2004.01.14 15:07 "Re: [Tiff] COLORMAP and byte padding", by Frank Warmerdam

I'm adding write support to the libtiff based OpenBeOS TIFF Translator. I want to support palette images. The format of the colormap that

TIFFSetField(tif, TIFFTAG_COLORMAP, ???);

expects is unclear to me. In the documentation, it just says

Tag Name           Count  Type      Notes

TIFFTAG_COLORMAP   3      uint16*   1<<BitsPerSample arrays

Ok, so there are 256 entries in the palette if I have 8 bits per sample. So much I follow, but what's with the uint16*?!? And why Count == 3? The BeOS colormap bitmaps all use the same system wide palette, which consists of 256 entries each representing an rgb color with 3 (4 actually) 8 bit values for r, g, b (and alpha). Am I supposed to fit a 24 bit RGB value into 16 bits and have 256 uint16s? Or do I have 3 * 256 uint16s with 16 bits for each r, g and b?

Stephan,

There is no support for alpha values in the TIFF colormaps, so you just need to deal with the red, green and blue components. The individual components are uint16's to give additional precision in color specification. I have typically computed the 16bit value by multiplying the 8bit value by 256, but that isn't really the most precise approach as 255 would not quite become the maximum 16bit value (65535).

So, you will have 3 uint16s for each rgb tuple with values ranging between 0 and 65535.

I have also a couple more questions about strip size and byte padding. There seem to be two alternating fields that determine strip size:

TIFFTAG_ROWSPERSTRIP       rows per strip of data
TIFFTAG_STRIPBYTECOUNTS    bytes counts for strips

Can both be used at the same time to tell libtiff that

strip byte counts != rows per strip * samples per pixel * width?

(So that there are some padding bytes at the end of each row...)

The stripbytecounts are normally written internally by the library and shouldn't be touched by application code. You should set rowsperstrip to control the number of scanlines per strip.

Good luck,

---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam@pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent