>From: Larry Riddle <address@hidden>
>Organization: UCSD
>Keywords: 200302211732.h1LHWat23001 LDM pqact.conf ldm-mcidas pnga2area
Hi Larry,
Please read this entire message as I have a question for you at the
bottom. Thanks...
re: pnga2area decoding of PNG-compressed Unidata-Wisconsin imagery
>This discussion brings up a couple of questions in my mind. The
>implication here is that these images are sent out in "png" format.
The reality is that the data portion of the images is compressed using
PNG compression. The header portions of the images are left intact.
>If that is true (question #1),
This is not specifically true.
>is it possible to get the LDM to write out
>these files in that format (question #2)?
A "graphical" depiction of what the ldm-mcidas PNG compression routine,
area2png, does to the Unidata-Wisconsin images is as follows:
McIDAS AREA UNIWISC PNG Compressed AREA
+-----------------------+ +-------------------------+
| Header | | Header |
+-----------------------+ +-------------------------+
| Nav | | Nav |
+-----------------------+ area2png +-------------------------+
| Cal | =====> | Cal |
+-----------------------+ +-------------------------+
| AUX | | AUX |
+-----------------------+ +-------------------------+
| | | Comment Cards |
| | +-------------------------+
| | | |
| | | PNG |
| Uncompressed | | Compressed |
| Data | | Data |
| | | |
| | | |
| | +-------------------------+
| |
| |
| |
| |
| |
+-----------------------+
| Comment Cards |
+-----------------------+
What you would have to do to end up with a file containing just the PNG
compressed image is: write a program/script that would crack the
header of each image in order to figure out how large the header
portions of the image actually are (pnga2area does just this). After
you know that, this program would need to strip the headers off and
write just the PNG compressed data portion out to a file. So, you
could take the pnga2area code in the ldm-mcidas source distribution and
modify it to do what you want.
Presumably, the objective here is to be able to then view the images in
a web browser without having to first convert them into a different
format. Remember that there will be no maps on the image, so their
usefulness will be limited.
OK, now for the question. We would like to install the most recent
release candidate for LDM 6 on your DEC Alpha for testing purposes. Is
this OK with you? I can provide more details about how LDM 6 differs
from LDM5 if you like.
Tom
NOTE: All email exchanges with Unidata User Support are recorded in the Unidata inquiry tracking system and then made publicly available through the web. If you do not want to have your interactions made available in this way, you must let us know in each email you send to us.