Subject:
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Re: LDLite Question
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Newsgroups:
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lugnet.cad
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Date:
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Tue, 19 Dec 2000 21:36:28 GMT
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Viewed:
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762 times
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In lugnet.cad, Mike Walsh writes:
>
> I want to view my model upside down, ..
OK...
> ... so essentially I want to rotate
> it 180 degrees around the Y axis.
Nope, not if you want to view it "upside down".
(Bad wording.)
When we say "rotate about the [whatever] axis",
we mean "hold the model *by*that*axis*,
and spin by however many degrees".
Now, when modifying the rotation matrix
(i.e., that string/array of nine numbers
for the "-A" option), you *will* want to multiply
the second set of three numbers by "minus one" (-1),
but you will *_also_* want to multiply either the first set
of three numbers *or* the third set of three numbers
by minus-one as well, in order to maintain the integrity
of the view of your model.
In order to better explain, I'm gonna *have* to use
a little bit of matrix algebra, so please don't panic.
Let's call these nine numbers "ax", "ay", "az",
"bz", "by", "bz", "cx", "cy", and "cz".
If you were to write these nine numbers
as a matrix, the matrix would look like this:
| ax ay az |
| bx by bz |
| cx cy cz |
(This should line up in nice column format.
If need be, you can click on "View Raw Message" above
to see this message in raw ASCII form.)
Notice that *each* column correspondes to one of the three
axes: "X", "Y", and "Z". Each _row_ also corresponds to
one of the three axes, but I labelled that part using "A", "B",
and "C" to make things a little easier to understand, OK?
If you multiply the second row (i.e., the "B" values)
ach by negative one (-1), you will get an upside-down view;
*however*, this upside down view will be the *mirror-image*
of the model's underside (that is, it will be the image
you would see in a *mirror* held under the model).
If you want to have the image of the underside view
with proper "facing forward" orientation, you would need
to multiply *both* the first *and* the second rows
(i.e., the "A" row and the "B" row) by minus-one (-1).
This view corresponds to holding the model in your hands,
and flipping it upside down, left over right.
On the other hand, if you want to have the image of the underside view
with the model's left side to your left, you would need to multiply
the second and third rows (i.e., the "B" row and the "C" row)
by negative one. This view corresponds to holding the model
in your hands, and flipping it upside down, front over back.
I hope this helps. :-)
Franklin
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Message is in Reply To:
 | | LDLite Question
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| I think this is the correct group for this question, if not, please direct me appropriately. Can anyone help me understand the arguments to LDLite's "-A" switch. I want to view my model upside down, so essentially I want to rorate it 180 degrees (...) (24 years ago, 19-Dec-00, to lugnet.cad)
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