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20000621: Unidata-Wisconsin datastream additions: CIMSS products (cont.)

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  • Subject: 20000621: Unidata-Wisconsin datastream additions: CIMSS products (cont.)
  • Date: Wed, 21 Jun 2000 16:15:34 -0600
>From: "D. J. Raymond" <address@hidden>
>Organization: New Mexico Tech
>Keywords: 200006210029.e5L0TGT14952 Unidata-Wisconsin

Dave,

>That helps a lot.  Indeed, I am using my own code.

OK.  So the decoding into native AREA format will not help.

>Given what you
>have sent me, I should be able to modify things to make them work
>sometime in the next month.

Super.  Again, if this is not enough time, please let me know so we can
adjust the conversion schedule.

>Is there a way for a program to tell whether it is looking at an
>old delta-encoded file or a png compressed file?

Yes.  The beginning of the PNG compressed block in the image is labeled
with the ASCII sequence PNG.  It is then followed by four 4-byte words
the third and fourth of which are the length of an image line and the
number of image lines.  Since the existence of a calibration
codicil, aux block, and comment cards is not guaranteed, the location of
the PNG string is not fixed.  A code snippit from pnga2area.c shows how
to locate the beginning of the PNG compressed image lines:

    /*
    ** Now figure out where the beginning of the PNG compressed image is.
    */

    datoff = imgdir[33];
    datlen = datoff - nbytes;
    cmtlen = 80 * imgdir[63];

    udebug( "datoff: %d", datoff );
    udebug( "datlen: %d", datlen );
    udebug( "cmtlen: %d", cmtlen );

'datoff' (area directory word 33 (zero based)) would be the beginning
of the image lines in an AREA except that I move the comment cards to
that location so that the compressed image lines will be at the end of
the product.

'cmtlen' (area directory word 63 (zero based) is the number of bytes of
comment cards which will be located after all of the
directories/codicils and just before the PNG compressed image.  This
value could be zero.

The beginning of the PNG stuff will be at 'datoff+cmtlen' bytes from the
beginning of the file.

I hope that this helps...

Tom

>From address@hidden Wed Jun 21 16:27:49 2000
>Subject: Re: 20000621: Unidata-Wisconsin datastream additions: CIMSS products 
>(cont.)

Thanks Tom! -- Dave