| Re: Object graphics axis [message #17441] |
Wed, 27 October 1999 00:00 |
davidf
Messages: 2866
Registered: September 1996
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Senior Member |
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Karri Kaksonen (karri.kaksonen@picker.fi) writes:
> I just came back from Davids course about object graphics and it
> occured to me that a reason why positioning the axis objects is
> so difficult is that the range depends on the location of the
> axis.
>
> The manual says that the range of the axis is set by a vector:
>
> [-Xmin/(Xmax-Xmin), 1/(Xmax-Xmin)]
>
> This may work if the length of the axis is 1.0 in normalized
> coordinates. In the course I chose the length to be 2.0 and in
> order to get it right I just tried out different values until
> I found it to be closer to:
>
> [-1-Xmin*2/(Xmax-Xmin), 2/(Xmax-Xmin)]
>
> I thought about this on my flight home last night and what I am
> afraid of is that the -1 in the first element may actually depend
> on where the axis is drawn on the screen. My location of the axis
> was at [-1.0, -1.0]. If this is the case then RSI should do
> something about it before version 5.3 comes out. Otherwise you have
> to update the range vector every time you reposition the axis.
>
> I run some tests at home and found out that the Location-keyword
> sets two coordinates out of three. And the coordconv sets the
> third coordinate, range and the scale.
>
> range = [-50 150]
> normalized_len = 1.6
>
> Location = [not_valid y_pos z_pos]
> scale = normalized_len/(range[1]-range[0])
> Xcoordconv = [x_pos - range[0]*scale, scale]
>
> This does not sound logical to me. I would rather see:
>
> Location = [x_pos, y_pos, z_pos]
> Scale = normalized_len/(range[1]-range0])
> Offset = -range[0]*scale
>
> Perhaps RSI could add "scale" and "offset" to make the objects behave
> like the user expects them to work.
I agree completely that this does not work like the
user expects it to work. You, Ben, me...everyone I know...
has found it incredibly difficult to understand. And the
IDL documentation is decidedly unhelpful. I found it
frustrating too that the NORM_COORD routine supplied
in the object/examples directory, and referred to in
the documentation, also only works with data that
starts at zero. In other words, with data created
with the ever popular DIST function. :-)
After spending literally hours trying to make *real*
data show up in object graphics plots I threw up my
hands and threw myself on the mercy of the IDL
programmers who wrote the code. They supplied me
with an algorithm, which with a few slight modifications
to make it actually work, reliably creates the
translation and scaling factors necessary to place
a real data range into a particular view.
The relevant piece of code is this:
scale = [((position[0]*range[1])-(position[1]*range[0])) / $
(range[1]-range[0]), (position[1]-position[0])/(range[1]-range[0])]
where "range" is a two-element vector with the minimum and
maximum data range, and "position" is a two-element vector
with the minimum and maximum location in the arbitrary
coordinate space of the view. This is the code that
can be found in NORMALIZE:
http://www.dfanning.com/programs/normalize.pro
Why this code hasn't found its way back to RSI is
beyond me, but I know for a fact that I can't
write object programs without it. I've tried.
Cheers,
David
P.S. This is not to say you *can't* write object
programs without NORMALIZE. I've seen plenty of
object programs from RSI which seem to be working
just fine. I'm only suggesting that the people who
wrote those programs seem to be a hell of a lot
smarter than I am. :-)
--
David Fanning, Ph.D.
Fanning Software Consulting
Phone: 970-221-0438 E-Mail: davidf@dfanning.com
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Toll-Free IDL Book Orders: 1-888-461-0155
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| Re: Object graphics axis [message #17460 is a reply to message #17441] |
Mon, 25 October 1999 00:00  |
<karri
Messages: 4
Registered: October 1999
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Junior Member |
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Thank you for the tip about Normalize. David did go through this
but I still wanted to understand how the location/coordconv stuff
works.
I run some tests at home and found out that the Location-keyword
sets two coordinates out of three. And the coordconv sets the
third coordinate, range and the scale.
range = [-50 150]
normalized_len = 1.6
Location = [not_valid y_pos z_pos]
scale = normalized_len/(range[1]-range[0])
Xcoordconv = [x_pos - range[0]*scale, scale]
This does not sound logical to me. I would rather see:
Location = [x_pos, y_pos, z_pos]
Scale = normalized_len/(range[1]-range0])
Offset = -range[0]*scale
Perhaps RSI could add "scale" and "offset" to make the objects behave
like the user expects them to work.
On Mon, 25 Oct 1999, Ben Tupper wrote:
> ... snip ...
> Fortunately, David F has written the NORMALIZE function for people like
> me who tend to shuffle along with the lost sometimes. NORMALIZE is
> available at his website. NORMALIZE accepts (as a keyword) the
> POSITION you intend to scale your axis into.
--
Regards,
Karri Kaksonen
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| Re: Object graphics axis [message #17464 is a reply to message #17460] |
Mon, 25 October 1999 00:00 |
Ben Tupper
Messages: 186
Registered: August 1999
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Senior Member |
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<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
Karri Kaksonen wrote:
<blockquote TYPE=CITE>The manual says that the range of the axis is set
by a vector:
<p>[-Xmin/(Xmax-Xmin), 1/(Xmax-Xmin)]
<p>This may work if the length of the axis is 1.0 in normalized
<br>coordinates. In the course I chose the length to be 2.0 and in
<br>order to get it right I just tried out different values until
<br>I found it to be closer to:
<p>[-1-Xmin*2/(Xmax-Xmin), 2/(Xmax-Xmin)]
<p>I thought about this on my flight home last night and what I am
<br>afraid of is that the -1 in the first element may actually depend
<br>on where the axis is drawn on the screen. My location of the axis
<br>was at [-1.0, -1.0]. If this is the case then RSI should do
<br>something about it before version 5.3 comes out. Otherwise you have
<br>to update the range vector every time you reposition the axis.</blockquote>
<p><br>I've had the same difficulty... it drove me crazy for the longest
time until David F bailed me out. The weakness inherent in
RSI's formula ([-Xmin/(Xmax-Xmin), 1/(Xmax-Xmin)]) is that it assumes that
you are going to scale your data into the positional space of [0,1].
In your case, you have decided to scale into positional space of [-1, 1].
Fortunately, David F has written the NORMALIZE function for people like
me who tend to shuffle along with the lost sometimes. NORMALIZE
is available at his website. NORMALIZE accepts (as a keyword)
the POSITION you intend to scale your axis into.
<p>I was so crazed by this poblem that I wrote a tutorial for figuring
out the difference between unscaled data coordinates and scaled data coordinates.
I don't have it handy right now, but I will send it along to you as soon
as possible.
<p>Ben
<pre>--
Ben Tupper
Bigelow Laboratory for Ocean Science
tupper@seadas.bigelow.org
Pemaquid River Company
pemaquidriver@tidewater.net</pre>
</html>
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