[rsf-user] Can someone explain color mapping in sfgrey?

[James J. <jen...@ma...>]

Hello everyone:

Can someone explain how color mapping works in sfgrey?

I can figure out a few things from the self doc and website:

(1) The parameter "color" controls the color scheme.  There is a nice explanation of that at http://egl.beg.utexas.edu/rsflog/index.php?/archives/14-Color-schemes.html

(2) I suppose the program makes a linear mapping from the input numbers to the color table, except for a parameter gpow which seems to be an exponent for a power law mapping.  How does that work?  Is the mapping linear unless gpow is supplied?

There are also parameters "gainstep" and "phalf" which seem to be involved in the "estimation" of an optimal(?) gpow.  How does that work?

(3) The parameters clip and pclip seem to be for data clipping prior to application of the color map, so that extreme values beyond the corresponding limits will all be assigned colors at the corresponding ends of the color table.  Yes?

(4) sfgrey seems to be designed mostly for data centered on zero.  I suppose seismic signals are like that (In case you haven't noticed, I'm not a geophysicist).  So I think the default behavior is to make a color map centered on zero.  That is, the data value mapped to one end of the color table is the negative of the value mapped to the other end.  Am I correct?

The parameter allpos changes this default behavior so that zero is mapped to the lower end of the color table, and a positive value is mapped to the upper end.  It looks like the data value mapped to the lower end of the color table will never be bigger than zero.

(5) What I really want is a way to independently control the end values of a linear color mapping.  For example, I would like to map the value 10 to the lower end and 20 to the upper end, with linear scaling in between.

But there doesn't seem to be a way to do that.

Let's say I have values between 9 and 21.  If I understand it correctly, I can clip my data to the range 10-20 and that will make the value 20 map to the upper end of the color table, but the lower end will be -20 with allpos=n and zero with allpos=y, yes?  If my data have values between 11 and 19, then the upper end of the color map will be 19 if I clip to 20 or not, yes?

(6) There are parameters minval and maxval, but they don't seem to control the color mapping.  They just control the portion of the color mapping displayed on the color bar.  Correct?


--
Jim Jennings