[GRASS5] [bug #1209] (grass) In nviz tha Z slider of Cutting Planes Panel seems not to work properly

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this bug's URL: http://intevation.de/rt/webrt?serial_num=1209
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Subject: In nviz tha Z slider of Cutting Planes Panel seems not to work properly

Platform: GNU/Linux/i386
grass obtained from: Mirror of Trento site
grass binary for platform: Downloaded precompiled Binaries
GRASS Version: 5.0pre3

Please enter error description here (and your name)
In the Cutting Planes Panel of nviz, the Z slider seems to to work correctly until new values changing the Z range are not introduced in the type-in box. If the Z range is changed, values displayed in the slider seem not to correspond to  the elevation values of the displayed surface(s).
The following simple experience will be probably useful in understanding the problem.
I loaded the elevation.dem surface (spearfish dataset)on nviz. The min. and max. values of the surface are 1066 and 1840 m. I entered the Cutting Planes panel and the Z slider shows to be correctly set with a range 1066-1840. As I wanted to use a cutting plane at an elevation of 3000m, I typed this value in the Z box. The top value of the Z slider changed correctly at 3000 (actually 3001). In order to visually check the position of the cutting plane, I constructed (by the Surfaces panel) an horizontal surface at a constant elevation of 3000m. Then I chose a Z value of 3000m by the Z slider and I began to tilt the cutting plane by the Tilt button. In tilting it was evident that rotation was not around an axis situated on the 3000m surface, but much more above. To find the elevation of the tilting axis, and so of the cutting plane, I fixed the tilting angle at 90° (horizontal cutting plane)and I moved progressively the Z slider upward until the 3000m surface was displayed. This happened when the Z value displayed in Z slider was 1161 !. The tilting of the cutting plane with the Z at 1161 confirmed that rotation is around an axes on the displayed 3000m surface, so it seems that a surface at an elevation of 3000m is considered at 1161m by the Z slider. I changed the Z max to 4000 and I introduced another horizontal surface at that elevation. The slider indicates a value of 2161m for that surface. I noticed also that differences in elevation change if the new surface is introduced before or after the Z range modification
I repeated the experiment with other Z ranges (and horizontal surfaces) but I wasn't able to find out a rule.
The final question is: is there some sort of scaling or a shift of the Z origin in changing the Z range, or some bug in the program? (or may be I did some wrong operation?).
Many thanks and sorry for the long report.
Aldo Clerici

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