Re: Converting to PNG8 loses transparency

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In GraphicsMagick, "PNG8" means indexed 8-bit PNG with optional binary
transparency.  PNG can represent 8-bit images with full transparency, but
that is not what we call PNG8.  "pngquant" is a handy third-party
application that can reduce a 32-bit RGBA PNG image to 8-bit with full
transparency (it reduces the palette to 256 or fewer RGBA entries, which
are represented in PNG via the PLTE and tRNS chunks).  You can get such
8-bit indexed images with full transparency via the -colors 255 option,
etc, but not by using the PNG8: format descriptor.  Note that PNG8 is not
defined by the PNG specification but rather by various other parties.  When
we defined "PNG8" in GraphicsMagick, the target for such images was a major
commercial browser that could not handle the tRNS chunk properly to decode
and display full transparency.


On Thu, Jun 20, 2013 at 12:18 PM, Kenneth Xu <[email protected]> wrote:

> Hello,
>
> This may be a dumb question but I did google as well as searching archive
> but cannot seem to find the answer. I'm trying to convert an PNG32 image to
> PNG8 using command below:
>
> gm convert source.png png8:target.png
>
> The command is useful to us as it reduces the image size to half but still
> yet good quality (see http://www.sitepoint.com/png8-the-clear-winner/).
> The problem is any transparency in source image is lost in the target
> image. The closest post I can find in the archive is this one:
> http://sourceforge.net/mailarchive/message.php?msg_id=20516411, but it
> was about creating an image. But that at least confirm that PNG8 in
> graphicsmagick can support transparency.Also, http://tinypng.org/ was
> able to do the same conversion but keeps the transparency intact, which
> means this is doable.
>
> So what I'm doing wrong? Thank you in advance for you help.I can provide
> test images if needed.
>
> Kenneth
>
>
>
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<div dir=3D"ltr">In GraphicsMagick, &quot;PNG8&quot; means indexed 8-bit PN=
G with optional binary transparency. =A0PNG can represent 8-bit images with=
 full transparency, but that is not what we call PNG8. =A0&quot;pngquant&qu=
ot; is a handy third-party application that can reduce a 32-bit RGBA PNG im=
age to 8-bit with full transparency (it reduces the palette to 256 or fewer=
 RGBA entries, which are represented in PNG via the PLTE and tRNS chunks). =
=A0You can get such 8-bit indexed images with full transparency via the -co=
lors 255 option, etc, but not by using the PNG8: format descriptor. =A0Note=
 that PNG8 is not defined by the PNG specification but rather by various ot=
her parties. =A0When we defined &quot;PNG8&quot; in GraphicsMagick, the tar=
get for such images was a major commercial browser that could not handle th=
e tRNS chunk properly to decode and display full transparency.</div>
<div class=3D"gmail_extra"><br><br><div class=3D"gmail_quote">On Thu, Jun 2=
0, 2013 at 12:18 PM, Kenneth Xu <span dir=3D"ltr">&lt;<a href=3D"mailto:sou=
[email protected]" target=3D"_blank">[email protected]</a>&gt;</span=
> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div><div><div>Hello,<br><b=
r></div>This may be a dumb question but I did google as well as searching a=
rchive but cannot seem to find the answer. I&#39;m trying to convert an PNG=
32 image to PNG8 using command below:<br>


<br></div>gm convert source.png png8:target.png<br><br>The command is usefu=
l to us as it reduces the image size to half but still yet good quality (se=
e <a href=3D"http://www.sitepoint.com/png8-the-clear-winner/" target=3D"_bl=
ank">http://www.sitepoint.com/png8-the-clear-winner/</a>). The problem is a=
ny transparency in source image is lost in the target image. The closest po=
st I can find in the archive is this one: <a href=3D"http://sourceforge.net=
/mailarchive/message.php?msg_id=3D20516411" target=3D"_blank">http://source=
forge.net/mailarchive/message.php?msg_id=3D20516411</a>, but it was about c=
reating an image. But that at least confirm that PNG8 in graphicsmagick can=
 support transparency.Also, <a href=3D"http://tinypng.org/" target=3D"_blan=
k">http://tinypng.org/</a> was able to do the same conversion but keeps the=
 transparency intact, which means this is doable.<br>


<br></div><div>So what I&#39;m doing wrong? Thank you in advance for you he=
lp.I can provide test images if needed. <br><span class=3D"HOEnZb"><font co=
lor=3D"#888888"><br></font></span></div><span class=3D"HOEnZb"><font color=
=3D"#888888"><div>
Kenneth<br></div><div><br></div></font></span></div>
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<br></blockquote></div><br></div>

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