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
encoding scheme is applied. Currently this field is used only with LZW
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