1999.08.22 17:34 "TIFF LZW : How select predictor", by Gilles Vollant
Sometime LZW compressed TIFF are smaller with Predictor=1 (or none Predictor Tag, 1 is the default value), sometime with Predictor=2
Tiff spec say:
<< A predictor is a mathematical operator that is applied to the image data before an encoding scheme is applied. Currently this field is used only with LZW (Com-pression= 5) encoding because LZW is probably the only TIFF encoding scheme that benefits significantly from a predictor step. See Section 13.
The possible values are:
1 = No prediction scheme used before coding.
2 = Horizontal differencing.
Default is 1.>>
Photoshop seem saved Paletted image with predictor 1, and RGB or grey image with predictor 2