Image::Magick Depth

Guildencrantz has asked for the wisdom of the Perl Monks concerning the following question:

I've found an interesting issue where under ImageMagick-6.0.7.1 I can resize an image and it remains 8-bit, however under ImageMagick-perl-6.2.8.0 resized images are written at 16-bit. I need this code to generate 8-bit thumbnails on both systems and I can't seem to find a way to force the 8-bit depth from within PerlMagick. As an example I wrote this script:

#!/usr/bin/perl

use Image::Magick;

my $thumbnail = Image::Magick->new;
$thumbnail->Read("full.bmp");
$thumbnail->Write("full.png");

$thumbnail->Resize(width=>'200', height=>'150');
$thumbnail->Write("tn_200x150.png");

$thumbnail->Resize(width=>'100', height=>'75');
$thumbnail->Write("tn_100x75.png");

undef $thumbnail;

exec "file /home/mhenkel/*.png";
[download]

On the ImageMagick-6.0.7.1 it outputs:

/home/mhenkel/full.png:       PNG image data, 240 x 294, 8-bit/color R
+GB, non-interlaced
/home/mhenkel/tn_100x75.png:  PNG image data, 100 x 75, 8-bit/color RG
+B, non-interlaced
/home/mhenkel/tn_200x150.png: PNG image data, 200 x 150, 8-bit/color R
+GB, non-interlaced
[download]

and the ImageMagick-perl-6.2.8.0 system:

/home/mhenkel/full.png:       PNG image data, 240 x 294, 8-bit/color R
+GB, non-interlaced
/home/mhenkel/tn_100x75.png:  PNG image data, 100 x 75, 16-bit/color R
+GB, non-interlaced
/home/mhenkel/tn_200x150.png: PNG image data, 200 x 150, 16-bit/color 
+RGB, non-interlaced
[download]

Ideas? Suggestions?

Comment on Image::Magick Depth Select or Download Code

Replies are listed 'Best First'.
Re: Image::Magick Depth by parv (Parson) on Sep 03, 2008 at 03:40 UTC
According to a 2006 reply on video.image-magick.bugs, specifying `depth` attribute (with `8` as the value) should solve your problem. See also the manual (6.4x).	[reply] [d/l] [select]
Re^2: Image::Magick Depth by Guildencrantz (Sexton) on Sep 03, 2008 at 04:20 UTC
Unfortunately the Depth attribute is immutable. All references I've found suggest some kind of SetDepth function which does the changing. I just can't seem to find what it is.	[reply]
Re^3: Image::Magick Depth by Guildencrantz (Sexton) on Sep 03, 2008 at 05:18 UTC
DUH! Okay, I should have seen this before, but there's a Set() function: `$thumbnail->Set(depth => 8);` [download] So, to ensure that my code performs as expected on both systems: `#!/usr/bin/perl use Image::Magick; my $thumbnail = Image::Magick->new; $thumbnail->Read("full.bmp"); $thumbnail->Set(depth => 8); $thumbnail->Write("full.png"); $thumbnail->Resize(width=>'200', height=>'150'); $thumbnail->Write("tn_200x150.png"); $thumbnail->Resize(width=>'100', height=>'75'); $thumbnail->Write("tn_100x75.png"); undef $thumbnail; exec "file /home/mhenkel/*.png";` [download]	[reply] [d/l] [select]
Re: Image::Magick Depth by kyle (Abbot) on Sep 03, 2008 at 03:41 UTC
I'd try this... `$thumbnail->Quantize( colors => 2**8 );` [download]	[reply] [d/l]
Re^2: Image::Magick Depth by Guildencrantz (Sexton) on Sep 03, 2008 at 04:59 UTC
While that changes the colors, it doesn't change the channel depth.	[reply]