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June 2005
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2005.06.07 18:43 "Group4 compression & alpha channel", by Bob Friesenhahn
It has been observed that when GraphicsMagick writes a min-is-black TIFF image with Group4 compression and an alpha channel (as an extra sample) that the right half of the image is lost. The image description is: TIFF Directory at offset 0x0 Image Width: 207 Image Length: 167 Bits/Sample: 1 Sample Format: unsigned integer Compression Scheme: CCITT Group 4 Photometric Interpretation: min-is-white Extra Samples: 1<unspecified> FillOrder: msb-to-lsb Orientation: row 0 top, col 0 lhs Samples/Pixel: 2 Rows/Strip: 167 Planar Configuration: single image plane DocumentName: muki.tif Software: GraphicsMagick 1.2 unreleased Q16 http://www.GraphicsMagick.org/ I strongly suspect that this loss of data is due to libtiff but have not done a thorough investigation yet to make absolutely sure. The same occurs when using FAX compression. By simply not requesting Group4 or FAX compression, the file is saved and read ok. This works: TIFF Directory at offset 0x0 Image Width: 207 Image Length: 167 Bits/Sample: 1 Sample Format: unsigned integer Compression Scheme: None Photometric Interpretation: min-is-black Extra Samples: 1<unspecified> FillOrder: msb-to-lsb Orientation: row 0 top, col 0 lhs Samples/Pixel: 2 Rows/Strip: 157 Planar Configuration: single image plane DocumentName: muki.tif Software: GraphicsMagick 1.2 unreleased Q16 http://www.GraphicsMagick.org/ Are there any known issues in libtiff with this scenario? Bob ====================================== Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/