2005.06.07 18:43 "[Tiff] 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/