For signed char, signed short, and signed int data types, all negative raw (uncalibrated) values are always set to zero when the file is loaded and therefore cannot be displayed in MultiTracer under any circumstances. Consequently, if negative values are meaningful and need to be visualized, it is recommended to store them in float or double format volumes. Float and double format volumes cannot be edited, but do allow display of negative values as described below.
Contrasts computed from (signed or unsigned) char, short or int data type timeseries can be negative and can be displayed by simply inverting the sign of the contrasts. In the contrast window "Define" menu, choose "Invert current contrasts" to toggle between corresponding positive and negative contast values.
For float and double data types, negative values can be displayed when a single volume is loaded by clicking the "Display negatives" checkbox in the Image Intensity panel. When this box is checked, the value in the "Unscaled values < ___ are black" field is applied after negative and positive values are interchanged (i.e, "Unscaled values < 10 are black" implies that displayed negative unscaled values > -10 are black). The image intensit(ies) displayed in the lower right hand corner of the main display window will still reflect the actual voxel intensities without negation applied.
When a floating point time series is loaded and therefore displayed as a contrast, the "Display negatives" checkbox is not available, but the negative values can be displayed by inverting the sign of the contrasts. In the contrast window "Define" menu, choose "Invert current contrasts" to toggle between corresponding positive and negative contast values.
©2001-2012 Roger P. Woods, M.D.
Modified: August 28, 2012