v.abs v.abs is intended to operate on signed images (where negative pixel values are allowed). v.abs is intended to operate on signed images (where negative pixel values are allowed). For int16, int32 and float32 images, the output is the absolute value of each input pixel. For 8-bit images, v.abs assumes that the image is 'pseudo-signed' in which the range 0 to 255 represents -128 to 127. It first subtracts 128 from the 0-255 value to produce a signed image and then performs the absolute value. (Objects like v.motion when used in signed mode set their output based on the same assumption)

from the library : softVNS Format : External Environment : jitter / max commercial Max 4.x http://homepage.mac.com/davidrokeby/softVNS.html