Logscale colormap support

Dear All

Do we have direct support for logscale colormaps support? The chlorophyll colormaps used in modis. Viirs, amd other ocean color missions are generally logscale.

As far as I remember VisAD has this support?
Kindly correct me.
It's about bringing this functionality into mcidas-v GUI.
Any thoughts on this?

Regards
Ghansham

Hi Ghansham,

There's no way currently through the Color Table Editor to do a logarithmic stretch. This is a part of Inquiry 1153. In the past we've suggested the following approaches to users looking to do a logarithmic stretch:

Display the log of the data. In this case, the color table will be logarithmic to match the data values. Here's a function you could try:

def logData(data):
    # replace any zero data values with NaN
    missData = setToMissing(data,0)
    # run log function on setToMissing output
    lData = log(missData)
    return lData

In this function, values of 0 are set to "missing" so the enhancement stretches from the true min to max values of the output data object.

You can approximate a logarithmic stretch using piecewise linear stretching. You would just need to make sure you have enough breakpoints at the correct locations to make things look correct.

I'm not overly familiar with standalone VisAD, but from what I've seen it looks like you can manually build a color table with log-spaced entries, but this isn't built into McIDAS-V's Color Table Editor.

Thanks,
Bob

Okay Sir. Thanks

https://github.com/visad/visad/blob/master/core/src/visad/BaseColorControl.java

There is a concept of function in VisAD basecolorcontrol class which can be used to implement this functionality. Only UI part is missing in mcidas-v
We can add an optional in UI for logscale and supply log function to set colorcontrol accordingly.

https://github.com/visad/visad/blob/156206052e72eaba306948bb4f9c5a961479a84d/core/src/visad/BaseColorControl.java#L298

Will try if some prototype is demonstrable

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Logscale colormap support #135

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rmcarp commented on Aug 13, 2025

ghansham commented on Aug 14, 2025

ghansham commented on Aug 14, 2025

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