Did you give your presentation yet, Alex?

Or did you already talk about that, and I missed it?

hey guys, do someone is familar with coordinate matrix rotation?

yeah

Nope but I'd like to learn more about it :)

look I'am making some kind of bouncing off an angle

using matrix coordinate rotation

about which axis ?

is it 3d?

x and y

2d

give me a minute

I'll explain

:)

Gotcha, very nice

so I have a line that is angled let's say 45 degrees or pi/4, and an object with x and y coordinates, so when the object collides with the line we rotate the whole system and make the calculations, that is working okay only when the angle of the line is in the right side, when I rotate it to the left side it works wrong

Look I'am using this in the android coordinate system, thus the top is negative angle, and the bottom is positive, so if the line is angled to the right side(which is less than pi/2 or more than -pi/2) the bouncing is done ok, but when the line is in the left side(which is more than pi/2 or less than -pi/2) the bounce is not right, cause it is rotating everything until the line is flat, and only then we make calculation

or please look at my question maybe it makes more sense gamedev.stackexchange.com/questions/100728/…

can't find enough information about this

I think it is about mathematics more than programming

but I doubt

maybe you've got some thoughts about this

@AlexClark Well, that doesn't sound so bad. He'll do all the boring stuff and you get the fun stuff. But are you implying you don't have a plan, or that you're worried he'll forget his lines?

@Kjelkor I tried to look, but I think it would be really helpful if you have a diagram of the issue going wrong, and for bonus points, a picture of what it should do

I have only 1 point there :D

well

1 moment reading this stuff @Kjelkor

I don't know if you understand the code

but I hope)

ye it looks like c++

it is java

@AlexClark Ah, I see. Well, you really can't worry about that too much. You could prepare a "baby" example if he goes too long, or elucidate on why your real example is actually cool, if he doesn't go long enough.

maybe the angle doesn't understand negative angles ?

that is positive only

so what happens exactly @Kjelkor when you go more than PI/2

I rotate everything to the right

until the line is at 0 angle

and the coordinates are rotated too

why not do it directly ?

as it is was flat

@AlexClark Put on the Rocky soundtrack, and get some math done, I like it!

like what you do when angle is less than PI

yes, no matter at what angle I'am what happens is that the line, the object, and the velocity of the object is rotated to the right, thus until the line is 0 degree

but

that works good only when the object is to the right side of the point where the line start

if it is to the left side

that rotation is not valid as I see

and the question is

its probably some programming issue how are you representing your rotation

no, this is exactly a mathematical problem

this looks good

I just need the idea

yes it is right

it works

no problem

but

how do think about it mathematically?

thus should I always rotate it to the 0 angle?

even if I wasn't used any programming language

can you tell me what is the idea of your game?

like what are you doing

just a ball

ball is an object

x y coordinates

alright

it bounces off an angle

oke

thus it bounces of a line

but that line is not flat, at may be at any angle

45

135

alrightt

so

I use this formula

yeah for rotations

cause it is the most efficient

so

if the line is at 45 degrees

we rotate it to 0 degrees and make the ball bounce

then we rotate everything back

if the line is at 135 degrees

the same is happening

but

the ball is falling from above

at 45 degrees it bounces okay

oh I see

but if the line is at 135 degrees

oh ok I understand

I guess the rotation then should be flipped

if it is at 135

thus at 180?

for example

when it is at 135

you want to rotate the thing to 0 back right?

yes

so you could either flip your rotation instead of rotating anti clockwise you rotate clockwise at angle of 135 degrees

or you could rotate it

but the rotation affect not only the line, the ball will be rotated too, but it will apear bellow the line, not above as you may see

at angle of 360 - 135 = 225

right?

I guessing what happens now to your game is that rotation isn't really done right

there are no 360

there are only 0, 180, -180, 0 again)

what is the glitch that happens now to your game if your ball act at negative angles ?

I am guessing the rotation still rotate but it doesn't go back to 0 right?

if the ball and the line is at the right side(I mean from the point where the line begins) it is all okay

yes

thus it may be negative or positive angle

but what happens to left side

when it is about the left side, the line is being rotated to 0 coordinates and the ball too, after that the ball appears below the line, even if visualy it was above it, this is the moment of problem

thus

no matter at angle we are

the same thing happens

give me a minute :)

I'll think

yeh because I guess we rotate your system at anti-clockwise angle

if you rotate it at clock-wise angle when you go to left side it will work out

just add an if statement there

and see what happens

ah

no no

but the ball will be rotated too

even if it is clock-wise

and again it will appear below

I can't figure this moment...

I don't know maybe if you could add a picture and show me what is happened

that would be good

sec I'll post

can I add here images?

postimage.org

so first: postimg.org/image/fzq2ta4yv

then we rotate it: postimg.org/image/knrf3vrqt

now imagine the same, but the line is rotated much more that is in the left side

at some moment the ball is above

and the whole thing is rotated

even if the ball is above at this moment

it will appear below after the rotation

but in the book its not said about that

I'am stuck(

or if we say it like this

let's assume that the line is at 180 degrees

thus is the same as 0 degrees

but the on the left side

the ball is above the line

but then we make the rotation and it is at 0 degree, the ball will be rotated too and no matter how we rotate it, clock-wise or anti-clockwise, it will appear below

and that is not right

my brain is blowing((

no need to abuse the enter key

sorry, it is because I play games, there people abbuse key enter))

bad habbit)

I can't got that all messages in my head and then write it all, I write step by step, thought by thought, isn't that right?)

Haven’t read what you wrote, but I can tell you are quite the poet

hm I don't know

lol

well

thats okay

no one know LOL

but if you ask maybe at mse post maybe some people could help you.

yeah

algebra is actually most fun even more than functional analysis

topology is very nice too

yeah

I didn't do so far just couple reading from munkrees very nice though the ideas of it

number theory too I liked it alot

algebraic number theory I didn't dive into analytic number theory yet.

you'll like it if you take algebraic number theory class

nice

I want to take algebraic topology in my winter semester

no it is 4 month semester

Paul Plummer any ideas?

did you read the above messages?

@AlexClark our year work as follows we have 2 semesters fall and winter each is 4 month interval. I don't know how is it different in Australia.

No

no, no ideas?

or didn't readed

Yes

emmm

yes what?)

@AlexClark Maybe a bit, but like the poem?

oh I see it is like us nice I guess seasons there are different so that is why you have it like this. @AlexClark

nice problem

If n = 2k is even and n $\geq$ 4, show that z = $r^k$ is an element of order 2 which commutes with all elements of $D_{2n}$. Show that z is the only non-identity element of $D_{2n} which commutes with all elements of $D_{2n}$

@Kjelkor supposing that your ball is traveling in direction x,y and the angle of your line is at angle t, To reflect it off the line the new direction a,b you could use: a=x cos(2t)+y sin(2t) and b=x sin(2t)-y sin(2t). Maybe this will help.

@AlexClark Hmm maybe "Coffee shop amateur"

let y be an arbitrarily element of $D_{2n}$ so $y = s^{i}r^{j}$ $\rightarrow$ $yz$ = $s^{i}r^{j}r^{k}$ = $s^{i}r^{k}r^{j}$ = $r^{-k}s^{i}r^{j}$ = $r^{k}s^{i}r^{j}$ = $zy$

now we need to show it is the only element such this happens but I will go eat something first then we will do it

I thought Paul's poem was free-form: There were so many un-closed left parentheses!

That represents my incomplete life @pjs36

is floored

why why @PaulPlummer

I thought your life is complete with us around

You guys are not quite right parentheses :D

lol

You guys are my left parentheses

what @paul says is right but he didn't leave anything left to say :))))

Hey guys it's fine :) you can actually say things if you want :) I didn't mean my joke to shut down the whole conversation lol.

Well I'm new to the chat rooms so I'm not totally sure of the rules. I did read the "Chat Guidelines".

@Paul and will it be poetry?

Well great! I love math and I also like "off topic".

@AlexClark I don't know. I have been thinking about what I want to lead in with, and think I have decided, so it will probably be a week or two. Although I would like to get the site "fully" designed before my first real post.

@MathMan Probably not :P

What David?

Oh yah, it has been a while

Moving and job is probably keeping him busy, he found his right parentheses probably

@PaulPlummer do you know what you're writing about?

I have an idea @mixedmath.

@mixedmath We talked about that crank mathematician (Prastaro) a few days ago. Its strange, because he seems to have actually done math research in the past. (Just thought of that when you came in)

@AlexClark here's some of the things I am interested in: Mathematics and programming. I am currently working on a degree in mathematics. I also like web development: php, html, javascript, regular expressions and so forth. I like fractals and the Chaos theory also.

To kinda introduce myself.

@PaulPlummer oh, you and I did? I totally forgot

sorry

yes, he's a big mystery to me

oh! right - you commented on that old answer of mine

I don't think you were here

when we were talking about it

I meant the chatroom

not you and I, haha

@AlexClark I definitely love algebra. especially fake proofs.

@mixedmath I think I will lead in with the first bullet point ( here ), something called selection principles, since it should give me more time to absorb the rest of the stuff.

@PaulPlummer Ah, I had to go find that old answer of mine. I assumed it was you. Just two days ago, someone commented on a 3 year old answer of mine about Agostino Prastaro

Hello/

Hello @Owatch

hiya @Owatch

So, is anyone familiar with modelling differential equations?

@PaulPlummer might you consider also putting your post on the MSE blog?

I really don't understand what the book is asking for when they tell me to show that every member of the family of function X is a solution of the differential equation Y.

In general, you come up with an idea. You write something down.

@AlexClark If it's reasonable, poof!

@mixedmath Oh, never thought about it. Are there any sort of requirement, or have to pitch the idea?

@AlexClark The blog doesn't really have a direction yet, so there's not much more to say. Some of the other sites' blogs have meaning and direction, so contributing to them is a bit more meaningful in some way

@Owatch Hmm what x and y?

@Owatch It means that for every function f in X, it satisfies the differential equation Y

for instance, the differential equation f' = f is satisfied by every function in the family e^x + c, for any constant c

They tell me to show that every member of the family of functions $ y = \frac{1 + ce^{t}}{1 - ce^{t}}$ is a solution of $y' = \frac{1}{2}(y^{2}-1)$

@mixedmath Or should I just come with something already written, and then have it reviewed.

They differentiate y =, which makes sense I guess

@PaulPlummer In principle, it's a good idea to make sure that the piece would fit before writing it. But in this case, you'll already have it written - so the absolute worst thing that could happen would be someone saying nope - doesn't fit

and that's really not bad at all

(and has only happened twice)

@PaulPlummer That would be totally fine

@AlexClark Yep! Go for it

Ah!

I get it

`The found $y'$ from $y$, then they plugged that into the initial $y'$ function they gave me. Then for y, they put in the original undifferentiated equation.

@AlexClark Actual questions of ownership are... unknown to me. I'd assume that contributions to the blog are under CC-BY-SA. You can do with it whatever you'd like

@AlexClark yeah, totally!

I didn't see that, I was wondering where the hell they were getting those equations. \

@Owatch handy. Are you starting a diff eq. class over the summer?

@mixedmath: I told you a while ago I'd write for the community blog, but instead I started my own blog, where there's no fear of audience considerations or length problems... oops

@mixedmath No.

I actually forgot about the community blog until just now.

@MikeMiller all fine to me. I write much more for myself than I do for the community blog. It's not yet clear that it will be successful

but it might be a nice way for someone who is just starting a blog to inject themselves into the mathematical blogosphere, as it were

Eh, still can't solve the problem.

@mixedmath: Looking at it again, I think both of those reasons for starting a separate blog are valid; what I want to write would be out of place

@MikeMiller would it?

What's a length problem?

Checking Tobias's post, there's actually no worry about being restricted by length. The more meaningful worry, then, is that I'm probably assuming some background in graduate-level topology.

@mixedmath I will definitely think about submitting something to the MSE blog. I am guessing the main content requirements is to be relatively self contained and cover something that should end up interesting. What caused those two previous ideas to not work?

@MikeMiller I understand this worry. In principle, there's no reason why it couldn't go on the blog (since we have no audience in mind). But in practice, I have also not thrown anything onto the community blog at graduate level, even though that's what I write on frequently

> What caused those two previous ideas to not work?

one was a crank post

so that was easy

the second (which came first chronologically) was less easy. I can actually find a copy of it - give me a moment

I've been to the blog room once or twice so can guess what that is

@PaulPlummer It's here

suffice it to say you probably won't have the same trouble, @Paul

tell me, would you have that on your blog?

@MikeMiller yes, he's still angry about it. Further, he places the blame solely on me. It's not conducive to good discussion

@mixedmath Haha, okay, I don't think I will be having something like that on my blog

what was the crank post about

@MikeMiller It was about twin primes, I believe.

I wonder if it was Prastero trying to get something past you!

@PaulPlummer Somehow, it all connects today, doesn't it?

Yes, yes it does

Yes, perentheses must always be completed.

:)

@mixedmath haven't done much blogging yourself lately, I see

@MikeMiller That's true. I actually have a string of posts lined up, but I'm waiting for a draft of my current paper to be ready to become public

I have a set whose goal is to allow this paper to be completely understandable, and that actually show the background computations

(analytic number theory - it's complicated and messy)

@paul /.*\ (.*\ ).*$/ to match the parentheses :)

can I assume the experts in your field won't need to read the blog post before understanding your paper...?

@MikeMiller Right. But there's rather a lot of computations that doesn't make it to final publications. I bet the reviewers will be thankful

for that matter, I'm reviewing a paper right now. I would certainly benefit from more explicit computations

ah, right

hi @MichaelA

@MikeMiller actually, I also see that a blog post is queued to appear on the blog tonight sometime, and it's actually mine

Hi @MikeMiller.

@MathMan Ahh, is that some code?

Was that the author of the post that left a minute ago?

That's the regular expression to balance sets of parentheses in a string. But here's a better regular expression - more versatile: /^\ ((?>[^()]|(?R))*\ )$/

I told the code guy to post an example of what his code does wrong, I'm not going to dig through it and see what's happening... but if I have an example of what's wrong, it's much easier to pinpoint. But, c'est la vie!

chat.SE says he hasn't been seen for four hours @Paul

formidablee

the guy paul was trolling

@pjs36 do you mean Kjelkor?

Aye, @MathMan. I would have tried to help, I just didn't want to take the time to read and understand all of the code

@MikeMiller Okay, I guess the gravitar just looked alike

Oh and Rick and Morty is genius @MikeMiller

How do I cite an answer to a question in an essay? Is it a website, or a correspondence with the answerer?

Your mixing me up with Kjelkor I think.

IOW what's the way to cite stack exchange in a scholarly work?

@PaulPlummer New season this summer, catch up before then

How much have you watched

Lol

That's Kjelkor who posted the question [here]{gamedev.stackexchange.com/questions/100728/…}

@MikeMiller I actually binged watched them yesturday

haha nice

Right! I didn't know how to do it. Thanks.

@PaulPlummer Did you have a favorite

Don't force your grammar on us, Incurrence! By the way, if I ping @Incurrence, what happens? :)

alex

how come you don't use your old account ?