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May 2004
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2004.05.10 18:56 "Re: Tiff questions", by Frank Warmerdam
James Carroll wrote:
> Hi, I have a bunch of questions.
>
> I am exploring using TIFF support for my image server that also supports
> FlashPIX. The general idea is that the images are very large, and
> people want to zoom in and out, so I am trying (pyramidal) multiple
> levels of downsampled versions of the same image, all stored as
> jpeg-compressed tiles.
>
> I notice Adobe Photoshop can save a pyramidal TIFF, but when I do so,
> then look at it with tiffinfo, I don't see multiple pages, or tiles. It
> seems to be one large image. The file size is 4x what the image would
> be normally, so something extra is stored somewhere. Normally the image
> pyramid makes the file 1.33x bigger (not 4x.) Something tells me this
> isn't going to help me.
Jim,
I can't answer what Photoshop is doing. Are you saying there is no
difference in the tiffinfo output before and after building pyramids but
the result file is 4x larger? Thats pretty odd.
> Can anyone point me to the details of the Photoshop Pyramidal format?
>
> I've got a small test program using libtiff that I'm using to test
> writing tiled tiffs, and it fails. I get a strange return code from the
> write, but haven't found a reference of what the return values mean. I
> notice there's an TIFFErrorHandler callback I can register, but I can't
> find any examples or specifics of how to do this. Maybe this would tell
> me what I'm doing wrong.
static void
unixErrorHandler(const char* module, const char* fmt, va_list ap)
{
if (module != NULL)
fprintf(stderr, "%s: ", module);
vfprintf(stderr, fmt, ap);
fprintf(stderr, ".\n");
}
...
TIFFSetErrorHandler( UnixErrorHandler );
...
The "va_list ap" is the generic variable arguments stuff used by printf
and so on.
> How do I make sense of the return codes when writes fail using libtiff?
I think you need to look at the various functions to see what the return
codes mean. For example the TIFFSetField() has the following return
values:
RETURN VALUES
1 is returned if the operation was successful. Otherwise, 0 is
returned if an error was detected.
> When I write tiff tiles, there are parameters for x, y, and z. Does the
> Z parameter correspond to different pages in a multi-tiff or is it a
> different mechanism?
No, the "Z" is not a page value. It is some sort of third dimension but
I have never used it. But it is very distinct from the page file handling.
> (Please CC: jim@microbrightfield.com, I'm trying to subscribe, but my
> subscrption is deferred for approval.)
Now approved. Sorry for the delay.
I would note that there is code related to overviews in the libtiff/contrib/addtiffo
directory. It includes a utility to build overviews (as seperate pages with the
ReducedResolution bit set). My GDAL library takes advantage of overviews for faster
display of data in some circumstances. (http://gdal.maptools.org). I can provide
sample files, and inspect pyramid files you produce if you would like. You can
even download a built kit for Windows that you can use to prepare/check pyramided
files. Let me know (directly) if you need details.
Best regards,
---------------------------------------+--------------------------------------
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