Pygame - Platformer jump when grounded is unresponsive

Question

I was recently making a platformer game in pygame (Python version: 3.8.5, Pygame version: 2.0.1).

I implemented a jump functionality where, whenever "a" key or up key is pressed and the player is grounded.

Problem

But this was too unresponsive:

• I had to press the key 2 to 3 times before the jump happened.

Code

Below is the whole code of the project (admittedly a little bit messy):

import os
import sys
import pygame
from pygame.locals import *

# utility functions

# for drawing the collision box
def draw_collision_box(screen, rect):
    pygame.draw.rect(screen, (0, 255, 0), rect, 1)

# getting collisions
def get_collisions(rect, tiles):
    collisions = []

    for tile in tiles:
        if rect.colliderect(tile):
            collisions.append(tile)
    
    return collisions

# moving the player
def move(rect, movement, tiles):
    collision_types = {"top": False, "bottom": False, "right": False, "left": False}

    rect.x += movement[0]
    collisions = get_collisions(rect, tiles)

    for collision in collisions:
        if movement[0] > 0:
            rect.right = collision.left
            collision_types["right"] = True
        elif movement[0] < 0:
            rect.left = collision.right
            collision_types["left"] = True

    rect.y += movement[1]
    collisions = get_collisions(rect, tiles)

    for collision in collisions:
        if movement[1] > 0:
            rect.bottom = collision.top
            collision_types["bottom"] = True
        elif movement[1] < 0:
            rect.top = collision.bottom
            collision_types["top"] = True

    return rect, collision_types

# constants
FPS = 60
TILE_SIZE = 32
MAX_GRAVITY_SCL = 3

# initialize pygame
pygame.init()

# initializing the game window
WINDOW_SIZE = (400, 400)
screen = pygame.display.set_mode(WINDOW_SIZE)
pygame.display.set_caption("Platformer")

# clock
clock = pygame.time.Clock()

# player
player_image = pygame.image.load(os.path.join(
    "assets", "platformer", "player.png"))  # loading the player image
player_image = pygame.transform.scale(player_image, (16, 32))  # resizing the player image
player_image.set_colorkey((255, 255, 255))  # making the bg of the player transparent by setting the color key 

moving_right = False  # checking if the player is moving right
moving_left = False  # checking if the player is moving left

player_xspeed = 4  # player speed on horizontal axis

player_ymomentum = 0  # initial momentum on vertical axis
gravitational_acc = 0.5  # constant gravitation acceleration
jump_force = 10  # jump force of the player
grounded = False   # checking if the player is grounded

player_rect = pygame.Rect(
    50, 50, player_image.get_width(), player_image.get_height())  # player rect for managing collisions

# tiles
grass_image = pygame.image.load(os.path.join(
    "assets", "platformer", "grass.png"))  # loading the grass image
dirt_image = pygame.image.load(os.path.join(
    "assets", "platformer", "dirt.png"))  # loading the dirt image

# resizing the tile images
grass_image = pygame.transform.scale(grass_image, (TILE_SIZE, TILE_SIZE))
dirt_image = pygame.transform.scale(dirt_image, (TILE_SIZE, TILE_SIZE))

# game map
game_map = [["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "2", "2", "2", "0", "0", "2", "2", "2", "0", "0", "0", "0", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "2", "2", "2", "0", "0", "0", "0", "0"],
            ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "0", "1", "1", "1", "0", "0", "0", "0", "0"],
            ["2", "2", "2", "2", "2", "2", "2", "2", "2", "2", "2", "2", "1", "1", "1", "2", "2", "2", "2", "2"],
            ["1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1"],
            ["1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1", "1"]]

# game loop
while True:
    # frames per second
    clock.tick(FPS)

    # filling the background with a solid color so that images doesn't make on top of each other
    screen.fill((146, 244, 255))

    # rendering images
    screen.blit(player_image, (player_rect.x, player_rect.y))
    
    # rendering the map
    tiles = []

    for y in range(len(game_map)):
        for x in range(len(game_map[y])):
            if game_map[y][x] == "1":
                screen.blit(dirt_image, (x * TILE_SIZE, y * TILE_SIZE))
            if game_map[y][x] == "2":
                screen.blit(grass_image, (x * TILE_SIZE, y * TILE_SIZE))
            if game_map[y][x] != "0":
                tiles.append(pygame.Rect(x * TILE_SIZE, y * TILE_SIZE, TILE_SIZE, TILE_SIZE))
    
    # drawing the collision boxes for the tiles
    # for tile in tiles:
    #     draw_collision_box(screen, tile)

    # updating the game
    movement = [0, 0]

    if moving_right == True:
        movement[0] += player_xspeed
    if moving_left == True:
        movement[0] -= player_xspeed

    # gravity for the player
    player_ymomentum += gravitational_acc
    player_ymomentum = min(player_ymomentum, MAX_GRAVITY_SCL)
    movement[1] += player_ymomentum

    # updating player's position
    player_rect, collision_types = move(player_rect, movement, tiles)
    if collision_types["top"] or collision_types["bottom"]:
        player_ymomentum = 0

    grounded = collision_types["bottom"]

    # drawing the collision rect of the player
    # draw_collision_box(screen, player_rect)

    # listening for events
    for event in pygame.event.get():
        if event.type == QUIT:  # quit event
            pygame.quit()  # stop pygame
            sys.exit()  # quit the program

        if event.type == KEYDOWN:  # key input event
            if (event.key == K_RIGHT or event.key == K_d):
                moving_right = True
            if (event.key == K_LEFT or event.key == K_a):
                moving_left = True
            if (event.key == K_UP or event.key == K_w) and grounded:
                player_ymomentum = -jump_force

        if event.type == KEYUP:  # keyup input event
            if (event.key == K_RIGHT or event.key == K_d):
                moving_right = False
            if (event.key == K_LEFT or event.key == K_a):
                moving_left = False

    # updating the display
    pygame.display.update()

Can someone help me?

Answer 1

From reading your code, this is what happens:

In this line of code

player_ymomentum += gravitational_acc

player_ymomentum is set to 0.5; therefore movement[1] is also set to 0.5.

In the move function, you run this line of code:

rect.y += movement[1]

Since movement[1] is < 1 and Rect.y is/has to be an integer value, rect.y does not actually change.

Since rect.y did not change, no collision is detected in the call to get_collisions that follows next.

The condition if movement[1] > 0 is True, but since no collision got detected, collision_types["bottom"] will never be set to True; hence grounded will not be True and you can't jump.

---

In the next frame, the line

player_ymomentum += gravitational_acc

is run again. This time, player_ymomentum is set to 1; therefore movement[1] is also set to 1.

In the move function, the line

rect.y += movement[1]

will actually move the rect this time. A collision will be detected, collision_types["bottom"] will be set to True, grounded will be set to True, allowing you to jump.

player_ymomentum will be reset to 0, and this cycle begins again.

So whenever you're trying to jump while standing, you have a 50/50 chance of grounded being actually False, so pressing the jump key will not work.

---

To solve this, either remove this part:

if collision_types["top"] or collision_types["bottom"]:
    player_ymomentum = 0

player_ymomentum will grow towards MAX_GRAVITY_SCL while still standing on the ground, but that shouldn't be a problem.

Or use a bigger Rect when trying to detect collisions between the player and the ground, like this:

...
rect.y += movement[1]
collisions = get_collisions(rect.inflate(1, 1), tiles)

for collision in collisions:
...

Answer 2

I solved the problem by making another function named check_grounded.

Problem found

The problem was that in each frame the grounded variable was assigned the value True and then False. See the excerpt from within the game-loop:

    # updating player's position
    player_rect, collision_types = move(player_rect, movement, tiles)
    if collision_types["top"] or collision_types["bottom"]:
        player_ymomentum = 0

    grounded = collision_types["bottom"]  # here we assign True or False

So the player has to make the jump at right moment - when grounded == True.

Solution

To fix this I made another function, just for the purpose of ground detection and it worked fine:

# check if grounded
def check_grounded(rect, tiles):
    rect.y += 1
    collisions = get_collisions(rect, tiles)
    rect.y -= 1
    return len(collisions) != 0

grounded = check_grounded(player_rect, tiles)