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Allen,<br>
<br>
I like the way you think. I have an application that shows best
using a logarithmic scale for the iso-surface displays/animations. I
now have to set-up by hand the log-limits, avoiding negative
numbers. Your suggested refinements would be helpful to me.<br>
<br>
Samuel W Key<br>
FMA Development, LLC<br>
1005 39th Ave NE<br>
Great Falls, Montana 59404<br>
USA<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 7/1/2014 11:09 AM, Scott, W Alan
wrote:<br>
</div>
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<div>I would like to propose a few changes in our log scaling
algorithm for painting a dataset by a variable. I
discussed this with Utkarsh, and he asked that I bounce it
off the e-mail list. So, here goes.</div>
<div> </div>
<div>Currently, when a user log scales a variable, if all data
is positive, ParaView just uses the normal min and max.
There are times when this is not proper – for instance when
looking at the temperature or density of material in a
supernova, or velocity
of outbound gas. Another example is large data, with noise
around zero. I would like to propose that we have a user
selectable option to set the minimum at maximum*10^-q, where
q is user defined but defaults to 4. In other words, the
minimum would be set
to 1*10^-4 of what the maximum is.</div>
<div> </div>
<div>If all of the user’s data is negative, ParaView grumbles,
and then seg faults using a current master git pull – not
optimal behavior. In PV 4.1, it just sets min and max to
0. I would like to propose that ParaView calculate the log
of the data, as follows:
Index= -(log(abs(Var))). Then, just draw the color legend
as normal – for instance, red at top, white in the middle
and blue at bottom. Tick marks will be the reverse of
positive log scaling – with the dense numbers, more negative
numbers at the bottom and
less dense, less negative numbers at the top.</div>
<div> </div>
<div>The problem arises with data that spans positive and
negative numbers. Since the log of 0 is infinity, we have
to deal with very small numbers in a special way. I propose
that we find maxVal = max(maximum, abs(minimum)). Then, we
set the color bar to
run from maxVal to –maxVal. We log scale the top half of
the color legend, running from maxVal to maxVal*10^-4, and
we reverse log scale the bottom half of the color legend,
running from –maxVal*10^-4 to –maxVal. We calculate this
negative range the same
as the all negative data section above. All data between
maxVal*10^-4 and -maxVal*10^-4 would remain white by
default, or user selectable black.</div>
<div> </div>
<div>User selectable functionality would be as follows:</div>
<ul style="margin:0;padding-left:36pt;">
<li>to allow/ not allow negative numbers (default allow)</li>
<li>to be able to change the q exponent (i.e., 4 above)
(default 4)</li>
<li>to be able to change the painting color that is too
small (default white)</li>
<li>to clamp minimum to some number (such as q == 4 above).
(default on).</li>
</ul>
<div> </div>
<div>Thoughts?</div>
<div> </div>
<div>Alan </div>
<div> </div>
<div> </div>
</span></font>
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