Here are the last few lines of the error log for reference:

end setValues
setValues 0 300
end setValues
setValues 0 280
end setValues
setValues 0 110
end setValues
setValues 0 90
end setValues
setValues 0 0
end setValues
ASSERT: "rgb[0] >= 0. && rgb[0] <= 1. && rgb[1] >= 0. && rgb[1] <= 1. && rgb[2] >= 0. && rgb[2] <= 1." in file /home/ej/software/public/Slicer/Slicer4-SuperBuild-Debug/CTK/Libs/Visualization/VTK/Core/ctkVTKColorTransferFunction.cpp, line 123
error: [/data/ej/software/public/Slicer/Slicer4-SuperBuild-Debug/Slicer-build/bin/./SlicerApp-real] exit abnormally - Report the problem.

Postponed to 4.4 because it's a minor issue:

• happens rarely
• only in debug (assert)

As of Slicer r23177 built with VTK6 enabled, I was able to reproduce this problem.

Issue is as line 260 of qSlicerScalarVolumeDisplayWidget.css:

https://github.com/Slicer/Slicer/blob/03d871adad9a7e8659b01f226baa8f672c4d2077/Modules/Loadable/Volumes/Widgets/qSlicerScalarVolumeDisplayWidget.cxx#L260

More specifically, the rgb colors are set to -0.00000238664 when upper and lower threshold are 0

low -> 0 [threshold low value]
upper -> 0 [threshold upper value]
min -> low + 0.001 -> 0.001
max -> 419
maxVal -> (upper - min)/(max-min) -> -0.001 / (419 - 0.001) -> -0.00000238664

After discussing with Julien, the solution would be to "properly" implement the transfer function by using the "nextHigher" function. see [1]

That way, we wouldn't have to use the value 0.001 that doesn't work in some case.

[1] https://github.com/commontk/CTK/blob/488631a0f5cefd2384e77bbc4dfe228c873541f6/Libs/Visualization/VTK/Widgets/ctkVTKThresholdWidget.cpp#L60

It works correctly in Slicer 4.7.0-2017-06-02. There were related fixes.

Error logs are still visible when using Slicer 4.7.0-2017-06-02, so the problem is not solved, but this is only a debug assertion, not an application crash.