Wed, Dec 8 2004
00:03:11
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Request created by jidanni@jidanni.org
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Subject |
display/drivers/PNG/description.html default GRASS_PNG_COMPRESSION
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From |
Dan Jacobson <jidanni@jidanni.org>
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Wed, 08 Dec 2004 00:16:16 +0800
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In display/drivers/PNG/description.html,
* GRASS_PNG_COMPRESSION=[0|1|9]
compression level of PNG files (0 = none, 1 = fastest, 9 = best)
Mention default=what?
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Wed, Dec 8 2004
14:33:27
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Mail sent by mneteler
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Here related details:
cd grass57/display/drivers/PNG/
grep GRASS_PNG_COMPRESSION *
description.html: <LI><B>GRASS_PNG_COMPRESSION=[0|1|9]</B><BR>
write.c: str = getenv("GRASS_PNG_COMPRESSION");
grep PNG_COMPRESSION_TYPE_DEFAULT *
write.c: PNG_COMPRESSION_TYPE_DEFAULT,
grep PNG_COMPRESSION_TYPE_DEFAULT /usr/include/png.h
#define PNG_COMPRESSION_TYPE_DEFAULT PNG_COMPRESSION_TYPE_BASE
grep PNG_COMPRESSION_TYPE_BASE /usr/include/png.h
png_byte compression_type; /* must be PNG_COMPRESSION_TYPE_BASE (IHDR) */
#define PNG_COMPRESSION_TYPE_BASE 0 /* Deflate method 8, 32K window */
#define PNG_COMPRESSION_TYPE_DEFAULT PNG_COMPRESSION_TYPE_BASE
... still no idea what to add to the HTML page.
Markus |
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Thu, Dec 9 2004
23:09:25
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Mail sent by glynn@gclements.plus.com
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Glynn Clements <glynn@gclements.plus.com>
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Date |
Thu, 9 Dec 2004 22:08:50 +0000
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To |
Markus Neteler via RT <grass-bugs@intevation.de>
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Cc |
jidanni@jidanni.org, grass5@grass.itc.it
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Subject |
Re: [GRASS5] [bug #2811] (grass) display/drivers/PNG/description.html default GRASS_PNG_COMPRESSION
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<20041208133327.C5067102BFC@lists.intevation.de>
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<20041208133327.C5067102BFC@lists.intevation.de>
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Markus Neteler via RT wrote:
> Here related details:
>
> cd grass57/display/drivers/PNG/
> grep GRASS_PNG_COMPRESSION *
> description.html: <LI><B>GRASS_PNG_COMPRESSION=[0|1|9]</B><BR>
> write.c: str = getenv("GRASS_PNG_COMPRESSION");
[snip]
> ... still no idea what to add to the HTML page.
The GRASS_PNG_COMPRESSION environment variable determines the level of
compression, with 0 being no compression and 9 being maximum
compression. If unset, the PNG library's default compression level
will be used.
The documentation for that function in png.h says:
/* Set the library compression level. Currently, valid values range from
* 0 - 9, corresponding directly to the zlib compression levels 0 - 9
* (0 - no compression, 9 - "maximal" compression). Note that tests have
* shown that zlib compression levels 3-6 usually perform as well as level
9
* for PNG images, and do considerably fewer caclulations. In the future,
* these values may not correspond directly to the zlib compression levels.
*/
extern PNG_EXPORT(void,png_set_compression_level) PNGARG((png_structp png_ptr,
int level));
Note: the PNG_COMPRESSION_TYPE_* stuff isn't relevant. It's present in
write.c because one of the required parameters to png_set_IHDR() is
the compression type, but PNG only actually understands one
compression type.
--
Glynn Clements <glynn@gclements.plus.com>
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Mon, Dec 12 2005
11:55:35
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Area changed to grass6 by msieczka
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Fri, Aug 4 2006
02:37:20
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Comments added by guest
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Cc: tutey@o2.pl
So what is the default compression level in the png library? The snippet you
posted didn't actually mention what it was, just that it uses its default
value, which is never stated.
~ Eric.
<epatton at nrcan dot gc dot ca> |
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Tue, Sep 26 2006
21:31:07
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Mail sent by guest
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From the png library manual (http://www.libpng.org/pub/png/libpng-manual.txt)
<snip>
Libpng uses zlib for its compression and decompression of PNG files.
Further information about zlib, and the latest version of zlib, can
be found at the zlib home page, <http://www.info-zip.org/pub/infozip/zlib/>.
The zlib compression utility is a general purpose utility that is
useful for more than PNG files, and can be used without libpng.
<snip>
Configuring zlib:
There are special functions to configure the compression. Perhaps the
most useful one changes the compression level, which currently uses
input compression values in the range 0 - 9. The library normally
uses the default compression level (Z_DEFAULT_COMPRESSION = 6). Tests
have shown that for a large majority of images, compression values in
the range 3-6 compress nearly as well as higher levels, and do so much
faster.
So it looks like the default compression level is set to 6 if
$GRASS_PNG_COMPRESSION is not set.
Maciek, can we close this one?
~ Eric.
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Thu, Sep 28 2006
13:38:25
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Mail sent by msieczka
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Good research Eric. Could you maybe update the doc in question and post the
patch to me?
Thanks,
Maciek
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Thu, Sep 28 2006
14:40:24
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Mail sent by epatton@nrcan.gc.ca
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From |
"Patton, Eric" <epatton@nrcan.gc.ca>
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To |
'Maciek Sieczka via RT ' <grass-bugs@intevation.de>
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Subject |
RE: [bug #2811] (grass) display/drivers/PNG/description.html defa ult GRASS_PNG_COMPRESSION
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Thu, 28 Sep 2006 09:40:13 -0300
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Sure. I think it's one of the Grass site webpages that needs to be updated.
I should be able to get this done today.
~ Eric.
-----Original Message-----
From: Maciek Sieczka via RT
To: jidanni@jidanni.org
Cc: Patton, Eric
Sent: 9/28/2006 7:38 AM
Subject: [bug #2811] (grass) display/drivers/PNG/description.html default
GRASS_PNG_COMPRESSION
Good research Eric. Could you maybe update the doc in question and post
the
patch to me?
Thanks,
Maciek
-------------------------------------------- Managed by Request Tracker
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Thu, Sep 28 2006
16:10:06
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Status changed to resolved by msieczka
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Thu, Sep 28 2006
16:10:06
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Mail sent by msieczka
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Closing it. Thanks Eric!
Maciek
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