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (bird_x, bird_y),
            (bird_width, 25)
        ),
        color=(224, 214, 68)
    )

    # etc.

Colliding with the bottom pipe segment

The bird is colliding with the bottom pipe segment if...

the bird is overlapping the pipe on the X axis (as checked for with the top pipe segment), and...
the bottom edge of the bird is above the top edge of the pipe segment (i.e. the bird is not completely above the pipe segment)

The bird's height is reused from drawing the bird, so it is made into a variable.

Full code at this point

# etc.

bird_height = 25

def update(dt):
    # etc.

    if (
        # Left edge of bird is to the left of the right edge of pipe
        bird_x < (pipe_x + pipe_width)
        and
        # Right edge of bird is to the right of the left edge of pipe
        (bird_x + bird_width) > pipe_x
        and (
            # Top edge of bird is above the bottom edge of first pipe segment
            bird_y < pipe_space_y
            or
            # Bottom edge of bird is below the top edge of second pipe segment
            (bird_y + bird_height) > (pipe_space_y + pipe_space_height)
        )
    ):
        # Temporary
        reset_pipe()

def draw():
    # etc.

    screen.draw.filled_rect(
        Rect(
            (bird_x, bird_y),
            (bird_width, bird_height)
        ),
        color=(224, 214, 68)
    )

Drawing two pipes

Each pipe has its own X position and space position.

For now, these are manually set for each pipe.

The pipe drawing code is turned into a function which takes a pipe's X position and space position.

Full code at this point

# etc.

pipe_1_x = 100
pipe_1_space_y = 100

pipe_2_x = 200
pipe_2_space_y = 200

def draw():
    # etc.

    def draw_pipe(pipe_x, pipe_space_y):
        screen.draw.filled_rect(
            Rect(
                (pipe_x, 0),
                (pipe_width, pipe_space_y)
            ),
            color=(94, 201, 72)
        )

        screen.draw.filled_rect(
            Rect(
                (pipe_x, pipe_space_y + pipe_space_height),
                (pipe_width, playing_area_height - pipe_space_y - pipe_space_height)
            ),
            color=(94, 201, 72)
        )

    draw_pipe(pipe_1_x, pipe_1_space_y)
    draw_pipe(pipe_2_x, pipe_2_space_y)

Randomizing space position for two pipes

Because each pipe has an individual X position, setting the X position is removed from the reset_pipe function, leaving only the random space position. The function is renamed to new_pipe_space_y to reflect this.

Instead of creating a global pipe_space_y variable, the function returns a new space position.

Because the variables pipe_x, pipe_space_y and reset_pipe no longer exist, the parts of the update function which use them are commented out.

To test the random space positions, the pipes get new space positions when a key is pressed.

Full code at this point

def new_pipe_space_y():
    # Removed: global pipe_space_y
    # Removed: global pipe_x

    pipe_space_y_min = 54
    pipe_space_y = random.randint(
        pipe_space_y_min,
        playing_area_height - pipe_space_height - pipe_space_y_min
    )

    # Removed: pipe_x = playing_area_width

    return pipe_space_y

# Removed: reset_pipe()

pipe_1_x = 100
pipe_1_space_y = new_pipe_space_y()

pipe_2_x = 200
pipe_2_space_y = new_pipe_space_y()

def update(dt):
    global bird_y
    global bird_y_speed
    # Removed: global pipe_x

    bird_y_speed += 516 * dt
    bird_y += bird_y_speed * dt

##    pipe_x -= 60 * dt
##
##    if (pipe_x + pipe_width) < 0:
##        reset_pipe()
##
##    if (
##        # Left edge of bird is to the left of the right edge of pipe
##        bird_x < (pipe_x + pipe_width)
##        and
##        # Right edge of bird is to the right of the left edge of pipe
##        (bird_x + bird_width) > pipe_x
##        and (
##            # Top edge of bird is above the bottom edge of first pipe segment
##            bird_y < pipe_space_y
##            or
##            # Bottom edge of bird is below the top edge of second pipe segment
##            (bird_y + bird_height) > (pipe_space_y + pipe_space_height)
##        )
##    ):
##        # Temporary
##        reset_pipe()

def on_key_down():
    # etc.

    # Temporary
    global pipe_1_space_y
    global pipe_2_space_y
    pipe_1_space_y = new_pipe_space_y()
    pipe_2_space_y = new_pipe_space_y()

Moving two pipes

The code which moves the pipe is uncommented and turned into a function which takes a pipe's X position and space position, and returns the updated positions.

When a pipe moves out of the playing area, its position is set to be at the right of the playing area.

Full code at this point

def update(dt):
    global bird_y
    global bird_y_speed
    global pipe_1_x
    global pipe_2_x
    global pipe_1_space_y
    global pipe_2_space_y

    bird_y_speed += 516 * dt
    bird_y += bird_y_speed * dt

    def move_pipe(pipe_x, pipe_space_y):
        pipe_x -= 60 * dt

        if (pipe_x + pipe_width) < 0:
            pipe_x = playing_area_width
            pipe_space_y = new_pipe_space_y()

        return pipe_x, pipe_space_y

    pipe_1_x, pipe_1_space_y = move_pipe(pipe_1_x, pipe_1_space_y)
    pipe_2_x, pipe_2_space_y = move_pipe(pipe_2_x, pipe_2_space_y)

##    if (
##        # Left edge of bird is to the left of the right edge of pipe
##        bird_x < (pipe_x + pipe_width)
##        and
##        # Right edge of bird is to the right of the left edge of pipe
##        (bird_x + bird_width) > pipe_x
##        and (
##            # Top edge of bird is above the bottom edge of first pipe segment
##            bird_y < pipe_space_y
##            or
##            # Bottom edge of bird is below the top edge of second pipe segment
##            (bird_y + bird_height) > (pipe_space_y + pipe_space_height)
##        )
##    ):
##        # Temporary
##        reset_pipe()

Initial X position for pipes

The first pipe's initial X position is set to the width of the playing area (i.e. the pipe's left edge is touching the right edge of the playing area).

The second pipe's initial X position is set so that there is an even space between it and the other pipe on either side.

The total distance that a pipe travels from the of the right of the playing area to left is the width of the playing area plus the width of the pipe.

The second pipe's initial X position is set to the playing area width plus half the total distance, so that the two pipes are spaced evenly apart.

Full code at this point

# etc.

pipe_1_x = playing_area_width
pipe_1_space_y = new_pipe_space_y()

pipe_2_x = playing_area_width + ((playing_area_width + pipe_width) / 2)
pipe_2_space_y = new_pipe_space_y()

Bird colliding with pipe

The code for the checking if the bird and pipe have collided is uncommented and turned into a function which takes a pipe's X position and space position and returns a boolean value.

The function is called for both pipes.

For now, when the bird collides with a pipe, the position of both pipes are reset.

Full code at this point

def update(dt):
    # etc.

    def is_bird_colliding_with_pipe(pipe_x, pipe_space_y):
        return (
            # Left edge of bird is to the left of the right edge of pipe
            bird_x < (pipe_x + pipe_width)
            and
            # Right edge of bird is to the right of the left edge of pipe
            (bird_x + bird_width) > pipe_x
            and (
                # Top edge of bird is above the bottom edge of first pipe segment
                bird_y < pipe_space_y
                or
                # Bottom edge of bird is below the top edge of second pipe segment
                (bird_y + bird_height) > (pipe_space_y + pipe_space_height)
            )
        )

    if (
        is_bird_colliding_with_pipe(pipe_1_x, pipe_1_space_y)
        or is_bird_colliding_with_pipe(pipe_2_x, pipe_2_space_y)
    ):
        pipe_1_x = playing_area_width
        pipe_2_x = playing_area_width + ((playing_area_width + pipe_width) / 2)

Resetting the game

A function is made which sets the initial state of the game.

This function is called before the game begins and when the bird collides with a pipe.

Full code at this point

bird_x = 62
bird_width = 30
bird_height = 25

playing_area_width = 300
playing_area_height = 388

pipe_space_height = 100
pipe_width = 54

def new_pipe_space_y():
    pipe_space_y_min = 54
    pipe_space_y = random.randint(
        pipe_space_y_min,
        playing_area_height - pipe_space_height - pipe_space_y_min
    )

    return pipe_space_y

def reset():
    global bird_y
    global bird_y_speed
    global pipe_1_x
    global pipe_1_space_y
    global pipe_2_x
    global pipe_2_space_y

    bird_y = 200
    bird_y_speed = 0

    pipe_1_x = playing_area_width
    pipe_1_space_y = new_pipe_space_y()

    pipe_2_x = playing_area_width + ((playing_area_width + pipe_width) / 2)
    pipe_2_space_y = new_pipe_space_y()

reset()

def update(dt):
    # etc.

    if (
        is_bird_colliding_with_pipe(pipe_1_x, pipe_1_space_y)
        or is_bird_colliding_with_pipe(pipe_2_x, pipe_2_space_y)
    ):
        reset()

Bird falling out of the playing area

The game is also reset if the bird has fallen out of the playing area (i.e. the bird's top edge is below the bottom edge of the playing area).

Full code at this point

def update(dt):
    # etc.

    if (
        is_bird_colliding_with_pipe(pipe_1_x, pipe_1_space_y)
        or is_bird_colliding_with_pipe(pipe_2_x, pipe_2_space_y)
        or bird_y > playing_area_height
    ):
        reset()

Drawing the score

The score is initially set to 0 and is drawn.

Full code at this point

def reset():
    # etc.
    global score

    # etc.

    score = 0

def draw():
    # etc.

    screen.draw.text(str(score), (15, 15))

Updating the score after passing the first pipe

If the bird's left edge is to the right of the pipe's right edge, then 1 is added to the score.

Currently, this happens every frame instead of just once, so the score will increase too much, and only the first pipe is checked.

Full code at this point

def update(dt):
    # etc.
    global score

    # etc.

    if bird_x > (pipe_1_x + pipe_width):
        score += 1

Updating the score once for both pipes

A number representing which pipe is upcoming is stored in a variable and the bird is checked to see if it has passed a pipe only when that particular pipe is the upcoming pipe.

When it has passed it, the upcoming pipe is set to the other pipe.

Full code at this point

def reset():
    global upcoming_pipe
    # etc.

    upcoming_pipe = 1

def update(dt):
    global upcoming_pipe
    # etc.

    if (
        upcoming_pipe == 1
        and bird_x > (pipe_1_x + pipe_width)
    ):
        score += 1
        upcoming_pipe = 2

    if (
        upcoming_pipe == 2
        and bird_x > (pipe_2_x + pipe_width)
    ):
        score += 1
        upcoming_pipe = 1

Simplifying code

The only differences between updating the score for the first and second pipes are the currently upcoming pipe, the pipe's X position, and the next upcoming pipe.

A function is made with these values as parameters.

Full code at this point

def update(dt):
    # etc.

    def update_score_and_closest_pipe(this_pipe, pipe_x, other_pipe):
        global upcoming_pipe
        global score

        if (
            upcoming_pipe == this_pipe
            and bird_x > (pipe_x + pipe_width)
        ):
            score += 1
            upcoming_pipe = other_pipe

    update_score_and_closest_pipe(1, pipe_1_x, 2)
    update_score_and_closest_pipe(2, pipe_2_x, 1)