Author Topic: Some questions about "Color by"  (Read 4369 times)

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Offline Roc

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Some questions about "Color by"
« on: January 2, 2009, 13:58 »
Dear everyone,

When the result document .vnl is visualized by VNL tool, there are three options " Value, Magnitude, Sign" in the "Color by" pull-down menu, how do I choose it, and why is the numerical value different in the "Show colorbar" sometimes when I choose  "Value" or others. Just choose the default? I haven't understood the explanation in the manuscript:
Magnitude: available for all types of functions.
Phase: available for complex functions, such as wave functions.
Sign: available for real functions.
Value: available for real functions.


Thanks.
« Last Edit: January 2, 2009, 14:15 by anyipeng »

Offline Anders Blom

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Re: Some questions about "Color by"
« Reply #1 on: January 2, 2009, 22:28 »
For a 3D grid with real data (such as the electron density) you can for each type of plot (isosurface, volume plot, contour plot) choose how the data values will be mapped to colors in the plot.

  • Value: The color is determined by the value of the data in each point; probably the most useful option in most cases.
  • Magnitude: The color is determined by the absolute value of the data in each point.
  • Sign: The color is determined by the sign of the data in each point; this make sense mostly for volume plots, or when it is of specific interest to see where the values are >0 or <0 (for instance if the data represents a difference density)

For complex data (Bloch functions), there is additionally an option to color the data by the phase. In this case the "Value" option is not available, only the Magnitude of a complex number makes sense.

For each option, the data values are mapped linearly onto the colormap (there are a couple of different ones available) to color the plot. The numerical max and min values of the data set are shown in the corresponding boxes (if Value is used), but by entering different values you can "zoom" in/out in color in order to highlight specific features in the data.

The best way to figure out which plot that works best for your specific case is typically to play around with the options and see how the plot changes (click the Show button to see the effect). Note that for performance reasons only one volume plot can be shown at the time.

Offline Roc

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Re: Some questions about "Color by"
« Reply #2 on: January 3, 2009, 01:43 »
Ok,thank you very much, Sir!  :)
« Last Edit: January 3, 2009, 04:23 by anyipeng »

Offline Roc

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Re: Some questions about "Color by"
« Reply #3 on: January 7, 2009, 01:40 »
Dear everyone,

However, for my same one .vnl result document, I polt it using the three choices, respectively, as follows,(the first one is that when I choose "sign",the second is the situation for "Magnitude").
We can see that, the numbers are different at the "Show colorbar". Especial for the "sign", It seems that only three colors describe the data extent.

So, I'm still confused with that how to decide the choice.
Which is  correct, how to explain it?

Thanks very much! :)
Regards!

Offline Anders Blom

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Re: Some questions about "Color by"
« Reply #4 on: January 7, 2009, 10:48 »
Actually, the number are not really different on the colorbar... The data set contains values between -11.1 and 12.2.

  • Using "sign" shows this full range on the colorbar, but uses blue (cyan) for all negative values and red for all positive. (The "third color" in the middle is a VTK artifact but has no real influence on the plot.)
  • Using "magnitude" is probably not proper for this plot. What you get in that case is that first all negative values are converted to positive ones; then we have only have values from 0.0 to 12.2, and these are then according to the colorbar.
  • Using "value" would be the correct alternative to "magnitude", and would color all data values differently, from -11.1 to 12.2. Try it :)

In general, there is no "correct" (and hence no "wrong") choice, it's a matter of how you want to present your results.

Offline Roc

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Re: Some questions about "Color by"
« Reply #5 on: January 7, 2009, 12:33 »
Thank you very very~~~~~~ much, you are very kind! :) ;)

Best wishes´╝ü