gdalgorithms-list

Re: [Algorithms] Can I do this?

[Todd S. <to...@so...>]

On Tue, 25 Jul 2000 20:02:25 +0200, you wrote:

>(I know that I can always use a popup to warn the user that resources are
>low, but I figured it would be nice behaviour if the application assists the
>user in freeing up the necessary resources...)

I would just show an icon that indicates low memory like network games
do to indicate a poor or lagged connection.

If you prompted "Do you want to close some applications?" and the user
prompted yes which causes you to kill the 100mb download they've been
downloading for the last day that's at 99.99%?

Todd

-----------------------------------------------------------------
Todd Smith               | Email:   to...@so...
Solid Concepts, Inc.     | WebSite: http://www.solidconcepts.com
Valencia, CA             | WebSite: http://www.solidview.com
Phone: (661) 257-9300    | Fax:     (661) 257-9311
-----------------------------------------------------------------

RE: [Algorithms] How to derive transformation matrices

[Steve W. <Ste...@im...>]

> -----Original Message-----
> From: Klaus Hartmann [mailto:k_h...@os...]
> Steve,
> I'm not native to the English language, but I think it's 
> possible that you
> are confusing "shear" with "sheer"!?
> 

No...I mean shear.

> According to my English dictionary, "sheer" can be used as an adverb:
> B.1 Completely, absolutely, directly,
> B.2 Perpendicularly, very steeply, straight up or down
> 
> The English dictionary uses the term "shear" mostly for 
> 'parallel' things,
> like:
> 1. The bar, or either of the two parallel bars, forming the 
> bed of a lathe
> 2. "shear plane". [Geol.] a boundary surface between bodies 
> of rock or ice
> which have experienced relative motion parallel to the surface.
> 3. and a lot more samples, that include the term "parallel".
> 
> Niki
> 

Yes, a "shear plane" describes the plane where two surfaces move by each
other.  A force causing these surfaces to move is called a shear force and
is parallel to the shear plane, however it is perpendicular to the surface
of contact.  A dictionary sometimes doesn't provide what you might learn in
an engineering textbook.

Consider the following box and a force acting on it perpendicular to the
surface of contact:

                                         -------------------------
                   shear force --------> |                       |
                                         |                       |
                                         |                       |
                                         -------------------------

This is what happens if the box breaks apart with the top half moving and
the bottom half staying:

                                         -------------------------
                   shear force --------> |                       |
                                        -------------------------  this is
the shear plane
                                        |                       |
                                        -------------------------

That is what I learned in engineering about shear forces...I could be wrong,
but I think the physics is correct. I have a friend that went to the same
class and he argued for hours that the force was parallel, with the same
argument that you gave...that the text used the word parallel.  It's not
always the definition that counts, it's mostly the application of the
knowledge that makes the money.  If you want to handle shear planes, then
you need to use only the component of a force that is perpendicular to the
surface of contact to determine if shear occurs...afterwards if it does
occur then you will need to use the parallel component to determine the
resultant forces on the separated portion.

But, again I don't think the original poster (OP) has an object box breaking
apart and just wants to deform it, like turning a box into a trapezoid...in
other words a geometry solution and not a physics solution...so my bringing
up shear forces has gotten this thread off track.

R&R

R&R

Re: [Algorithms] Can I do this?

[Adam W. <ad...@ar...>]

You can request other apps to quit with WM_QUIT (much like windows does when
it performs a shutdown), but you can't force them short of a
TerminateProcess (which leaves all sorts of loose ends). Most well behaved
apps will comply, some of them performing their "Unsaved work" type dialogs.
Some ignore you though. Depends on the app.

adamw

----- Original Message -----
From: "Giovanni Bajo" <ba...@pr...>
To: <gda...@li...>
Sent: Tuesday, July 25, 2000 9:48 PM
Subject: RE: [Algorithms] Can I do this?


> apart "moral" aspects, can someone confirm that an application can
shutdown
> (via WM_QUIT) other apps running under Win2K?
> ---
> Giovanni Bajo
> Lead Programmer
>
> Protonic Interactive
> www.protonic.net
>
> a brand of Prograph Research S.r.l.
> www.prograph.it
>
>
> > -----Original Message-----
> > From: gda...@li...
> > [mailto:gda...@li...]On Behalf Of jason
> > watkins
> > Sent: Tuesday, July 25, 2000 10:46 PM
> > To: gda...@li...
> > Subject: Re: [Algorithms] Can I do this?
> >
> >
> > Bad.. bad.. bad..
> >
> > you quickly need to get out of the frame of mind that you and your
program
> > know what to do better than the user. the role of your program is to be
> > subserviant to the users wishes and demands.
> >
> > picture this.. I'm a 28.8k modem user, downloaded the lastest 250+meg
demo
> > in the background.. I fire up your game to pass the time, since
> > the download
> > will take all day. Your game spots my "save as" as a resource hog
> > because it
> > keeps doing lots of GDI stuff to make the little animation go on,
> > and so on.
> > So, it sends a WM_QUIT to it, and my Save As prompted dies,
> > leaving me with
> > nothing to show for the last 5 hours my download ran.
> >
> > at this point, i'd do 3 things: uninstall your game, tell every website
in
> > the universe just how crappy it is, and then research the feasability of
> > comming and killing you. after all, I could accomplish all that
> > in less time
> > than it'll take me to redownload 1/2 of that danged demo.
> >
> > now, as for things useful.. what you *can* do is inform the
> > user.. if you're
> > really worried about the situation, throw up a error or a simple notices
> > that says "it appears system resources are low due to many background
> > applications. please close some of them and running mygame.exe again for
> > better performance."
> >
> > then the user knows what's going on, and can make the call.
> >
> >
> > _______________________________________________
> > GDAlgorithms-list mailing list
> > GDA...@li...
> > http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list
> >
> >
>
>
>
> _______________________________________________
> GDAlgorithms-list mailing list
> GDA...@li...
> http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

RE: [Algorithms] How to derive transformation matrices

[Steve W. <Ste...@im...>]

> From: Ron Levine [mailto:ro...@do...]
> 
> But again, this is a huge digression from the issue of deriving
> transformation matrices. although the ideas are certainly 
> important in the sort of physical modeling that people are now doing in
games. 
> 

Roger that...I show the shear force is perpendicular to the surface of the
object, then I get shown that it's parallel to the shear plane...both
correct, but doesn't help solve the problem of deforming a 3D object using
transformation matrices...which I know nothing about.

R&R

RE: [Algorithms] Can I do this?

[Giovanni B. <ba...@pr...>]

apart "moral" aspects, can someone confirm that an application can shutdown
(via WM_QUIT) other apps running under Win2K?
---
Giovanni Bajo
Lead Programmer

Protonic Interactive
www.protonic.net

a brand of Prograph Research S.r.l.
www.prograph.it


> -----Original Message-----
> From: gda...@li...
> [mailto:gda...@li...]On Behalf Of jason
> watkins
> Sent: Tuesday, July 25, 2000 10:46 PM
> To: gda...@li...
> Subject: Re: [Algorithms] Can I do this?
>
>
> Bad.. bad.. bad..
>
> you quickly need to get out of the frame of mind that you and your program
> know what to do better than the user. the role of your program is to be
> subserviant to the users wishes and demands.
>
> picture this.. I'm a 28.8k modem user, downloaded the lastest 250+meg demo
> in the background.. I fire up your game to pass the time, since
> the download
> will take all day. Your game spots my "save as" as a resource hog
> because it
> keeps doing lots of GDI stuff to make the little animation go on,
> and so on.
> So, it sends a WM_QUIT to it, and my Save As prompted dies,
> leaving me with
> nothing to show for the last 5 hours my download ran.
>
> at this point, i'd do 3 things: uninstall your game, tell every website in
> the universe just how crappy it is, and then research the feasability of
> comming and killing you. after all, I could accomplish all that
> in less time
> than it'll take me to redownload 1/2 of that danged demo.
>
> now, as for things useful.. what you *can* do is inform the
> user.. if you're
> really worried about the situation, throw up a error or a simple notices
> that says "it appears system resources are low due to many background
> applications. please close some of them and running mygame.exe again for
> better performance."
>
> then the user knows what's going on, and can make the call.
>
>
> _______________________________________________
> GDAlgorithms-list mailing list
> GDA...@li...
> http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list
>
>

Re: [Algorithms] Can I do this?

[jason w. <jas...@po...>]

Bad.. bad.. bad..

you quickly need to get out of the frame of mind that you and your program
know what to do better than the user. the role of your program is to be
subserviant to the users wishes and demands.

picture this.. I'm a 28.8k modem user, downloaded the lastest 250+meg demo
in the background.. I fire up your game to pass the time, since the download
will take all day. Your game spots my "save as" as a resource hog because it
keeps doing lots of GDI stuff to make the little animation go on, and so on.
So, it sends a WM_QUIT to it, and my Save As prompted dies, leaving me with
nothing to show for the last 5 hours my download ran.

at this point, i'd do 3 things: uninstall your game, tell every website in
the universe just how crappy it is, and then research the feasability of
comming and killing you. after all, I could accomplish all that in less time
than it'll take me to redownload 1/2 of that danged demo.

now, as for things useful.. what you *can* do is inform the user.. if you're
really worried about the situation, throw up a error or a simple notices
that says "it appears system resources are low due to many background
applications. please close some of them and running mygame.exe again for
better performance."

then the user knows what's going on, and can make the call.

RE: [Algorithms] Can I do this?

[Steve W. <Ste...@im...>]

> From: Jim Offerman [mailto:j.o...@in...]
> 
> (I know that I can always use a popup to warn the user that 
> resources are low, but I figured it would be nice behaviour if the 
> application assists the user in freeing up the necessary resources...)
> 

Your approach is commendable...please be sure to send your idea to the
microsoft wish list...I think it's wi...@mi... or something like
that.  An algorithmic answer to your question might be that you're doing the
right thing by not causing that gray "out of resources" dialog box to pop
up.  I've never found that dialog box to go away no matter if I close down
every little process running except for kernel and winexplorer...so avoiding
it would be the best algorithm...however, checking for it would slow your
app down.

R&R

RE: [Algorithms] Tris intersection

[Robert D. <RD...@ac...>]

In that case go back to the posting which gave the reference, I can't
remember where it was to, but the coplanar part was only a small section of
the stuff, it was in reality a full blown three dimensional triangle
intersection routine.

Rob

-----Original Message-----
From: Steve Wood [mailto:Ste...@im...]
Sent: 25 July 2000 18:48
To: 'gda...@li...'
Subject: RE: [Algorithms] Tris intersection


> From: Robert Dibley [mailto:RD...@ac...]
> 
> Yup, thats what that suggestion from someone else does, in 
> coplanar cases -
>

Code word there is coplanar...that's why I brought up the area test since
that's what I use in my 3D collision detection, but I'm sooo glad you asked
that question; cause now that I think about it I'm not testing for the case
where the line is in the same plane...DOH!  

R&R

_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

Re: [Algorithms] How to derive transformation matrices

[Ron L. <ro...@do...>]

----- Original Message -----
From: "Steve Wood" <Ste...@im...>
To: <gda...@li...>
Sent: Tuesday, July 25, 2000 10:58 AM
Subject: RE: [Algorithms] How to derive transformation matrices


> > From: ro...@do... [mailto:ro...@do...]
>
> [snip]
> >
> > > If that is what you meant then a shear is
> > >the perpendicular component of force on a surface.
> >
> > Although incompletely stated,  I believe you have this wrong.  The
> > shear comes from the force component parallel to the surface, not
> > perpendicular.
> >
>
> uh...no, it's the perpendicular.  Forces parallel to a surface do not
> interact with the surface (actually, there would be no resulting force for
> any actions parallel to a surface) unless the perpendicular force causes
> penetration so the parallel component then has something to "grab onto".
>

Not in the classical picture (as opposed to the quantum mechanical picture).
Consider the force of friction of a flat table on a rectangular block being
pushed along it.  That frictional force is parallel to the interface, not
perpendicular.  True, in the quantum mechanical detail of the intersurface
forces at the molecular level , there is some interpenetration of the outer
layers of the electromagnetic fields of the surface molecules of the two
interacting materials, but at that level of detail, the notion of "surface"
becomes fuzzy.  Also true that, in the classical picture,  the magnitude of
that frictional force (parallel to the interface) does depend on the normal
force.

Now,  if the force pushing the block along is applied above the level of the
interface, then it is the couple of the pushing force and the frictional
resistance, both parallel to the table, that produces the shear force, and
the shear deformation if the body is deformable.  The perpendicular force is
simply equal and opposite to the weight of the block and produces no shear
deformation.

But again, this is a huge digression from the issue of deriving
transformation matrices. although the ideas are certainly important in the
sort of physical modeling that people are now doing in games.  In those
models, frictional force is parallel to the interface.

Re: [Algorithms] Can I do this?

[Michael S. H. <mic...@ud...>]

This should really be on a design list, but the short answer to your question is that any program that does this is going to be un-installed so fast, it's little pointy head will spin.

As far as I'm concerned, no program has permission to shut down any other program *for me*.

On the other hand, a program which will go to the effort to figure out what other program is hogging resources and *recommend* that I shut it down would be welcome as long as I can tell the complaining program to shut up.

At 08:02 PM 7/25/00, you wrote:
>I'll save you guys the lengthy situation introduction (my situation
>introductions tend to end up being very logn, but it is available on request
>though...) and get right down to the point:
>
>Can I have my application request a few applications to shut down if my
>resources are below a certain level? (I.e. my application is running on a
>machine with 64 Megs-o-RAM, but since the user has also running, say, 3D
>Studio MAX in the background, nearly 90% of system's resources are already
>used and locked away - trust me these things happen and if you have a little
>bad luck will even completely trash the system, even if all running
>applications are essentially stable) I stress the 'request' bit, as in
>sending some applications (.exe files only) the same window message Windows
>uses when you choose to switch off your computer, there is no way an
>application could get away with just 'killing' other applications.
>
>Is this a reasonable solution, or I am just being selfish?
>
>(I know that I can always use a popup to warn the user that resources are
>low, but I figured it would be nice behaviour if the application assists the
>user in freeing up the necessary resources...)
>
>Jim Offerman
>
>Innovade
>- designing the designer
>
>
>_______________________________________________
>GDAlgorithms-list mailing list
>GDA...@li...
>http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list 



  Michael Harrison
  United Developers / Inertia, LLC
  Director of Development
  Development Lead for RC Simulator
  214-855-5955 x 102
  Work log @ http://lynx.inertiagames.com

Re: [Algorithms] How to derive transformation matrices

[Klaus H. <k_h...@os...>]

----- Original Message -----
From: Steve Wood <Ste...@im...>
To: <gda...@li...>
Sent: Tuesday, July 25, 2000 7:58 PM
Subject: RE: [Algorithms] How to derive transformation matrices
>
> uh...no, it's the perpendicular.  Forces parallel to a surface do not
> interact with the surface (actually, there would be no resulting force for
> any actions parallel to a surface) unless the perpendicular force causes
> penetration so the parallel component then has something to "grab onto".

Steve,
I'm not native to the English language, but I think it's possible that you
are confusing "shear" with "sheer"!?

According to my English dictionary, "sheer" can be used as an adverb:
B.1 Completely, absolutely, directly,
B.2 Perpendicularly, very steeply, straight up or down

The English dictionary uses the term "shear" mostly for 'parallel' things,
like:
1. The bar, or either of the two parallel bars, forming the bed of a lathe
2. "shear plane". [Geol.] a boundary surface between bodies of rock or ice
which have experienced relative motion parallel to the surface.
3. and a lot more samples, that include the term "parallel".

Niki

RE: [Algorithms] How to derive transformation matrices

[Steve W. <Ste...@im...>]

> From: Andrew Howe [mailto:an...@co...]
> 
> "begging the question" is a logical fallacy, also known as circular
> reasoning.
> (See 
> http://www.nizkor.org/features/fallacies/begging-the-question.html).
> 
> I can't continue to parse your post because you seem to have 
> used incorrect
> terms.
> 
> etc. ;)
> 

I hope my sincere appologies are accepted for any ill feelings I caused.
Apparently I should not use the phrase "begging the question" since many
people now consider it to be something other than it's intended
construction, sorry.  I use the phrase "begging the question" to mean that
the subject matter of a solution includes a topic that raises another
question...the solution is said to "beg the question".  Rons answer brought
up the question in my mind that he might be asking about shear forces.

At first I thought the web site you gave was loonie tooney, however, I also
found some of the same info on the phrase at:
http://search.britannica.com/frm_redir.jsp?query=begging+the+question&redir=
http://www.drury.edu/faculty/Ess/Logic/Informal/Overview.html


R&R

RE: [Algorithms] Can I do this?

[Brian S. <Bri...@Re...>]

That's generally considered to be bad practice (a faux pas if you will).
You don't really know what the user is doing with essentially anonymous (to
you) applications, and if they want them to be closed.  I believe there is a
rule in Windows certification (logo qualification or somesuch) that mentions
this, or at least mentions something tangentially related.

If there's a serious resource problem, the user is most likely to be aware
of it.  If you absolutely must, then you shouldn't do any more than indicate
that there might be a resource problem that could cause poor performance (a
dialog consisting of "Continue" and "Quit" buttons, and a "Don't Show Me
This Crap Again, I Know What I'm Doing" check box).

"Helping" the user is really just silly, as they might not want to close all
their running applications.  Besides, it's probable that some applications
are going to pop up a Save As/Save Changes, Confirm Quit, or some other form
of dialog asking for user input anyways, so let them take care of it.

Finally, it's not your problem (i.e., not a support issue) if they've
decided to tie up the majority of system resources and thereby have problems
running your program.  Add a FAQ entry to your manual if it makes you feel
better. :p

-Brian

-----Original Message-----
From: Jim Offerman [mailto:j.o...@in...]
Sent: Tuesday, July 25, 2000 1:02 PM
To: gda...@li...
Subject: [Algorithms] Can I do this?


I'll save you guys the lengthy situation introduction (my situation
introductions tend to end up being very logn, but it is available on request
though...) and get right down to the point:

Can I have my application request a few applications to shut down if my
resources are below a certain level? (I.e. my application is running on a
machine with 64 Megs-o-RAM, but since the user has also running, say, 3D
Studio MAX in the background, nearly 90% of system's resources are already
used and locked away - trust me these things happen and if you have a little
bad luck will even completely trash the system, even if all running
applications are essentially stable) I stress the 'request' bit, as in
sending some applications (.exe files only) the same window message Windows
uses when you choose to switch off your computer, there is no way an
application could get away with just 'killing' other applications.

Is this a reasonable solution, or I am just being selfish?

(I know that I can always use a popup to warn the user that resources are
low, but I figured it would be nice behaviour if the application assists the
user in freeing up the necessary resources...)

Jim Offerman

Innovade
- designing the designer


_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

RE: [Algorithms] How to derive transformation matrices

[Steve W. <Ste...@im...>]

> From: ro...@do... [mailto:ro...@do...]

[snip]
>    
> > If that is what you meant then a shear is
> >the perpendicular component of force on a surface. 
> 
> Although incompletely stated,  I believe you have this wrong.  The
> shear comes from the force component parallel to the surface, not
> perpendicular.
> 

uh...no, it's the perpendicular.  Forces parallel to a surface do not
interact with the surface (actually, there would be no resulting force for
any actions parallel to a surface) unless the perpendicular force causes
penetration so the parallel component then has something to "grab onto".

[Algorithms] Can I do this?

[Jim O. <j.o...@in...>]

I'll save you guys the lengthy situation introduction (my situation
introductions tend to end up being very logn, but it is available on request
though...) and get right down to the point:

Can I have my application request a few applications to shut down if my
resources are below a certain level? (I.e. my application is running on a
machine with 64 Megs-o-RAM, but since the user has also running, say, 3D
Studio MAX in the background, nearly 90% of system's resources are already
used and locked away - trust me these things happen and if you have a little
bad luck will even completely trash the system, even if all running
applications are essentially stable) I stress the 'request' bit, as in
sending some applications (.exe files only) the same window message Windows
uses when you choose to switch off your computer, there is no way an
application could get away with just 'killing' other applications.

Is this a reasonable solution, or I am just being selfish?

(I know that I can always use a popup to warn the user that resources are
low, but I figured it would be nice behaviour if the application assists the
user in freeing up the necessary resources...)

Jim Offerman

Innovade
- designing the designer

RE: [Algorithms] Tris intersection

[Steve W. <Ste...@im...>]

> From: Robert Dibley [mailto:RD...@ac...]
> 
> Yup, thats what that suggestion from someone else does, in 
> coplanar cases -
>

Code word there is coplanar...that's why I brought up the area test since
that's what I use in my 3D collision detection, but I'm sooo glad you asked
that question; cause now that I think about it I'm not testing for the case
where the line is in the same plane...DOH!  

R&R

Re: [Algorithms] How to derive transformation matrices

[<ro...@do...>]

Andrew Howe  wrote:

>From: "Ron Levine" <ro...@do...>
>Subject: Re: [Algorithms] How to derive transformation matrices
>
>> Guilty of begging the question,  not because I didn't know the answer,
>> but because it was getting late.  I can't necessarily answer every
>> question in onepost or one evening.
>
>"begging the question" is a logical fallacy, also known as circular
>reasoning.
>(See http://www.nizkor.org/features/fallacies/begging-the-question.html).
>
>I can't continue to parse your post because you seem to have used incorrect
>terms.
>
>etc. ;)
>

Right.  I was confusing the term with "begging off".  I am actually
proud of unbegging the question by insisting that a derivation has to
begin with a definition.  

[Algorithms] Cyrus-beck line clipping algorithm

[Phil Y. <ph...@cl...>]

I have just been looking through the definition of the Cyrus-Beck line
clipping algorithm, as defined in 'Procedural elements for computer
graphics' by David Rogers.

On talking about deriving the normals to the top, bottom and side planes of
the viewing frustrum, he talks about using the 'cross products of the
vectors from the centre of projection to the corners at z=0, the plane of
projection'.

However, he previously defines the view frustrum as a 'perspective volume
with (xl, xr, yb, yt, zh, zy ) = (-1, 1, -1, 1, 1, -1), with a centre of
projection at zcp = 5'.

He lists the vectors as:
	v1 = [ 1, 1, -5 ]
	v2 = [ -1, 1, -5 ]
	v3 = [ -1, -1, -5 ]
	v4 = [ 1, -1, -5 ]

Surely the plane of projection should be z = -1, and therefore the z
component in the above vectors should be -6?

Or am I (more likely!) missing something obvious here?

_____________________________________
Phil Yard, Lead Programmer, Climax (Brighton) Ltd
01273 764109  mob: 0771 258 1164  web: www.climax.co.uk

Re: [Algorithms] How to derive transformation matrices

[Andrew H. <an...@co...>]

From: "Ron Levine" <ro...@do...>
Subject: Re: [Algorithms] How to derive transformation matrices

> Guilty of begging the question,  not because I didn't know the answer,
> but because it was getting late.  I can't necessarily answer every
> question in onepost or one evening.

"begging the question" is a logical fallacy, also known as circular
reasoning.
(See http://www.nizkor.org/features/fallacies/begging-the-question.html).

I can't continue to parse your post because you seem to have used incorrect
terms.

etc. ;)

Re: [Algorithms] How to derive transformation matrices

[<ro...@do...>]

I  wrote:

>The point of nomenclature:  What you are calling the "invariant"
>plane, would be called the "fixed plane" by most math authors. Each
>point of the fixed plane is mapped to itself by the transformation, so
>it consists entirely of fixed points of the mapping.   Then each of
>the planes parallel to it would be called "invariant" under the
>transformation, because each is mapped to itself, that is each point
>in such a plane is mapped to another point in the same plane..
>

By way of clarifciation:  The fixed plane of the shear is also an
invariant plane of the shear.  But in standard mathematical
nomenclature, the other non-fixed planes which are mapped to
themselves are called "invariant" 

Re: [Algorithms] How to derive transformation matrices

[<ro...@do...>]

Robert Dibley  wrote:

>I think most people in this case take shear to mean a transformation in
>which there is an invariant plane, but everything outside that plane is
>moved within its own parallel plane, so viewing through the said invariant
>plane, you would see this :
>
>
>  pre-shear            post-shear
>
>  +------+             +------+
>  |      |            /      /
>--+------+----- -----+------+-------- invariant plane
>  |      |          /      /
>  +------+         +------+
>

That's pretty close to the way a mathematician would define it, but
your statement  needs a little sharpening  and a nomenclature
adjustment.   The sharpening: it's understood that "transformation"
implies "linear" of "affine", so that the displacements in all the
parallel planes are mutually parallel (or antiparallel) and are
proportional to the signed distance from the fixed plane. Your ASCII
art suggest the mutually parallel property  but does not in fact imply
it in the 3D case, because the artwork lacks one dimension.  Your art
does give a pretty solid indication of the displacement
proportionality property (at least in the font in which I am viewing
it), but again, art is not math.

The point of nomenclature:  What you are calling the "invariant"
plane, would be called the "fixed plane" by most math authors. Each
point of the fixed plane is mapped to itself by the transformation, so
it consists entirely of fixed points of the mapping.   Then each of
the planes parallel to it would be called "invariant" under the
transformation, because each is mapped to itself, that is each point
in such a plane is mapped to another point in the same plane..

OK, as promised, given the intrinsic definition I'll show you how to
get the matrix.  Again, the matrix depends on the coordinate system.
It's straightforward to write down the matrix in the case that the
mapping is nicely aligned with the coordinate system, i.e the case
that the fixed plane is a coordinate plane and the mutually parallel
displacement direction is in the direction of one of the coordinate
axes in that plane. Just as Martin indicated, you write down the
linear equations that express what we have just described in words,
and pick off the coefficients to get the matrix.  So it would be, for
the case that the fixed plane is the xy plane and the translation
direction is in the direction of the x axis

	1  0   c
                0  1   0
                0  0   1 

where c is a non-zero constant giving the amount of shear, the
proportionality constant relating the displacement in the invariant
plane to its z coordinate.  A lot simpler than for the axis-aligned
rotation, isn't it?.   
   
But of course, we want to be able to express the matrix  in ANY
coordinate system, not just a system aligned with the intrinsic
parameters of the mapping.  Just as with rotations, there are two
ways.  The first, for which it is messy to write down a general
formula but not hard to derive in a particular case: 

(1)  Find a transformation T that maps the xy plane of the coordinate
system to the fixed plane of the shear and the x axis to the
displacement direction of the shear, try to make it simple, e.g. (a
rotation, if the origin is in the fixed plane, a rotation and
translation if it is not), and work out the  similarity transformation
T^-1 S T, where S is the above matrix for the axis-aligned shear (or
its obvious extension to a 4x4).

The second, which I prefer because of its manifest coordinate
invariance

(2) Go back to the verbal description of the mapping and write it as a
vector geometric  equation.  Intrinsically, the shear mapping is
described by
	 a fixed plane, which is described by a point with position
vector P0 and a unit normal vector N,
	 a unit displacement vector U which is parallel to that plane,
so perpendicular to N, 
	and a scalar shear constant c, which gives the rate at which
the displacement grows as you move  perpendicular to the fixed plane.

Let V be the position vector of any point.  Writing down the vector
equation described by the above verbal description is fairly direct.
The result is that  V is mapped to a point with position vector V'
given by 

	V' = V +  c ((V - P0) dot N) U 

Voila.  A manifestly coordinate invariant vector expression for the
shear mapping.  You get the 4x4 matrix  with respect to ANY coordinate
system as follows.  Express P0, N and U in their components with
respect to the given coordinate system.  For the translation part of
the 4x4  matrix, substitute (0,0,0) for V. The resulting components of
V' = -c(P0 dot N)U give the first three elements of the last row or
column of the 4x4 affine matrix.  For the upper left 3x3 linear part,
successively substitute the basis vectors (1,0,0), (0,1,0), (0,0,1)
for V.  Then the rows or columns of the upper left 3x3 part are formed
by the respective resulting components of V'.

>Useful for certain effects, but in general not a good thing because unlike a
>rotation matrix it doesn't preserve your normals.
>

True.  Also, the inverse is no longer the transpose (of the 3x3 linear
part), and other nice properties of orthogonal matrices are also lost.
But you can still  recover the normal of a transformed surface by
applying the inverse of the transpose of the 3x3 linear part to the
corresponding normal of the untransformed surface.

Re: [Algorithms] How to derive transformation matrices

[<ro...@do...>]

Steve Wood  wrote:

>> From: ro...@do... [mailto:ro...@do...]
>> 
>> Having dealt with rotations, what about shear?  I'll leave that aside
>> until someone asks the question using an intrinsic geometric
>> definition of "shear" (And it does exist).  
>> 
>
>Which begs the question to Lorrimar...What did you mean by shear? 

Guilty of begging the question,  not because I didn't know the answer,
but because it was getting late.  I can't necessarily answer every
question in onepost or one evening.

> I'm not a
>matrix guy at all (well, except for the movie which rules) but I do know a
>few things about shear forces. 

There is no question of forces here, but just of affine mappings.
True, a shear force on a deformable object tends to produce a
deformation that would be described by a shear mapping.  I think the
engineers say "Stress is proportional to strain".  The stress is the
shear force, the strain is the deformation.  

But on a rigid body a shear just produces a force couple, which
results in rotation, not a shear deformation, so not a shear
transformation..  But all this is a digression from the issue of
transformations, i.e. mappings.
   
> If that is what you meant then a shear is
>the perpendicular component of force on a surface. 

Although incompletely stated,  I believe you have this wrong.  The
shear comes from the force component parallel to the surface, not
perpendicular.

> In engineering we need
>material that can withstand shear forces so things won't rip apart.  In 3D
>graphics I'm assuming that it will create a moment in the object receiving
>the force and instead of determining the deflection and deformation of the
>material or illustrating the breakage of the material I also assume you will
>use shear to cause the object to move or spin.
>

See above the comment on the difference between shear qua force and
shear qua mapping.

RE: [Algorithms] Where is the enemy?

[Aldo . <al...@ho...>]

It works fine, now!
Thanks a lot, Steve.
And thanks to the other messages, too, guys!

Aldo



>From: Steve Wood <Ste...@im...>
>Reply-To: gda...@li...
>To: "'gda...@li...'"  
><gda...@li...>
>Subject: RE: [Algorithms] Where is the enemy?
>Date: Mon, 24 Jul 2000 10:48:44 -0700
>
>Come to think of it...the vector can be eliminated since it's in both
>equations...just this test need be done:
>
>if Px + Py + Pz > P1x + P1y + P1z then it's to the right
>if Px + Py + Pz < P1x + P1y + P1z then it's to the left
>if Px + Py + Pz = P1x + P1y + P1z then it's straight ahead
>
>R&R
>
> > From: Aldo . [mailto:al...@ho...]
> > How can I decide wich side must I turn?
> >
> > I've tried cross product, but it doesn't worked fine.
> >
> > Any sugestions?
> >
>
>Hmm, how about using the equation of a plane which has at it's normal a
>vector V pointing at a right angle to the up and forward vectors and
>positive to the right:
>
>VxP1x + VyP1y + VzP1z = C1
>
>Where C1 would be an index to compare with C in
>
>VxPx + VyPy + VzPz = C
>
>that tells you if point P is to the right (C>C1) or left (C<C1).
>
>R&R
>
>_______________________________________________
>GDAlgorithms-list mailing list
>GDA...@li...
>http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

________________________________________________________________________
Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com

RE: [Algorithms] How to derive transformation matrices

[Akbar A. <sye...@ea...>]

> Posting  wrong answers with an air of authority makes you
>fair game for chastisement.
very, very true.

but, actually i don't think it's a but.
but :-)
somebody once _important_ said something of the nature.
"to be a master one must learn from his teachers and the ones below him,
then one is a true master"

i think this philosophies applies to the _VAST_ field of,
yes; computer graphics. :-D
i have actually learned _quite_ a bit just observing my cousins play with
legos', interact with the "world", swimming in the pool, playing in the tub,
things of this nature. imho usually when you are doing the activities it
becomes _difficult_ to really apperciate the _technical_ point of view.

"nature is an epic beaty from a aesthetic point of veiw _as_ well as a
technical standpoint."
first time i said it 8-D
<...snip>

besides that, what ron says is very true though.
by telling people false information it becomes _manytimes_ harder to
understand the material.
but; you do learn a lot more when you challenge something.
for ex. at the opengl gamedev list before i posted about richard's wright's
first edition book OpenGL superbible, i sat for many hours trying to
_understand_ the _wrong_ code. I read through many books trying to _prove_
the code. after many hours the code was wrong. But, i didn't stop, checked
more books, sites, etc..

follow this philosphy and you will learn much more and remeber things in
general. once you get _really_ good at your _BASICS_ the rest is a breeze.
can i say albert einstein? but many people, including some on this list
don't understand that the _BASICS_ are _VERY_ important.
the next generation of programmers are going to _live_ off api's. this is
"not a good thing for humanity" :-|

a generation of mindless idiots that feel _good_ when they call a library
function properly? and a trivial one at that?
well back to philosphy.
	o Challenge EVERYTHING! -f = -ma
	o yes, books are _always_ right unless _YOU_ find otherwise. just don't
	  trust what joe schmoe says. dig up your own dirt.

in the end (book dillema) i gave up cause i could _not_ understand it (the
bad code)
It turned out it was a error and iirc, 2 people had found it since the books
first publishing.


i have been _learning_ a lot about linux and the kernel in general so please
forgive my abnormal use of underscores 8-) mucho thanks go out to steve for
showing me the light 8-)

quoting gl.
>Many of us learn as we go
the _best_ programmers do this. the linux kernel imho is the best example i
can think of, and the programmers that develop the core os.

a few of the best physists come from a poor educational background.
read, read, read! coding is the quick part (see Code Complete for diagrams)

and no,
schools like mit, cornell, berkley, yes-- rice as well are not doomed.
there are _very_ few people that write e-mails about a
mathamateical-philosphical to an algo list,
and stay up "enginerring" software all night and researching all day.


eeek. i have no idea how i wrote all thise, just about incorrect ..
advice..<snip>



peace.
akbar A.

"We want technology for the sake of the story, not for its own sake. When
you look back, say 10 years from now, current technology will seem quaint"
Pixars' Edwin Catmull.


-----Original Message-----
From: gda...@li...
[mailto:gda...@li...]On Behalf Of Ron
Levine>
Sent: Monday, July 24, 2000 8:56 AM
To: gda...@li...
Subject: Re: [Algorithms] How to derive transformation matrices


Martin Gladnishki  wrote:

>
>. We all can
>learn from you, so please be more forgiving with 'newbies' like me.
>

I am never contemptuous of newbies for showing their ignorance by
posting questions--everyone was once ignorant of everything;
everyone, myself included,  remains perpetually ignornant of almost
everything. I do frequently express annoyance at newbies' seeking help
in the wrong forum.  For example, I don't think that this list is  the
right place to seek tutoring on elementary math topics for which there
exist myriad textbooks and courses that everyone ought to have taken
before setting out to do 3D applications.

What sets off my "flamethrower", as you put it, is when people post,
with an air of authority, wildly incorrect responses to the newbies'
earnest questions.  It seems to me that unless you are certain in your
heart that your answer is correct, helpful, or interesting, you should
keep quiet.  Posting  wrong answers with an air of authority makes you
fair game for chastisement.

_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list

RE: [Algorithms] How to derive transformation matrices

[Robert D. <RD...@ac...>]

I think most people in this case take shear to mean a transformation in
which there is an invariant plane, but everything outside that plane is
moved within its own parallel plane, so viewing through the said invariant
plane, you would see this :


  pre-shear            post-shear

  +------+             +------+
  |      |            /      /
--+------+----- -----+------+-------- invariant plane
  |      |          /      /
  +------+         +------+

Useful for certain effects, but in general not a good thing because unlike a
rotation matrix it doesn't preserve your normals.

Robert

-----Original Message-----
From: Steve Wood [mailto:Ste...@im...]
Sent: 24 July 2000 17:49
To: 'gda...@li...'
Subject: RE: [Algorithms] How to derive transformation matrices


> From: ro...@do... [mailto:ro...@do...]
> 
> Having dealt with rotations, what about shear?  I'll leave that aside
> until someone asks the question using an intrinsic geometric
> definition of "shear" (And it does exist).  
> 

Which begs the question to Lorrimar...What did you mean by shear?  I'm not a
matrix guy at all (well, except for the movie which rules) but I do know a
few things about shear forces.  If that is what you meant then a shear is
the perpendicular component of force on a surface.  In engineering we need
material that can withstand shear forces so things won't rip apart.  In 3D
graphics I'm assuming that it will create a moment in the object receiving
the force and instead of determining the deflection and deformation of the
material or illustrating the breakage of the material I also assume you will
use shear to cause the object to move or spin.

R&R

_______________________________________________
GDAlgorithms-list mailing list
GDA...@li...
http://lists.sourceforge.net/mailman/listinfo/gdalgorithms-list