[web-team] feedback from SVLUG website: PNG fixes

Heather star at betelgeuse.starshine.org
Wed Sep 5 17:26:02 PDT 2001


Apologies for the delay.  Your updated images have been posted - a copy
of the previosu images has been kept, in case anybody reports that these
cause other problems.

Thanks bunches for your investigation of the problem though ... and even
more for applying its solution!

  . | .   Heather Stern
--->*<--- Starshine Technical Services
  ' | `   SVLUG Web Content Coordinator

> The Netscape 4.x PNG problem ("may have difficulty rendering this") on
> your maps page (http://www.svlug.org/directions/cisco-9.shtml) is well-
> known and has been documented for some time:
> 
>      http://www.libpng.org/pub/png/pngapbr.html#nn
> 
> The fix is to remove the bKGD chunks from your (opaque) PNG images, or
> not to add them in the first place.  The simplest way to do the former
> is with pngcrush, for which there's a link here:
> 
>      http://www.libpng.org/pub/png/pngapcv.html
> 
> For example:
> 
>     pngcrush -d tmpdir -rem bkgd north-sj.png cisco-9-lrt-path-2001.png
>     mv tmpdir/*.png .
> 
> Before:
> 
>     File: svlug.org-north-sj.png (25161 bytes)
>       chunk IHDR at offset 0x0000c, length 13
>         390 x 245 image, 24-bit RGB, non-interlaced
>       chunk gAMA at offset 0x00025, length 4: 0.45455
>       chunk bKGD at offset 0x00035, length 6: red = 255 green = 255 blue = 255
>       chunk pHYs at offset 0x00047, length 9: 2834x2834 pixels/meter (72 dpi)
>       chunk tIME at offset 0x0005c, length 7:  1 Jun 2001 17:23:29 GMT
>       chunk IDAT at offset 0x0006f, length 8192
>         zlib:  deflated, 32K window, maximum compression
>       chunk IDAT at offset 0x0207b, length 8192
>       chunk IDAT at offset 0x04087, length 8192
>       chunk IDAT at offset 0x06093, length 418
>       chunk IEND at offset 0x06241, length 0
>     No errors detected in svlug.org-north-sj.png (91.2% compression).
> 
> After:
> 
>     File: svlug.org-north-sj-cr.png (17422 bytes)
>       chunk IHDR at offset 0x0000c, length 13
>         390 x 245 image, 24-bit RGB, non-interlaced
>       chunk gAMA at offset 0x00025, length 4: 0.45455
>       chunk tIME at offset 0x00035, length 7:  1 Jun 2001 17:23:29 GMT
>       chunk pHYs at offset 0x00048, length 9: 2834x2834 pixels/meter (72 dpi)
>       chunk IDAT at offset 0x0005d, length 17309
>         zlib:  deflated, 32K window, maximum compression
>       chunk IEND at offset 0x04406, length 0
>     No errors detected in svlug.org-north-sj-cr.png (93.9% compression).
> 
> You'll note that this also reduces their sizes by about 25-30%.  Of course,
> converting them to palette mode makes them smaller still, although it re-
> quires other tools (e.g., XV or pngtopnm/pnmtopng).  And you can shave off
> a little more also by using additional pngcrush -rem options to nuke the
> pHYs and tIME chunks, which are not especially necessary:
> 
>     File: svlug.org-north-sj-8-cr.png (14394 bytes)
>       chunk IHDR at offset 0x0000c, length 13
>         390 x 245 image, 8-bit colormap, non-interlaced
>       chunk gAMA at offset 0x00025, length 4: 0.45455
>       chunk PLTE at offset 0x00035, length 96: 32 palette entries
>       chunk IDAT at offset 0x000a1, length 14213
>         zlib:  deflated, 32K window, maximum compression
>       chunk IEND at offset 0x03832, length 0
>     No errors detected in svlug.org-north-sj-8-cr.png (84.9% compression).
> 
> For your convenience, I'll send the fixed, 8-bit images in my next two
> messages.  (I'd append them to this one, but my ancient, cheesy-but-fully-
> virus-proof mailer doesn't do MIME.  Sorry.)
> 
> Btw, if you have comments or questions, please respond to my other account
> (newt at pobox.com).  Many thanks.
> 
> Regards,
> --
> Greg Roelofs           newt at pobox.com           http://pobox.com/~newt/
> Newtware, PNG Group, Info-ZIP, Philips Semiconductors, ...
> 
> 
> 
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