How to rotate an object by changing it's point coordinates (Python, Pygame)

Question

I've been working on a basic game using Pygame in Python, but I can't for the life of me figure out a formula to rotate a shape by changing it's base coordinates. Here's what I have so far:

pygame.init()
screen = pygame.display.set_mode(size)
pygame.display.set_caption("Singleplayer Mode")
done = False
clock = pygame.time.Clock()

p11 = 100
p12 = 100
p21 = 100 
p22 = 150
p31 = 150
p32 = 125

upspeed = 0
rightspeed = 0
topspeed = 25

pause = "false"

while not done:
    screen.fill(WHITE)        
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            done = True
        elif event.type == pygame.KEYDOWN:
            if event.key == K_w:
                upspeed -= 1
            if event.key == K_s:
                upspeed += 1
            if event.key == K_d:
                rightspeed += 1
            if event.key == K_a:
                rightspeed -= 1
            if event.key == K_SPACE:
                pass
            if pause == "true":
                if event.key == K_p:
                    pause = "false"
                    print("Game is unpaused.")
                if event.key == K_q:
                    done = True
            if event.key == K_p:
                pause = "true"
                print("Game is paused.")

    if rightspeed > 0:
        rightspeed -= 0.01
    if rightspeed < 0:
        rightspeed += 0.01
    if upspeed > 0:
        upspeed -= 0.01
    if upspeed < 0:
        upspeed += 0.01

    if pause != "true":
        p11 += rightspeed
        p21 += rightspeed
        p31 += rightspeed
        p12 += upspeed
        p22 += upspeed
        p32 += upspeed

    if p12 < 0:
        upspeed = 0
    if p11 < 0:
        rightspeed = 0
    if p31 > w:
        rightspeed = 0
    if p22 > h:
        upspeed = 0

    pygame.draw.polygon(screen, BLACK, [[p11, p12], [p21, p22], [p31, p32]], 0)

    pygame.display.flip()
    clock.tick(60)

pygame.quit()

Essentially what I want to do is take that triangle that I create, and rotate it instead of making it move right or left when I press the d and a keys. Is there any formula I could use to change the values of p11,p12,p21,p22,p31, and p32 to accomplish this?

Have you tried anything? For example, find your triangles centroid, then draw and rotate the points around that? — Shawnic Hedgehog, Feb 23 '15 at 18:00
See [this](http://stackoverflow.com/questions/20023209/python-function-for-rotating-2d-objects/20025232#20025232) answer. — martineau, Feb 23 '15 at 18:23
Sorry about it taking so long to get back to this, I tried finding the triangle's centroid and creating a formula, but it ended up just making the object move diagonally. I'm just having trouble finding a formula that will work for all 6 numbers. Here's what I tried: **2 * math.pi * 25 * (turnspeed / 360)** (turnspeed is just a variable I used to define how fast the object was turning.) — 1adog1, Feb 25 '15 at 17:22

beiller · Answer 1 · 2015-02-23T20:18:33.580

1

If you represent your coordinates as 2d coordinates [100,100], [100, 150], and [150, 125], you can rotate these coordinates by applying a matrix

[  0, -1, 
   1,  0   ] (90 degree clockwise rotation)

You can use some libraries to help you, EG numpy

x = np.array( ((100,100), (100, 150), (150, 125)) )
y = np.matrix( ((0,-1), (1, 0)) )
print (x * y)

And to get an arbitrary rotation matrix:

def get_matrix(angle) :
     return np.matrix( ((math.cos(angle),-math.sin(angle)), (math.sin(angle), math.cos(angle))) )

edited Feb 23 '15 at 20:18

answered Feb 23 '15 at 20:07

beiller

3,105
1
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Thanks for the advice, but I'm a bit confused, does the np module come with Python, or is it an outside one? – 1adog1 Feb 26 '15 at 13:12
its used a lot. It sometimes comes standard. `import numpy as np` is the usual way to import it. – beiller Feb 26 '15 at 17:58

How to rotate an object by changing it's point coordinates (Python, Pygame)

1 Answers1