Python sin and cosine giving incorrect values

问题

I have code that draws a line from 2 points; One on the middle bottom of the screen, and the other to the mouse pointer. I am trying to constrain the point by not exceeding a length parameter that I set. Heres the code:

import pygame
import Resources as r
import math as m

pygame.init()

class Segment():



    def __init__(self, _screen, _id, _start_pos, _length):
        self.screen = _screen
        self.id = _id
        self.start_pos = _start_pos
        self.length = _length

    def update(self):
        if self.id == 1:
            mouse_pos = pygame.mouse.get_pos()


            self.angle = m.atan2(mouse_pos[1]-self.start_pos[1],mouse_pos[0]-self.start_pos[0])
            self.a = self.start_pos
            self.b = (m.cos(self.angle)*self.length, m.sin(self.angle)*self.length)


            self.draw_line(self.a, self.b, r.black, 4)




    def draw_line(self, start, end, color, width):
        if self.id == 1:
            pygame.draw.line(self.screen, color, start, end, width)

    def get_data(self):
        return (self.start_pos, self.end_)

I am seeing very different results when I run this that I would expect, it doesnt line up with my mouse and often just oscillates back and forth when the mouse is moved.

回答1:

self.b is calculated based on origin 0, 0, not self.start_pos.

Add coordinates in self.a to self.b.

回答2:

EDIT: as skrx pointed out in comment: mouse position doesn't have to be converted to Vector2 because tuple-Vector2 gives Vector2.

You can do the same with python.math.Vector2

Your start point on the middle bottom of the screen

screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
screen_rect = screen.get_rect()

start = pygame.math.Vector2(screen_rect.centerx, screen_rect.bottom)

And end point using mouse positon and length

#mouse = pygame.math.Vector2(pygame.mouse.get_pos()) # edited
mouse = pygame.mouse.get_pos()
end = start + (mouse - start).normalize() * length

And now you can draw

pygame.draw.line(screen, (255,0,0), start, end)

Working example

import pygame

# === CONSTANS === (UPPER_CASE names)

BLACK = (  0,   0,   0)
WHITE = (255, 255, 255)

RED   = (255,   0,   0)
GREEN = (  0, 255,   0)
BLUE  = (  0,   0, 255)

SCREEN_WIDTH  = 600
SCREEN_HEIGHT = 400

# === MAIN === (lower_case names)

# --- init ---

pygame.init()

screen = pygame.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
screen_rect = screen.get_rect()

# --- objects ---

start = pygame.math.Vector2(screen_rect.centerx, screen_rect.bottom)
end = start
length = 150

# --- mainloop ---

clock = pygame.time.Clock()
is_running = True

while is_running:

    # --- events ---

    for event in pygame.event.get():

        # --- global events ---

        if event.type == pygame.QUIT:
            is_running = False
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_ESCAPE:
                is_running = False
        elif event.type == pygame.MOUSEMOTION:
            #mouse = pygame.math.Vector2(pygame.mouse.get_pos()) # edited
            mouse = pygame.mouse.get_pos()
            end = start + (mouse - start).normalize() * length

        # --- objects events ---

            # empty

    # --- updates ---

        # empty

    # --- draws ---

    screen.fill(BLACK)

    pygame.draw.line(screen, RED, start, end)

    pygame.display.update()

    # --- FPS ---

    clock.tick(25)

# --- the end ---

pygame.quit()

Red line has always the same length and it shows direction to cursor.

来源：https://stackoverflow.com/questions/47644279/python-sin-and-cosine-giving-incorrect-values

标签

python

math

pygame

trigonometry