LibTiff Mailing List
TIFF and LibTiff Mailing List Archive
April 2005
Previous Thread
Next Thread
Previous by Thread
Next by Thread
Previous by Date
Next by Date
Contact
The TIFF Mailing List Homepage
This list is run by Frank Warmerdam
Archive maintained by AWare Systems
Thread
2005.05.02 17:55 "Re: Color TIFF/JPEG compression questions", by Joris Van Damme
> tiff2rgba -c jpeg -r 16 quad-jpeg.tif quad-jpeg-rgb.tif > > This gave me a file with the following tags: > > tiffinfo quad-jpeg-rgb.tif > TIFF Directory at offset 0xdb28 > Image Width: 512 Image Length: 384 > Bits/Sample: 8 > Compression Scheme: JPEG > Photometric Interpretation: RGB color > Extra Samples: 1<assoc-alpha> > YCbCr Subsampling: 2, 2 > Orientation: row 0 top, col 0 lhs > Samples/Pixel: 4 > Rows/Strip: 16 > Planar Configuration: single image plane > Software: LIBTIFF, Version 3.7.2 > Copyright (c) 1988-1996 Sam Leffler > Copyright (c) 1991-1996 Silicon Graphics, Inc. > JPEG Tables: (289 bytes) tiff2rgba most certainly produces a buggy image here. > My source image has these tags: > TIFF Directory at offset 0x5b6e > Image Width: 512 Image Length: 384 > Position: 0, 0 > Bits/Sample: 8 > Compression Scheme: JPEG > Photometric Interpretation: YCbCr > YCbCr Subsampling: 2, 2 > Samples/Pixel: 3 > Rows/Strip: 16 > Planar Configuration: single image plane > Reference Black/White: > 0: 0 255 > 1: 128 255 > 2: 128 255 > JPEG Tables: (574 bytes) Interestingly, the tagdump produced by AsTiffTagViewer is slightly different: ImageWidth (1 Short): 512 ImageLength (1 Short): 384 BitsPerSample (3 Short): 8, 8, 8 Compression (1 Short): JPEG Technote #2 Photometric (1 Short): YCbCr StripOffsets (24 Long): 8, 175, 342, 633, 2227, 3683, 5112, 6553,... SamplesPerPixel (1 Short): 3 RowsPerStrip (1 Short): 16 StripByteCounts (24 Long): 167, 167, 291, 1594, 1456, 1429, 1441, 1114,... PlanarConfig (1 Short): Contig XPosition (1 Rational): 0 YPosition (1 Rational): 0 JpegTables (574 Undefined): ReferenceBlackWhite (6 Rational): There's no YCbCrSubsampling tag here. Thus, tiffinfo clearly brings defaults into the picture, which is not ideal when the purpose is discovering what tags are there. > It looks like tiff2rgba retained the YCbCr field. And, for some reason > the source file has JPEG Tables, which I thought were used under Type 6 > compressions. I don't know if that's expected, but I thought I'd pass > it on. JpegTables is a valid compression 7 new-style jpeg-in-tiff tag. It contains a 'tables-only jpeg'. In this case, that is an SOI marker (start of image), 2 DQT markers (quantization tables), 4 DHT markers (huffman tables), and an EOI marker (end of image). The actual strips are then allowed to contain jpegs without tables, so-called 'abbreviated' jpeg streams. In this case, first strip contains SOI marker, immediatelly followed by a SOF0 and SOS marker, the compressed data, and trailing EOI. This tag is used to group common tables, used in all strips/tiles, into one single tables-only jpeg, to save space. Such usuage is actually even supported by the JPEG specification, and the JpegTables tag thus is a quite logical element of compression 7, even though it is not mandatory, tables can also be specified inside each tile/strip jpeg stream. Thus, quad-jpeg.tif is perfectly fine, as far as I can see. Joris Van Damme info@awaresystems.be http://www.awaresystems.be/ Download your free TIFF tag viewer for windows here: http://www.awaresystems.be/imaging/tiff/astifftagviewer.html