[Tutor] Fwd: circular movement in pygame

[diliup gabadamudalige]

---------- Forwarded message ----------
From: diliup gabadamudalige <***@gmail.com>
Date: Tue, Apr 28, 2015 at 6:22 PM
Subject: circular movement in pygame
To: pygame-***@seul.org


Looking at the code on this page lines 47 & 48

http://programarcadegames.com/python_examples/show_file.php?file=sprite_circle_movement.py

is there a way to do
self.rect.x +*= some value*
self.rect.y += some value

rather than

self.rect.x = self.radius * math.sin(self.angle) + self.center_x
self.rect.y = self.radius * math.cos(self.angle) + self.center_y

?

I tired it in the code attached and strangely it works ok but only OUTSIDE
THE class. When I implement the exact code in a class it does something
weird. I can't find where I have gone wrong.

Please help.

Many thanks in advance.

Diliup Gabadamudalige


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--
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http://www.diliupg.com
http://soft.diliupg.com/

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[Alan Gauld]

Post by diliup gabadamudalige
---------- Forwarded message ----------
Date: Tue, Apr 28, 2015 at 6:22 PM
Subject: circular movement in pygame

It's good that you tried the python-game list for a
pygame question. But if pygame-users can't give a
definitive answer then its even less likely that the
tutor list will. Although you might need to allow
a full 24 hours to get answers, email is not an
instant delivery mechanism.

Post by diliup gabadamudalige
Looking at the code on this page lines 47 & 48
http://programarcadegames.com/python_examples/show_file.php?file=sprite_circle_movement.py

I didn't look at the code in that tutorial, but...

Post by diliup gabadamudalige
is there a way to do
self.rect.x +*= some value*
self.rect.y += some value

Yes, that's just normal Python syntax. It adds/multiplies
the current value of self.rect with "some value"...

Post by diliup gabadamudalige
rather than
self.rect.x = self.radius * math.sin(self.angle) + self.center_x
self.rect.y = self.radius * math.cos(self.angle) + self.center_y

But its unlikely to have the same effect as this since the
above two lines are calculating a new value based on radius,
angle, and centre rather than one based on the current
values of self.rect

Post by diliup gabadamudalige
I tired it in the code attached and strangely it works ok but only OUTSIDE
THE class. When I implement the exact code in a class it does something
weird. I can't find where I have gone wrong.

I can't see your code but are you sure you are using self in
all the appropriate places in your class based code?
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[Oscar Benjamin]

Post by diliup gabadamudalige
Looking at the code on this page lines 47 & 48
http://programarcadegames.com/python_examples/show_file.php?file=sprite_circle_movement.py
is there a way to do
self.rect.x +*= some value*
self.rect.y += some value
rather than
self.rect.x = self.radius * math.sin(self.angle) + self.center_x
self.rect.y = self.radius * math.cos(self.angle) + self.center_y

There's no way to do that. The second version ignores the previous
value of self.rect.x/y.

I'm going to guess that the angle has only changed by a small amount
delta_angle between iterations in which case there would be a way to
do it approximately.

The real question is just why though? The second version is correct
and will be correct for ever. The first version would only be
approximately correct and over time you'd probably find that the
position would drift so that the ball was effectively at a different
radius.

In any case if
x = r * sin(theta)
then
dx/dt = r * cos(theta) * dtheta/dt.
Since
y = r * cos(theta) that's
dy/dt = x * dtheta/dt.
So you could update y approximately with:
y += x * dtheta/dt * deltat
where deltat is your timestep. In your code that would be:
self.rect.x += self.rect.y * self.speed
self.rect.y -= self.rect.x * self.speed

Really what you have now is better though.
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[Oscar Benjamin]

I thank all those who responded to my question
Here is the code that I had written.
When updating is applied to a surface object the rotation works but when it
is applied through a class to an object it goes wrong in about 3 rotations.
As far as I can see the code is the same. What is wrong? If you can correct
some code and show me would help.

Your code is too long and complicated for me to read through it all.
You should simplify your problem before posting it somewhere like
this. Make a simpler program that does something similar but without
all the pygame stuff. For example your program might just loop through
printing out the positions of the object at different times e.g.:

# game.py
#
# This program simulates a ball moving at constant speed
# in a two dimensional space.
#

# Initial position of ball
xpos = 0
ypos = 0

# Horizontal and vertical speed
xspeed = 1
yspeed = 2

# Timestep
deltat = 0.125

# Run through 10 iterations of simulation
for n in range(10):
# Update state
xpos += xspeed * deltat
ypos += yspeed * deltat

# Output current state
print('Position = (%.3f, %.3f)' % (xpos, ypos))

$ python game.py # Show the output
Position = (0.125, 0.250)
Position = (0.250, 0.500)
Position = (0.375, 0.750)
Position = (0.500, 1.000)
Position = (0.625, 1.250)
Position = (0.750, 1.500)
Position = (0.875, 1.750)
Position = (1.000, 2.000)
Position = (1.125, 2.250)
Position = (1.250, 2.500)

Once you have made a simpler program carefully explain what it is that
you want it to do and show us how what it does is different. Don't
expect people on this list to download your code and run it.

Also please put your code in the email and not in an attachment and
please reply back to the tutor list rather than directly to me.


--
Oscar
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[Dave Angel]

I thank all those who responded to my question
Here is the code that I had written.
When updating is applied to a surface object the rotation works but when it
is applied through a class to an object it goes wrong in about 3 rotations.
As far as I can see the code is the same. What is wrong? If you can correct
some code and show me would help.

By top-posting, you're messing up the readability of your response. And
by trying to use an attachment, you're messing up a large portion of the
people reading this thread.

Post a simplified example, inline in your message, and *following* any
quote you're using.

If your symptom is that the data diverges eventually from the intended
trajectory, the problem is that you're accumulating errors. Each point
you do you're rounding the calculation by storing it in finite
precision. After enough roundoffs, the error becomes visible.

If you need to reliably move an object in a circle, you'll want to store
the location in angular terms, center, radius, and angle.

Then each time around the loop, increment the angle by a non-rounded
amount, and recalculate the x/y coordinates.

[diliup gabadamudalige]

Thanks all for the responses.
Charles Cossé - yes I can write a simple pygame program that makes a
sprite move in a circle but it may not be the style and order that many may
approve or accept. I consider myself a student. :)
No one has pointed out why the object moves in a circle properly in the bit
of code where it is addressed directly and why it does not when the same
code is applied thru a class.
I know that an object can be made to move around the circumference of a
circle by constantly calculating the point and giving the x and y values

1. x = some code

That is what I used and it worked. I was trying to find how we can do the
same thing by updating the current position by doing

2. x += some code
Above 1 works. 2 does not. Why is that?

Both the same code. The same variable is used with different names cause
some others were confused when they were the same.

I hope this time I have answered correctly. No more attachments. (e mail
or otherwise. :) )
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[Alan Gauld]

Post by diliup gabadamudalige
1. x = some code
That is what I used and it worked. I was trying to find how we can do the
same thing by updating the current position by doing
2. x += some code
Above 1 works. 2 does not. Why is that?

Because they do completely different things!

1 is simply

x = some code

2 is

x = x + some code

In 2 you are adding x to some code.

Of course they don't work the same.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[Alan Gauld]

Thank you Alan. Understood. I already knew that. My question is
How to to do it the second way.
some code = what?

The point is you cannot do it.
The second approach always uses the current value of x.
The first approach may or may not use the current value
of x. If it does not use x then the += style cannot be
made to work (unless you tag a -x on at the end!

x = 42

x += 42-x

Which is just stupid.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos

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[diliup gabadamudalige]

x=42 good
x += 42 -x ?

That does not go by a mile of what I asked.
Alan, are you telling me that there is no way that one can arrive at X2
which is the NEXT place that X1 will be after "some increment"?
old x position = X1

new x position= X1 + some increment value
which can be written as :new X = oldX + some increment value

which can be simplified to X += some increment value?

my orginal question was

*Why does the object NOT move in a circle when implemented as a class?*

All the code was provided.


The above is what I asked. (I hope this is clear enough)

Post by Alan Gauld

Thank you Alan. Understood. I already knew that. My question is
How to to do it the second way.
some code = what?
The point is you cannot do it.

The second approach always uses the current value of x.
The first approach may or may not use the current value
of x. If it does not use x then the += style cannot be
made to work (unless you tag a -x on at the end!
x = 42
x += 42-x
Which is just stupid.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
http://www.flickr.com/photos/alangauldphotos

--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

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[Alan Gauld]

Post by diliup gabadamudalige
x=42 good
x += 42 -x ?
That does not go by a mile of what I asked.

You've asked three different things:

1) Why the += form gives different results from the assignment form,
2) Why some code you apparently wriotre worked outside a class
but not in a class
3) A specific question about some code for moving an object
in PyGame around in a circle

Number 1 is the only one I can comment on as I can't see your
attachments. That is what I (and several others) have answered.
They are fundamentally different. There is no general way
to make an augmented assignment such that

x = f(y) and
x += f(y) (or even g(y))

produce the same result except by the trivial

x += g(y) - x

Post by diliup gabadamudalige
Alan, are you telling me that there is no way that one can arrive at X2
which is the NEXT place that X1 will be after "some increment"?
old x position = X1

No, that is a completely separate question and has nothing
to do with using augmented assignment. But it does depend
on the specifics of the situation and is mainly a mathematical
problem rather than a Python one..

Post by diliup gabadamudalige
new x position= X1 + some increment value
which can be written as :new X = oldX + some increment value
which can be simplified to X += some increment value?

That may or may not be possible, it depends on what the
original equation in the pure assignment model is.

Post by diliup gabadamudalige
my orginal question was
*Why does the object NOT move in a circle when implemented as a class?*

Your original question was:

is there a way to do
self.rect.x +*= some value*
self.rect.y += some value

rather than

self.rect.x = self.radius * math.sin(self.angle) + self.center_x
self.rect.y = self.radius * math.cos(self.angle) + self.center_y

Which is about the use of augmented assignment.

Post by diliup gabadamudalige
All the code was provided.

No it wasn't. I have not seen any of your code because you persist
in sending it as attachments which (as you've been told) do not
work reliably in a text based mailing list. If you want
comments on the code either post it inline in your message
or as a link to a web site where we can all see it.

Post by diliup gabadamudalige
The above is what I asked. (I hope this is clear enough)

Your class based code should work but without sight of it
I cannot begin to guess about that aspect of your problem.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[diliup gabadamudalige]

I do not understand how Alan does not get the code that is in this thread.
Anyway I can conclude by saying that a friend of mine who is not in this
thread solved the problem and I would like to share it with you. He has
changed only a very smal portion in the original code that i posted in this
thread. and it works perfectly. Apparently the problem is in the way the
rect assignment is done in the Python/Pygame rect class. So as conclusion
the code below works perfectly. No drifting. Class implemented. I hope
everyone gets updated. Thanks for your time.


import sys, os, pygame, itertools
from math import sin,cos,pi
from pygame.locals import *
SCREENW = 800
SCREENH = 700
class object_factory(pygame.sprite.Sprite):
def __init__(self, image, xpos = 0, ypos = 0):
"""Constructor"""
pygame.sprite.Sprite.__init__(self)
self.image = image
self.mask = pygame.mask.from_surface(self.image) # pixelmask
self.rect = self.image.get_rect()
self.rect.x = xpos
self.rect.y = ypos
self.x=xpos
self.y=ypos
pygame.init()
clock = pygame.time.Clock()
os.environ['SDL_VIDEO_WINDOW_POS'] = "%d,%d" % (50,50) #Set window position
SCREEN = pygame.display.set_mode((SCREENW, SCREENH))
clock = pygame.time.Clock()
pygame.display.set_caption('rotating object')
ball = pygame.image.load("ball.jpg")
pin=pygame.image.load("center.jpg");
platforms = pygame.sprite.Group()
center_x = SCREENW/2
center_y = SCREENH/2
radius = 200
angle = pi/4 # starting angle 45 degrees
omega = 0.1 #Angular velocity

#---------------------------------------------------------------------------------------
for _ in itertools.repeat(None, 6):
xpos = center_x + radius * cos(angle) #Starting position x
ypos = center_y - radius * sin(angle) #Startinh position y
obj = object_factory(ball, xpos, ypos)
obj.angle = angle
obj.omega = omega #angula velocity
obj.radius = radius
platforms.add(obj)
angle += pi/3
#----------------------------------------------------------------------
while True:
clock.tick(24)
pygame.event.pump()
keys = pygame.key.get_pressed()
for event in pygame.event.get():
if event.type == QUIT or (event.type == KEYDOWN and event.key ==
K_ESCAPE):
pygame.quit()
sys.exit()
SCREEN.fill((255,255,255))
SCREEN.blit(pin,(center_x, center_y))

#Class
implementation--------------------------------------------------------------------------------------------------------------
for b in platforms:
b.angle+=b.omega
#Rect Base Rotation
#b.rect.x += b.radius * b.omega * cos(b.angle + pi / 2)
#b.rect.y -= b.radius * b.omega * sin(b.angle + pi / 2)
#SCREEN.blit(ball,(b.rect.x,b.rect.y))

# Normal
Rotation-------------------------------------------------------------------------------------------------------------
b.x+=b.radius * b.omega * cos(b.angle + pi / 2)
b.y-= b.radius * b.omega * sin(b.angle + pi / 2)
SCREEN.blit(ball,(b.x,b.y))
pygame.display.update()

Post by Alan Gauld

Post by diliup gabadamudalige
x=42 good
x += 42 -x ?
That does not go by a mile of what I asked.

1) Why the += form gives different results from the assignment form,
2) Why some code you apparently wriotre worked outside a class
but not in a class
3) A specific question about some code for moving an object
in PyGame around in a circle
Number 1 is the only one I can comment on as I can't see your attachments.
That is what I (and several others) have answered.
They are fundamentally different. There is no general way
to make an augmented assignment such that
x = f(y) and
x += f(y) (or even g(y))
produce the same result except by the trivial
x += g(y) - x
Alan, are you telling me that there is no way that one can arrive at X2

Post by diliup gabadamudalige
which is the NEXT place that X1 will be after "some increment"?
old x position = X1

No, that is a completely separate question and has nothing
to do with using augmented assignment. But it does depend
on the specifics of the situation and is mainly a mathematical
problem rather than a Python one..
new x position= X1 + some increment value

Post by diliup gabadamudalige
which can be written as :new X = oldX + some increment value
which can be simplified to X += some increment value?

That may or may not be possible, it depends on what the
original equation in the pure assignment model is.
my orginal question was

Post by diliup gabadamudalige
*Why does the object NOT move in a circle when implemented as a class?*

is there a way to do
self.rect.x +*= some value*
self.rect.y += some value
rather than
self.rect.x = self.radius * math.sin(self.angle) + self.center_x
self.rect.y = self.radius * math.cos(self.angle) + self.center_y
Which is about the use of augmented assignment.
All the code was provided.
No it wasn't. I have not seen any of your code because you persist
in sending it as attachments which (as you've been told) do not
work reliably in a text based mailing list. If you want
comments on the code either post it inline in your message
or as a link to a web site where we can all see it.
The above is what I asked. (I hope this is clear enough)
Your class based code should work but without sight of it
I cannot begin to guess about that aspect of your problem.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
http://www.flickr.com/photos/alangauldphotos
_______________________________________________
https://mail.python.org/mailman/listinfo/tutor

--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

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and all copies from your system and notify the sender immediately by return
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dissemination of this e-mail or its contents is strictly prohibited and may
be unlawful. Internet communications cannot be guaranteed to be timely,
secure, error or virus-free. The sender does not accept liability for any
errors or omissions.
**********************************************************************************************
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[diliup gabadamudalige]

Thanks Alan.

It is partly due to curiosity but I also wanted to have a common update
method. And as I almost always use the x+= some code, y+=some code for
animation stuff I wanted to know how it could be done using this method
rather than finding each position separately and assigning with x= some
code, y = some code (new position).

But here is STILL something curious in this whole matter.

If I update the rect positions with

obj.rect.x+ = some code
obj.rect.y+ = some code

then the rotation are NOT circular.
BUT
if I use

obj.x += some code
obj.y += some code

and then do

obj.rect.x, obj.rect.y = obj.x, obj.y

then the rotation works correct!!
This is really weird and I have no explanation as to why this happens!!!
Can you or anyone else explain this strange behaviour? After all a position
is a position.(at least that;s how I understand as long as you stick with
the same coordinate through out.)
The code is below.

import sys, os, pygame, itertools
from math import sin,cos,pi
from pygame.locals import *
SCREENW = 800
SCREENH = 700
class object_factory(pygame.sprite.Sprite):
def __init__(self, image, xpos = 0, ypos = 0):
"""Constructor"""
pygame.sprite.Sprite.__init__(self)
self.image = image
self.mask = pygame.mask.from_surface(self.image) # pixelmask
self.rect = self.image.get_rect()
self.rect.x = xpos
self.rect.y = ypos
self.x=xpos
self.y=ypos
pygame.init()
clock = pygame.time.Clock()
os.environ['SDL_VIDEO_WINDOW_POS'] = "%d,%d" % (50,50) #Set window position
SCREEN = pygame.display.set_mode((SCREENW, SCREENH))
clock = pygame.time.Clock()
pygame.display.set_caption('rotating object')
ball = pygame.image.load("ball.jpg")
pin=pygame.image.load("center.jpg");
platforms = pygame.sprite.Group()
center_x = SCREENW/2
center_y = SCREENH/2
radius = 200
angle = pi/4 # starting angle 45 degrees
omega = 0.1 #Angular velocity
for _ in itertools.repeat(None, 6):
xpos = center_x + radius * cos(angle) #Starting position x
ypos = center_y - radius * sin(angle) #Startinh position y
obj = object_factory(ball, xpos, ypos)
obj.angle = angle
obj.omega = omega #angula velocity
obj.radius = radius
platforms.add(obj)
angle += pi/.5
#----------------------------------------------------------------------
while True:
clock.tick(24)
pygame.event.pump()
keys = pygame.key.get_pressed()
for event in pygame.event.get():
if event.type == QUIT or (event.type == KEYDOWN and event.key ==
K_ESCAPE):
pygame.quit()
sys.exit()
SCREEN.fill((0, 0, 0))
SCREEN.blit(pin,(center_x - pin.get_width() / 2, center_y -
pin.get_height() / 2))
pygame.draw.circle(SCREEN, (0, 0, 255), (center_x, center_y), radius, 2)

for b in platforms:
b.angle+=b.omega
#Rect Base Rotation
#b.rect.x += b.radius * b.omega * cos(b.angle + pi / 2)
#b.rect.y -= b.radius * b.omega * sin(b.angle + pi / 2)
#SCREEN.blit(ball,(b.rect.x,b.rect.y))
#Normal Rotation
b.x+= b.radius * b.omega * cos(b.angle + pi / 2)
b.y-= b.radius * b.omega * sin(b.angle + pi / 2)

b.rect.x, b.rect.y = b.x - b.rect.width / 2, b.y - b.rect.height / 2

#SCREEN.blit(ball,(b.x,b.y)) # this and the line below gives the
same result
#SCREEN.blit(ball, (b.rect.x, b.rect.y))

platforms.update()
platforms.draw(SCREEN)

pygame.time.wait(100)
pygame.display.update()

Post by diliup gabadamudalige
I do not understand how Alan does not get the code

Replying off list.
I don't get it because text based email systems often strip out attachments
since they are a security risk. As the name says they are *text* based.
However, now that I've seen it and I see your solution I can better
understand
what you were trying to ask, which was the solution to a very specific
scenario.
I'm still not sure why you wanted to use augmented assignment, other
than curiosity perhaps, but at least I now see what you were trying to do.
Thanks for posting the solution.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
http://www.flickr.com/photos/alangauldphotos

--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

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[Alan Gauld]

Post by diliup gabadamudalige
It is partly due to curiosity but I also wanted to have a common update
method. And as I almost always use the x+= some code, y+=some code for
animation stuff I wanted to know how it could be done using this method
rather than finding each position separately and assigning with x= some
code, y = some code (new position).

OK, Curiosity is good.
But the conversion is mainly a math challenge rather than a
programming one. And its a very seductive but dangerous mind
set to try to force a problem into a pattern that fits how
you usually do it. Especially if the transformed code looks
less intuitive than the original.

When I started in Python it didn't even have the += style
of augmented assignment, it was mainly introduced to save
some typing, nothing more.

Remember that += is still an assignment to x. It is not
significantly different or in any way 'better' than
the standard assignment. In languages like C '+=' (and
even more so '++' which doesn't exist in Python) used to
result in performance improvements but optimising
compilers have removed even that reason for using it.
Now it should just be considered a typing shortcut.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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[Danny Yoo]

On Wed, Apr 29, 2015 at 11:37 AM, diliup gabadamudalige

Post by diliup gabadamudalige
I do not understand how Alan does not get the code that is in this thread.

Hi Dilliup,

Please try to avoid using the phrase, "I don't understand how <X> does
not get <Y>...".


You might not realize it, but it's radioactive: it's has a very
negative connotation that is toxic to collaborative communities.

For more information on this, there's a very good book called Team
Geek that talks about these issues:

https://books.google.com/books/about/Team_Geek.html?id=Iwk_pKeBc9gC&hl=en

Also see Hacker School's Social Rules:

https://www.recurse.com/manual#sub-sec-social-rules


Thanks!
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[Dave Angel]

Post by diliup gabadamudalige
I do not understand how Alan does not get the code that is in this thread.

There are at least 3 ways of posting to "this thread":
A) email
B) newsgroup
C) googlegroups

and at least 5 ways of looking at "this thread"
A) email
B) email digest
C) newsgroup
D) googlegroups
E) various archives

To get messages from one region to another involves going through a
gateway, and most of them damage some of the messages going through. To
minimize the likelihood that what looks good on your screen will be
missing or different on mine or on Alan's, follow a few rules:

1) avoid html
2) avoid attachments

There are others, but those seem to be the biggies.

Many other guidelines will help readability and consistency, like not
top-posting, using proper quoting and attribution, giving enough
information but not too much, specifying the whole environment in the
FIRST message of a thread, etc.

[diliup gabadamudalige]

Thanks all for the very informative responses especially to Alan for being
descriptive.

I am now going to make my movement linear and move away from my current
circular one.

I hope a little bit of fun and "Thank you" emails fall into the order of
the day..

:)

Post by Dave Angel

Post by diliup gabadamudalige
I do not understand how Alan does not get the code that is in this thread.

A) email
B) newsgroup
C) googlegroups
and at least 5 ways of looking at "this thread"
A) email
B) email digest
C) newsgroup
D) googlegroups
E) various archives
To get messages from one region to another involves going through a
gateway, and most of them damage some of the messages going through. To
minimize the likelihood that what looks good on your screen will be missing
1) avoid html
2) avoid attachments
There are others, but those seem to be the biggies.
Many other guidelines will help readability and consistency, like not
top-posting, using proper quoting and attribution, giving enough
information but not too much, specifying the whole environment in the FIRST
message of a thread, etc.
--
DaveA
_______________________________________________
https://mail.python.org/mailman/listinfo/tutor

--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

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[diliup gabadamudalige]

for what ever it's worth I'd like to share the code below with you.
I got the code to do what I wanted. Object falls onto a rotating platform
and stays there without falling off.

video is here ->

import sys, os, pygame
from pygame.locals import *
from standard_object_creator import *
from math import sin, cos, pi, radians

SCREENW = 800
SCREENH = 700

pygame.init()
FPSCLOCK = pygame.time.Clock()

FONT1= "data\Cookie-Regular.ttf"

if sys.platform == 'win32' or sys.platform == 'win64':
#os.environ['SDL_VIDEO_CENTERED'] = '2'# center of screen
os.environ['SDL_VIDEO_WINDOW_POS'] = "%d,%d" % (10,30)#top left corner

SCREEN = pygame.display.set_mode((SCREENW, SCREENH))
pygame.display.set_caption('rotating object')

BLUE = (0, 50, 255)
BLACK = (0, 0, 0)
RED = (255, 0, 0)
PURPLE = (145, 0, 100)
yellow = (220,220, 0)

FPS = 160 # frames per second


platforms = pygame.sprite.Group()
boxes = pygame.sprite.Group()

## this is the center of rotation. objects will be placed on the radius
according to the step
cx = SCREENW / 2 # x pos in relation to screen width
cy = SCREENH / 2 # y pos in relation to screen height

## self, posx, posy, imagelist, speedx = 0, speedy = 0, value = 0
plat = pygame.image.load("platform.png").convert_alpha()
box = pygame.image.load("box.png").convert_alpha()

box = object_factory ([box], cx, cy - 300, 0, 2)

boxes.add(box)

RADIUS = 100 # distance from the center
angle = radians(0) # angular distance between objects
omega = 1
for x in xrange(1):
xpos = (cos(angle * x) * RADIUS) + cx
ypos = (sin(angle * x) * RADIUS) + cy
obj = object_factory([plat], xpos, ypos)
obj.cx = cx
obj.cy = cy
obj.difx = 0
obj.dify = 0
obj.angle = radians(60 * x) # position each object around (cx,cy)
obj.radius = RADIUS # distance from the center
obj.omega = radians(omega)
obj.offsetx = cos(obj.angle) * obj.radius
obj.offsety = -sin(obj.angle) * obj.radius
platforms.add(obj)

mouseposlist = []
okkoma = [platforms, boxes]
x = 0
l = []
while True:

SCREEN.fill(BLACK)
x += 1
##--------------------------------------------------------------
pygame.event.pump()
keys = pygame.key.get_pressed()
for event in pygame.event.get():
if event.type == MOUSEBUTTONDOWN:
if event.button == 1:
pos = pygame.mouse.get_pos()
val = [pos[0], pos[1], 0, 0]
print val
mouseposlist.append(val)
elif event.button == 3 and mouseposlist != []:
mouseposlist.pop(-1)

if event.type == KEYDOWN and event.key == K_ESCAPE:
print mouseposlist
pygame.quit()
sys.exit()

for hp in boxes:
hp.move()
hp.collide(platforms)

## MOVE THE SPRITE IN A CIRCLE center of rotation = cx,cy. Each object
is placed by varying the step)

pygame.draw.circle(SCREEN, BLUE, (cx, cy), RADIUS, 2)

for obj in platforms:

obj.angle += obj.omega # larger value increases speed ( angle gets
larger)
#obj.radius += .1 # this will make the object move in an expanding
spiral
obj.offsetx = cos(obj.angle) * obj.radius
obj.offsety = -sin(obj.angle) * obj.radius
obj.difx = obj.offsetx - obj.rect.x
obj.dify = obj.offsety - obj.rect.y
obj.speedx = round(obj.cx + obj.difx, 2)
obj.speedy = round(obj.cy + obj.dify, 2)
l.append([obj.speedx, obj.speedy])

obj.move()

for ekak in okkoma:
ekak.update()
ekak.draw(SCREEN)

## --------------------------------------------------------------------
pygame.draw.line(SCREEN, BLUE, (0, SCREENH / 2), (SCREENW, SCREENH /
2), 2)
pygame.draw.line(SCREEN, BLUE, (SCREENW / 2, 0), (SCREENW / 2,
SCREENH), 2)

pygame.display.update()
FPSCLOCK.tick(FPS)

pygame.time.wait(10)

print l

Post by diliup gabadamudalige
Thanks all for the very informative responses especially to Alan for being
descriptive.
I am now going to make my movement linear and move away from my current
circular one.
I hope a little bit of fun and "Thank you" emails fall into the order of
the day..
:)

Post by Dave Angel

Post by diliup gabadamudalige
I do not understand how Alan does not get the code that is in this thread.

A) email
B) newsgroup
C) googlegroups
and at least 5 ways of looking at "this thread"
A) email
B) email digest
C) newsgroup
D) googlegroups
E) various archives
To get messages from one region to another involves going through a
gateway, and most of them damage some of the messages going through. To
minimize the likelihood that what looks good on your screen will be missing
1) avoid html
2) avoid attachments
There are others, but those seem to be the biggies.
Many other guidelines will help readability and consistency, like not
top-posting, using proper quoting and attribution, giving enough
information but not too much, specifying the whole environment in the FIRST
message of a thread, etc.
--
DaveA
_______________________________________________
https://mail.python.org/mailman/listinfo/tutor

--
Diliup Gabadamudalige
http://www.diliupg.com
http://soft.diliupg.com/
**********************************************************************************************
This e-mail is confidential. It may also be legally privileged. If you are
not the intended recipient or have received it in error, please delete it
and all copies from your system and notify the sender immediately by return
e-mail. Any unauthorized reading, reproducing, printing or further
dissemination of this e-mail or its contents is strictly prohibited and may
be unlawful. Internet communications cannot be guaranteed to be timely,
secure, error or virus-free. The sender does not accept liability for any
errors or omissions.
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--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

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[Dave Angel]

Post by diliup gabadamudalige
Thanks all for the responses.
Charles Cossé - yes I can write a simple pygame program that makes a
sprite move in a circle but it may not be the style and order that many may
approve or accept. I consider myself a student. :)
No one has pointed out why the object moves in a circle properly in the bit
of code where it is addressed directly and why it does not when the same
code is applied thru a class.
I know that an object can be made to move around the circumference of a
circle by constantly calculating the point and giving the x and y values
1. x = some code
That is what I used and it worked. I was trying to find how we can do the
same thing by updating the current position by doing
2. x += some code

2. x += some different code

Post by diliup gabadamudalige
Above 1 works. 2 does not. Why is that?

Because there is always some little error in the value you're adding to
x, since it's a floating point value calculated using transcendentals.
If you're going to loop thousands of times, those little errors add up.

Sometime try measuring a mile using two 12-inch rulers, placing each one
before picking up the previous. By the time you're done, you'll probably
be off a hundred feet.

Post by diliup gabadamudalige
Both the same code.

Not at all the same.

Post by diliup gabadamudalige
The same variable is used with different names cause
some others were confused when they were the same.
I hope this time I have answered correctly. No more attachments. (e mail
or otherwise. :) )

I can't necessarily understand the code since you use pygame and I'm not
familiar with pygame. In your itertools loop, you always reset the
angle back to 100 degrees.

In your while loop, you issue a call to draw.circle(). Perhaps that's
the working code you're talking about.

[diliup gabadamudalige]

question to Lucas.

Not sure if this is what you're looking for, but I would also use cos for
the x coordinate as suggested. If:

self.rect.x = self.radius * math.cos(self.angle) + self.center_x
self.rect.y = self.radius * math.sin(self.angle) + self.center_y

Take a derivative (assume: angle += angular_vel * dt for every increment
in time dt), so that:

self.rect.x += -self.radius * math.sin(self.angle) * self.angular_vel * dt
self.rect.y += self.radius * math.cos(self.angle) * self.angular_vel * dt

what is dt?

Post by diliup gabadamudalige
Thanks all for the responses.
Charles Cossé - yes I can write a simple pygame program that makes a
sprite move in a circle but it may not be the style and order that many may
approve or accept. I consider myself a student. :)
No one has pointed out why the object moves in a circle properly in the
bit of code where it is addressed directly and why it does not when the
same code is applied thru a class.
I know that an object can be made to move around the circumference of a
circle by constantly calculating the point and giving the x and y values
1. x = some code
That is what I used and it worked. I was trying to find how we can do the
same thing by updating the current position by doing
2. x += some code
Above 1 works. 2 does not. Why is that?
Both the same code. The same variable is used with different names cause
some others were confused when they were the same.
I hope this time I have answered correctly. No more attachments. (e mail
or otherwise. :) )

--
Diliup Gabadamudalige

http://www.diliupg.com
http://soft.diliupg.com/

**********************************************************************************************
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not the intended recipient or have received it in error, please delete it
and all copies from your system and notify the sender immediately by return
e-mail. Any unauthorized reading, reproducing, printing or further
dissemination of this e-mail or its contents is strictly prohibited and may
be unlawful. Internet communications cannot be guaranteed to be timely,
secure, error or virus-free. The sender does not accept liability for any
errors or omissions.
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