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

Marc MERLIN marc_news at valinux.com
Wed Aug 29 00:11:03 PDT 2001


On Sat, Aug 11, 2001 at 08:16:08AM -0700, Marc MERLIN wrote:
> On Wed, Aug 01, 2001 at 05:47:38PM -0700, greg.roelofs at philips.com wrote:
> > 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:
>  
> Sorry, I didn't get to follow this very closely. Were you guys able to
> answer him and/or use the pngs he sent us?

Could someone confirm that he's been answer so that I can archive this in my
"dealt with" folder.

Thanks
Marc

> >      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, ...
> > 
> > 
> > 
> > _______________________________________________
> > web-team mailing list
> > web-team at lists.svlug.org
> > http://lists.svlug.org/lists/listinfo/web-team
> 
> -- 
> Microsoft is to operating systems & security ....
>                                       .... what McDonalds is to gourmet cooking
>   
> Home page: http://marc.merlins.org/   |   Finger marc_f at merlins.org for PGP key
> 
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-- 
Microsoft is to operating systems & security ....
                                      .... what McDonalds is to gourmet cooking
  
Home page: http://marc.merlins.org/   |   Finger marc_f at merlins.org for PGP key




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