Arm visualization with PyGame

So, in my neverending quest for better arm visualizations so I can make prettier pictures / videos and improve my submission acceptance rates I’ve started looking at PyGame. I started out hoping to find something that was quick and easy in Python for animating using SVG images, and PyGame is as close as I got. All in all, I’m decently happy with the result. It runs slower than the Matplotlib animation, and you can’t zoom in like you can on a Matplotlib graph, but it’s prettier. So, tradeoffs!

There were a few things that I ran into that tripped me up a bit when I was doing this implementation, so I thought that I would share, and that’s what this post is going to be about. Below is an animation showing what the arm visualization looks like, and as always the code for everything can be found on my Github. That links you to the control folder, and you can find all of the code developed / used for this post in this folder up on my GitHub.

Arm visualization using PyGame

Alright, there were a few parts in doing this that were a bit tricky. Let’s go through them one at a time.

Rotations in PyGame

Turns out rotating images is a pain right off the bat, but once you get over the initial hurdles it looks pretty slick.

Centering your image

To start we’re just going to attempt to rotate an image in place, here’s a first pass:

import pygame
import pygame.locals

white = (255, 255, 255)

pygame.init()

display = pygame.display.set_mode((300, 300))
fpsClock = pygame.time.Clock()

image = pygame.image.load('pic.png')

while 1:

    display.fill(white)

    image = pygame.transform.rotate(image, 1)
    rect = image.get_rect()

    display.blit(image, rect)

    # check for quit
    for event in pygame.event.get():
        if event.type == pygame.locals.QUIT:
            pygame.quit()
            sys.exit()

    pygame.display.update()
    fpsClock.tick(30)

First thing you notice upon running this is that the image flies off to the side very quickly, as shown below:
rotation-bad1
This is because we need to recenter the image after each rotation. To do that we can add in this after line 21:

    rect.center = (150, 150)

and so now our animation looks like:
rotation-bad2
At which point a very egregious problem becomes clear: the image is destroying itself as it rotates.

Transforming from a base image

Basically what’s happening is that every transformation the image gets distorted a little bit, and continues moving farther and farther away from the original. To prevent this we’re going save the original and also track the total degrees to rotate the image. Then we’ll perform one rotation (with the minimal distortion caused by one transformation) and plot that every time step. To do this we’ll introduce a degrees variable to track the total rotations. The changes to the code start on line 12:

radians = 0

while 1:

    display.fill(white)

    radians += .5

    rotated_image = pygame.transform.rotate(image, radians)
    rect = rotated_image.get_rect()
    rect.center = (150, 150)

And the resulting animation looks like:
rotation-bad3
Which is significantly better! Pretty good, in fact. Looking close, however, you can notice that it gets a little choppy. And this is because the pygame.transform.rotate method doesn’t use anti-aliasing to smooth out the image. pygame.transformation.rotozoom does, however! So we’ll use rotozoom and set the zoom level to 1, changing line 20:

    rotated = pygame.transform.rotozoom(image, degrees, 1)

And now we have a nice smooth animation!
rotation-good

At this point, we’re going to create a class to take care of this rotation business for us, storing the original image and providing a function that rotates smoothly and recenters the image.

import numpy as np
import pygame

class ArmPart:
    """
    A class for storing relevant arm segment information.
    """
    def __init__(self, pic, scale=1.0):
        self.base = pygame.image.load(pic)
        # some handy constants
        self.length = self.base.get_rect()[2]
        self.scale = self.length * scale
        self.offset = self.scale / 2.0

        self.rotation = 0.0 # in radians

    def rotate(self, rotation):
        """
        Rotates and re-centers the arm segment.
        """
        self.rotation += rotation 
        # rotate our image 
        image = pygame.transform.rotozoom(self.base, np.degrees(self.rotation), 1)
        # reset the center
        rect = image.get_rect()
        rect.center = (0, 0)

        return image, rect

Everything is just what we’ve seen so far, except the offset, which is going to be very useful for the trig we’re about to get into.

Trig

With rotations working and going smoothly one of the major hurdles is now behind us. At this point we can get our arm images and use some trig to make sure that they rotate around the joint ends rather than in the center. Using the above class now we’ll write a script that loads in a picture of an upper arm, and then rotates it around the shoulder.

import numpy as np
import pygame
import pygame.locals

from armpart import ArmPart

black = (0, 0, 0)
white = (255, 255, 255)

pygame.init()

width = 500
height = 500
display = pygame.display.set_mode((width, height))
fpsClock = pygame.time.Clock()

upperarm = ArmPart('upperarm.png', scale=.7)

base = (width / 2, height / 2)

while 1:

    display.fill(white)

    ua_image, ua_rect = upperarm.rotate(.01) 
    ua_rect.center += np.asarray(base)
    ua_rect.center -= np.array([np.cos(upperarm.rotation) * upperarm.offset,
                                -np.sin(upperarm.rotation) * upperarm.offset])

    display.blit(ua_image, ua_rect)
    pygame.draw.circle(display, black, base, 30)

    # check for quit
    for event in pygame.event.get():
        if event.type == pygame.locals.QUIT:
            pygame.quit()
            sys.exit()

    pygame.display.update()
    fpsClock.tick(30)

So far the only trig we need is simply calculating the offset from the center point. Here it’s calculated as half of the length of the image multiplied by a scaling term. The scaling term is because we don’t want the rotation point to be the absolute edge of the image but rather we want it to be somewhere inside the arm. We calculate the x and y offset values using the cos and sin functions, respectively, with a negative sign in from of the sin function because the y axis is inverted, using the top of the screen as 0. The black circle is just for ease of viewing while we’re figuring all the trig out, to make it easier to verify it’s rotating around the point we want. The above code results in the following
arm1

Going ahead and adding in the other links gives us

import numpy as np
import pygame
import pygame.locals

from armpart import ArmPart

black = (0, 0, 0)
white = (255, 255, 255)

pygame.init()

width = 750
height = 750
display = pygame.display.set_mode((width, height))
fpsClock = pygame.time.Clock()

upperarm = ArmPart('upperarm.png', scale=.7)
forearm = ArmPart('forearm.png', scale=.8)
hand = ArmPart('hand.png', scale=1.0)

origin = (width / 2, height / 2)

while 1:

    display.fill(white)

    ua_image, ua_rect = upperarm.rotate(.03) 
    fa_image, fa_rect = forearm.rotate(-.02) 
    h_image, h_rect = hand.rotate(.03) 

    # generate (x,y) positions of each of the joints
    joints_x = np.cumsum([0, 
                          upperarm.scale * np.cos(upperarm.rotation),
                          forearm.scale * np.cos(forearm.rotation),
                          hand.length * np.cos(hand.rotation)]) + origin[0]
    joints_y = np.cumsum([0, 
                          upperarm.scale * np.sin(upperarm.rotation),
                          forearm.scale * np.sin(forearm.rotation), 
                          hand.length * np.sin(hand.rotation)]) * -1 + origin[1]
    joints = [(int(x), int(y)) for x,y in zip(joints_x, joints_y)]

    def transform(rect, origin, arm_part):
        rect.center += np.asarray(origin)
        rect.center += np.array([np.cos(arm_part.rotation) * arm_part.offset,
                                -np.sin(arm_part.rotation) * arm_part.offset])

    transform(ua_rect, joints[0], upperarm)
    transform(fa_rect, joints[1], forearm)
    transform(h_rect, joints[2], hand)
    # transform the hand a bit more because it's weird
    h_rect.center += np.array([np.cos(hand.rotation), 
                              -np.sin(hand.rotation)]) * -10

    display.blit(ua_image, ua_rect)
    display.blit(fa_image, fa_rect)
    display.blit(h_image, h_rect)

    # check for quit
    for event in pygame.event.get():
        if event.type == pygame.locals.QUIT:
            pygame.quit()
            sys.exit()

    pygame.display.update()
    fpsClock.tick(30)

So basically the only thing we’ve done here that was a little more complicated was setting up the centers of the forearm and hand images to be at the end of the upper arm and forearm, respectively. We do that in the joints_x and joints_y variables, and the trig for that is straight from basic robotics.
The above code results in the following animation (which is a little choppy because I dropped some frames to keep the file size small):
animation

Plotting transparent lines

The other kind of of tricky thing that was done in the visualization code was the plotting of transparent lines of the ‘skeleton’ of the arm. The reason that this was kind of tricky was because you can’t just use the pygame.draw.lines method, because it doesn’t allow for transparency. So what you end up having to do instead is generate a set of Surfaces of the shape that you want for each of the lines, and then transform and plot them appropriately, which is thankfully pretty straightforward after working things out for the images above.

To generate a transparent surface you use the pygame.SRCALPHA variable, so it looks like

surface = pygame.Surface((width, height), pygame.SRCALPHA, 32)
surface.fill((100, 100, 100, 200))

where the variable passed in to fill is a 4-tuple, with the first 3 parameters the standard RGB levels, and the fourth parameter being the transparency level.

Then there’s one more snag, in that when you blit a surface it goes by the top left position of the rectangle that contains it. So doing the same transformations for the images we then have to transform the surface further because with the images we were specifying the desired center point. This is easy enough; just get a handle to the rect for the surface and subtract out half of the width and height (post-rotation).

The code with the transparent lines (and some circles at the joints added in for prettiness) then is

import numpy as np
import pygame
import pygame.locals

from armpart import ArmPart

black = (0, 0, 0)
white = (255, 255, 255)
arm_color = (50, 50, 50, 200) # fourth value specifies transparency

pygame.init()

width = 750
height = 750
display = pygame.display.set_mode((width, height))
fpsClock = pygame.time.Clock()

upperarm = ArmPart('upperarm.png', scale=.7)
forearm = ArmPart('forearm.png', scale=.8)
hand = ArmPart('hand.png', scale=1.0)

line_width = 15
line_upperarm = pygame.Surface((upperarm.scale, line_width), pygame.SRCALPHA, 32)
line_forearm = pygame.Surface((forearm.scale, line_width), pygame.SRCALPHA, 32)
line_hand = pygame.Surface((hand.scale, line_width), pygame.SRCALPHA, 32)

line_upperarm.fill(arm_color)
line_forearm.fill(arm_color)
line_hand.fill(arm_color)

origin = (width / 2, height / 2)

while 1:

    display.fill(white)

    # rotate our joints
    ua_image, ua_rect = upperarm.rotate(.03) 
    fa_image, fa_rect = forearm.rotate(-.02) 
    h_image, h_rect = hand.rotate(.03) 

    # generate (x,y) positions of each of the joints
    joints_x = np.cumsum([0, 
                          upperarm.scale * np.cos(upperarm.rotation),
                          forearm.scale * np.cos(forearm.rotation),
                          hand.length * np.cos(hand.rotation)]) + origin[0]
    joints_y = np.cumsum([0, 
                          upperarm.scale * np.sin(upperarm.rotation),
                          forearm.scale * np.sin(forearm.rotation), 
                          hand.length * np.sin(hand.rotation)]) * -1 + origin[1]
    joints = [(int(x), int(y)) for x,y in zip(joints_x, joints_y)]

    def transform(rect, base, arm_part):
        rect.center += np.asarray(base)
        rect.center += np.array([np.cos(arm_part.rotation) * arm_part.offset,
                                -np.sin(arm_part.rotation) * arm_part.offset])

    transform(ua_rect, joints[0], upperarm)
    transform(fa_rect, joints[1], forearm)
    transform(h_rect, joints[2], hand)
    # transform the hand a bit more because it's weird
    h_rect.center += np.array([np.cos(hand.rotation), 
                              -np.sin(hand.rotation)]) * -10

    display.blit(ua_image, ua_rect)
    display.blit(fa_image, fa_rect)
    display.blit(h_image, h_rect)

    # rotate arm lines
    line_ua = pygame.transform.rotozoom(line_upperarm, 
                                        np.degrees(upperarm.rotation), 1)
    line_fa = pygame.transform.rotozoom(line_forearm, 
                                        np.degrees(forearm.rotation), 1)
    line_h = pygame.transform.rotozoom(line_hand, 
                                        np.degrees(hand.rotation), 1)
    # translate arm lines
    lua_rect = line_ua.get_rect()
    transform(lua_rect, joints[0], upperarm)
    lua_rect.center += np.array([-lua_rect.width / 2.0, -lua_rect.height / 2.0])

    lfa_rect = line_fa.get_rect()
    transform(lfa_rect, joints[1], forearm)
    lfa_rect.center += np.array([-lfa_rect.width / 2.0, -lfa_rect.height / 2.0])

    lh_rect = line_h.get_rect()
    transform(lh_rect, joints[2], hand)
    lh_rect.center += np.array([-lh_rect.width / 2.0, -lh_rect.height / 2.0])

    display.blit(line_ua, lua_rect)
    display.blit(line_fa, lfa_rect)
    display.blit(line_h, lh_rect)

    # draw circles at joints for pretty
    pygame.draw.circle(display, black, joints[0], 30)
    pygame.draw.circle(display, arm_color, joints[0], 12)
    pygame.draw.circle(display, black, joints[1], 20)
    pygame.draw.circle(display, arm_color, joints[1], 7)
    pygame.draw.circle(display, black, joints[2], 15)
    pygame.draw.circle(display, arm_color, joints[2], 5)

    # check for quit
    for event in pygame.event.get():
        if event.type == pygame.locals.QUIT:
            pygame.quit()
            sys.exit()

    pygame.display.update()
    fpsClock.tick(30)

And the resulting arm visualization now looks like this!
arm3wlines

From here you can then blit on an image in the background, and the little zoom in I have in the top left is just another surface, it’s all pretty straight forward. And that’s pretty much everything! Hopefully this helps someone trying to get some fancier PyGame things going, and if you have any means of generating fancier PyGame animations yourself please drop any tips below in the comments!

Again, the code from this post is all up on my GitHub.

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2 thoughts on “Arm visualization with PyGame

  1. kjk says:

    Great blog. I stumbled on here when I read your post on jacobians. I was curious if you’ve done this arm simulation (with images overlaid) in matlab.

    • travisdewolf says:

      Thanks!
      Ah, I moved out of Matlab a few years ago and do my damndest to avoid going back in.
      Haven’t tried anything fancier than plotting with basic lines in Matlab, sorry!

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