Jumping in pygame + understanding pygame math (vector)

Question

sprites.py:

import pygame as pg
from settings import *
vec = pg.math.Vector2
# for movement

class Player(pg.sprite.Sprite):
    def __init__(self, game):
        pg.sprite.Sprite.__init__(self)
        self.game = game
        self.image = pg.Surface((30, 40))
        self.image.fill((255, 255, 153))
        self.rect = self.image.get_rect()
        self.rect.center = WIDTH / 2, HEIGHT / 2
        self.pos = vec(WIDTH / 2, HEIGHT / 2)
        self.vel = vec(0, 0)
        self.acc = vec(0, 0)
def jump(self):
    self.rect.x += 1
    hits = pg.sprite.spritecollide(self, self.game.platforms, False)
    self.rect.x -= 1
    if hits:
        self.vel.y = -20

def update(self):
    self.acc = vec(0, PLAYER_GRAV)
    # pouze zryhlení
    keys = pg.key.get_pressed()
    if keys[pg.K_LEFT]:
        self.acc.x = - PLAYER_ACC
    if keys[pg.K_RIGHT]:
        self.acc.x = PLAYER_ACC

    # apply friction
    self.acc.x += self.vel.x * PLAYER_FRICTION
    # pohyb tělesa
    self.vel += self.acc
    self.pos += self.vel + 0.5 * self.acc

    self.rect.midbottom = self.pos

    if self.pos.x > WIDTH:
        self.pos.x = 0
    if self.pos.x < 0:
        self.pos.x = WIDTH

class Platform(pg.sprite.Sprite): def init (self, x, y, w, h): pg.sprite.Sprite. init (self) self.image = pg.Surface((w, h)) self.image.fill(GREEN) self.rect = self.image.get_rect() self.rect.x = x self.rect.y = y

main.py:

import random
import pygame as pg
from settings import *
from sprites import *

class Game:
    def __init__(self):
        # initialize the game window
        self.running = True
        pg.init()
        pg.mixer.init()
        self.screen = pg.display.set_mode((WIDTH, HEIGHT))
        pg.display.set_caption('My game')
        self.clock = pg.time.Clock()

    def new(self):
        # to start a new game, reset the pieces
        self.all_sprites = pg.sprite.Group()
        self.platforms = pg.sprite.Group()
        self.player = Player(self)
        self.all_sprites.add(self.player)
        p1 = Platform(0, HEIGHT - 40, WIDTH, 40)
        p2 = Platform(WIDTH / 2 - 50, HEIGHT * 3 / 4, 100, 20)
        self.platforms.add(p2)
        self.platforms.add(p1)
        self.all_sprites.add(p1)
        self.all_sprites.add(p2)
        self.run()

    def run(self):
        self.playing = True
        while self.playing:
            self.clock.tick(FPS)
            self.events()
            self.update()
            self.draw()

    def update(self):
        self.all_sprites.update()
        hits = pg.sprite.spritecollide(self.player, self.platforms, False)
        if hits:
            self.player.pos.y = hits[0].rect.top
            self.player.rect.midbottom = self.player.pos
            self.player.vel.y = 0

    def events(self):
        for event in pg.event.get():
            if event.type == pg.QUIT:
                if self.playing:
                    self.playing = False
                self.running = False
            if event.type == pg.KEYDOWN:
                if event.key == pg.K_SPACE:
                    self.player.jump()

    def draw(self):
        self.screen.fill(BLACK)
        self.all_sprites.draw(self.screen)
        pg.display.flip()

    def show_start_screen(self):
        pass

    def show_go_screen(self):
        pass


game = Game()
game.show_start_screen()

while game.running:
    game.new()
    game.show_go_screen()

pg.quit()

settings.py:

# game settings
WIDTH = 480
HEIGHT = 600
FPS = 60

# player settings
PLAYER_ACC = 0.5
PLAYER_FRICTION = -0.12
PLAYER_GRAV = 0.8

WHITE = (255, 255, 255)
BLACK = (0, 0, 0)
RED = (255, 0, 0)
GREEN = (0, 255, 0)
BLUE = (0, 0, 255)
YELLOW = ((255, 255, 153))

I have a few questions:

Question n. 1:

Why doesn't the jump method work? And why is it using velocity -20 to jump?

Question n. 2: I'm not quite sure if I understand the vector coordinates, can someone please try to explain it to me? I think that this is exactly why I have problems understanding the code.

Answer 1

In pygame, the screen coordinates start at the top left (0,0). Positive Y goes down. Jump is -20 because the player is going up (toward zero). It looks like the player bounces up when it hits an object.

Concerning the jump issue, the game does not start for (Platform not defined) so I can't help there.