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August 1999
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1999.08.22 18:59 "Re: TIFF LZW : How select predictor", by Gilles Vollant
Tiff 6.0 doc say << LZW without differencing works well for 1-bit images, 4-bit grayscale images, and many palette-color images. But natural 24-bit color images and some 8-bit grayscale images do much better with differencing.>> So Adobe Photoshop does like Adobe Tiff Spec -----Message d'origine----- De : owner-tiff@olympiakos.com [mailto:owner-tiff@olympiakos.com]De la part de Gilles Vollant Envoyé : dimanche 22 août 1999 19:35 À : tiff@olympiakos.com Objet : TIFF LZW : How select predictor 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 Any tips ?