 |
|
|
|
|
|
| |
| |
|
|
|
|
| |
| |
|
|
On 16 Nov 1998 12:05:54 -0500, par### [at] my-dejanews com (Ron Parker)
wrote:
>On Sun, 15 Nov 1998 06:10:38 -0800, Ken <tyl### [at] pacbellnet> wrote:
>>Dan Connelly wrote:
>>
>>> I respectfully disagree. The documentation is incredibly clear.
>>> A simple "translate <-0.5, -0.5, -0.5>" centers things -- this is
>>> so trivial there is no reason to change the source code.
>>>
>>> --
>>> http://www.flash.net/~djconnel/
>>
>>I respectfully disagree with your respectful disagreement.
>>Your simple example is 28 characters long. That's a lot
>>of simple typing. Nor does it redily explain the ambiguity.
>
>I respectfully disagree with your characterization of the
>situation as an ambiguity, since it's consistent among things
>that use images. The real question is why image maps are on
>the x-y plane and height fields on the x-z plane. I also
>respectfully disagree with your contention that it's 28
>characters... "translate -.5" is only 13.
I have also wondered why image ,maps and height fields were located on
two different planes. It always bothered me to add the extra code
required to place an image map over a height field. It could have
been *so* much simpler, if they were both on the same plane.
My other pet peeve about image maps is not having an option to
automatically maintain the original aspect ratio of the image. It
would be cool to set the x or y dimension to "1" and let POV
automatically scale the other dimension to keep the aspect ratio
intact. Something like "constrain_x" or "constrain_y" added to the
image map statement would be one possible syntax.
Later,
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Ron Parker <par### [at] my-dejanewscom> wrote in article
<36505b72.0@news.povray.org>...
> ... The real question is why image maps are on
> the x-y plane and height fields on the x-z plane...
>
'cause David Buck wrote planar image maps that way. Slide projector
casting an image on a wall (since most of us thought of Y as up and put the
camera somewhere on the -Z axis). Height fields are obviously on the
ground and stick up, so they are in the X/Z plane. I never cared for the
fact that height fields were squashed into a 1x1x1 box, but since I didn't
do that code, I can't complain. I wrote the TTF stuff, so I did it like I
wanted...
In general the objects are oriented they way they are because different
people wrote them at different times and had different ideas of how they
should go.
Xander
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Dan Connelly wrote:
>
> Ron Parker wrote:
> > Whoops, lathe and sor too. What about TTFs that are created from the
> > lower left corner?
>
> ... and superelipsoids, of course, which raises the question of why
> superellipsoids are symmetric in x and y while almost everything else
> is symmetric in x and z.
>
> It's clear the only consistency in POV is there is no consistency.
> It's like UNIX and basically every other piece-meal project.... the
> inconsistency just adds to its attractive character.
>
> > what's the cross-product of two 4-d vectors?
>
> Good question! I expect it would take 3 vectors to yield a cross
> product :
>
> y_i = a_j b_k c_l e_ijkl
>
> (a, b, c are 4-vectors, e is a matrix)
>
> where e_ijkl = 0 for and two elements of {i,j,k,l} equal,
> +1, or -1 in other cases.
>
> But cross products of 3-d vectors don't yield vectors, but pseudovectors,
> as the result depends on handedness. Either this or my physics background
> is too faded for such details.
The cross product of two 3d vectors certainly does yield another vector.
If it didn't, half of my code wouldn't work!
Regards,
John
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
John VanSickle wrote:
> The cross product of two 3d vectors certainly does yield another vector.
> If it didn't, half of my code wouldn't work!
I said "pseudovector". A velocity vector, for example, is a real
thing. If I have a right-hand or left-hand coordinate system
doesn't matter. But if I take the cross product of two sides
of a cube side in a fashion which yields a normal pointing
outside the box in a LHS, when I switch to a RHS the same
cross product will yield a normal pointing inside the box.
Since it is non-physical for a quantity to depend on the
coordinate system used, the term "pseudovector" is used.
The same thing happens with angular momentum, which is a
pseudovector (it's defined as a cross-product). It is
fundamentally different from linear momentum.
In Physics, this is important when considering conserved
quantities like parity.
Dan
--
http://www.flash.net/~djconnel/
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Dan Connelly <djc### [at] flashnet> writes:
> I said "pseudovector". A velocity vector, for example, is a real
> thing. If I have a right-hand or left-hand coordinate system
> doesn't matter. But if I take the cross product of two sides
> of a cube side in a fashion which yields a normal pointing
> outside the box in a LHS, when I switch to a RHS the same
> cross product will yield a normal pointing inside the box.
Yes, sure. The whole point is:
POV-Ray uses a right-handed coordinate system whatever your camera
, period. I know this is going to make the whole crowd hurling mad and
becoming epileptic and throwing rotten tomatoes at me, but that's the way
it is. The coordinate system has *nothing* to do with the way you display
it. The cross-product of <x1, y1, z1> by <x2, y2, z2> is <y1*z2-z1*y2,
z1*x2-x1*z2, x1*y2-y1*x2>, whatever your camera is. The confusing thing is
that the POV-Doc says that you can change the coordinate system handedness
by giving different combinations of up, right and direction vectors, which
is not true. I repeat, POV-Ray uses a right-handed coordinate system
whatever your camera. You can only change the way the camera looks at the
scene, in which you can choose to look at it as it is (that is, choose a
right-handed *camera*) of to flip it left-right (or upside-down, or
front-back, depending on the camera component you change into minus
itself). This is a rendering-only characteristic and has nothing to do
with the real handedness of the coordinate system, which stays the real
right-handed system used by every mathematician in the world (or at least,
every mathematician I have heard of).
> In Physics, this is important when considering conserved
> quantities like parity.
I won't deny it. In fact, I keep saying it: we are in a right-handed
coordinate system, so the Physics theorems and laws are safe. Now, you
just have to take care when you look at your scene...
Okay, throw the tomatoes. I won't read here till next Tuesday, so please
don't waste your ammunition till then, or till you're *really* sure I'm
wrong, in which case we will probably have an interesting discussion next
week >:-)
Roland.
--
Les francophones m'appellent Roland Mas,
English speakers call me Rowlannd' Mass,
Nihongode hanasu hitoha [Lolando Masu] to iimasu.
Choisissez ! Take your pick ! Erande kudasai !
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
I could have sworn the Pov-Ray docs explicitly state that
the left handed system is used by default contrary to what
the rest of the world uses. I'm may be getting old buy my
memory isn't that bad.
Ken Tyler
Roland Mas wrote:
> Dan Connelly <djc### [at] flashnet> writes:
>
> > I said "pseudovector". A velocity vector, for example, is a real
> > thing. If I have a right-hand or left-hand coordinate system
> > doesn't matter. But if I take the cross product of two sides
> > of a cube side in a fashion which yields a normal pointing
> > outside the box in a LHS, when I switch to a RHS the same
> > cross product will yield a normal pointing inside the box.
>
> Yes, sure. The whole point is:
>
> POV-Ray uses a right-handed coordinate system whatever your camera
>
> , period. I know this is going to make the whole crowd hurling mad and
> becoming epileptic and throwing rotten tomatoes at me, but that's the way
> it is. The coordinate system has *nothing* to do with the way you display
> it. The cross-product of <x1, y1, z1> by <x2, y2, z2> is <y1*z2-z1*y2,
> z1*x2-x1*z2, x1*y2-y1*x2>, whatever your camera is. The confusing thing is
> that the POV-Doc says that you can change the coordinate system handedness
> by giving different combinations of up, right and direction vectors, which
> is not true. I repeat, POV-Ray uses a right-handed coordinate system
> whatever your camera. You can only change the way the camera looks at the
> scene, in which you can choose to look at it as it is (that is, choose a
> right-handed *camera*) of to flip it left-right (or upside-down, or
> front-back, depending on the camera component you change into minus
> itself). This is a rendering-only characteristic and has nothing to do
> with the real handedness of the coordinate system, which stays the real
> right-handed system used by every mathematician in the world (or at least,
> every mathematician I have heard of).
>
> > In Physics, this is important when considering conserved
> > quantities like parity.
>
> I won't deny it. In fact, I keep saying it: we are in a right-handed
> coordinate system, so the Physics theorems and laws are safe. Now, you
> just have to take care when you look at your scene...
>
> Okay, throw the tomatoes. I won't read here till next Tuesday, so please
> don't waste your ammunition till then, or till you're *really* sure I'm
> wrong, in which case we will probably have an interesting discussion next
> week >:-)
>
> Roland.
> --
> Les francophones m'appellent Roland Mas,
> English speakers call me Rowlannd' Mass,
> Nihongode hanasu hitoha [Lolando Masu] to iimasu.
> Choisissez ! Take your pick ! Erande kudasai !
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Ken <tyl### [at] pacbellnet> writes:
> I could have sworn the Pov-Ray docs explicitly state that
> the left handed system is used by default contrary to what
> the rest of the world uses. I'm may be getting old buy my
> memory isn't that bad.
Section 4.1.1
Understanding POV-Ray's Coordinate System
First, we have to tell POV-Ray where our camera is and where it is
looking. To do this, we use 3D coordinates. The usual coordinate system for
POV-Ray has the positive y-axis pointing up, the positive x-axis pointing
to the right, and the positive z-axis pointing into the screen as follows:
[Here is a picture of a left-handedly *displayed* coordinate system]
Section 7.4.4.5.2
Handedness
The right vector also describes the direction to the right of the
camera. It tells POV-Ray where the right side of your screen is. The sign
of the right vector can be used to determine the handedness of the
coordinate system in use. The default right statement is:
right <1.33, 0, 0>
[So the handedness described here is only the one of the camera, as stated
above.]
I confirm my position. The handedness of a coordinate system is given by
the way it calculates cross-products, not by the way it looks because it
has no canonical way to look. A coordinate system is abstract, and if you
want to make it look like something you have to choose a camera
orientation. This does not change the handedness of the space. And
POV-Ray uses a right-handed cross-product. The default *camera* is
left-handed, that's all.
Other suggestions?
Roland.
--
Les francophones m'appellent Roland Mas,
English speakers call me Rowlannd' Mass,
Nihongode hanasu hitoha [Lolando Masu] to iimasu.
Choisissez ! Take your pick ! Erande kudasai !
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
| |
|
|
Roland Mas <mas### [at] acratrcojp> wrote in article
<m3s### [at] rpc66acratrcojp>...
> I confirm my position. The handedness of a coordinate system is given
by
> the way it calculates cross-products, not by the way it looks because it
> has no canonical way to look...
>
> Other suggestions?
The basic coordinate system used by POV-Ray (and many other renderers,
contrary to an earlier assertion that POV-Ray was somehow unique) is
definitely left-handed. Also, inverting one of the coordinate axes of the
camera does transform the world from right to left handed. The only
coordinate system of interest is that of the observer, which is defined by
the camera.
The rational for a left-handed coordinate system in computer graphics is
pretty easy to understand. Given a camera sitting at <0,0,0> and looking
along the positive Z axis, larger Z values correspond to larger depth
values. So, when removing hidden surfaces, I simply keep the one with the
smallest Z value. For non-raytracers, the camera is almost always
transformed into this canonical orientation before rendering.
Why would a left-handed cross product look like a right-handed one? Easy -
right thumb along one positive axis, fingers along a second positive one.
Fingers curl in the direction of the third (in the positive direction).
Look into a mirror as you do this. It still works, but now it's a left
hand doing the pointing and curling. It's an excellent example of parity,
since someone in a mirror universe would describe their mathematics in
exactly the same terms as you.
The biggest problems with left vs. right have to do with importing models.
If a different coordinate system was used, the model could be mirrored,
rotated, or both.
Xander
Post a reply to this message
|
|
| |
| |
|
|
|
|
| |
|
|
