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.
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.
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.
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.
User selectable functionality would be as follows:
- to allow/ not allow negative numbers (default allow)
- to be able to change the q exponent (i.e., 4 above)
(default 4)
- to be able to change the painting color that is too
small (default white)
- to clamp minimum to some number (such as q == 4 above).
(default on).
Thoughts?
Alan
