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88 bytes added, 11:16, 14 December 2017
clarify not for plots; keep the most frequent warning message, to bring searches here.
Octave will use [ GraphicsMagick] for reading and writing your images (not for plots, unless you are actually doing only used for image processing). This means that the GraphicsMagick configuration you have on you system will determine what your Octave installation can do. Most systems will have it on their own repositories. Unfortunately, their configuration is not always the best for image analysis as it can limit the bit depth when reading and writing of images.
To solve the problem, GraphicsMagick needs to be rebuilt with the appropriate settings. See below for tracking dependencies and the reasoning behind each flag. Once done, the following commands should work to build GraphicsMagick.
The most common problem is the following warning when using {{Codeline|imread()}} or {{Codeline|imwrite()}}:
<pre>warning: your version of GraphicsMagick limits images to <N> bits per pixel</pre>
...where {{codeline|N}} can be 8, 16 or 32. For instance:<pre>warning: your version of GraphicsMagick limits images to 8 bits per pixel</pre>This warning means that GraphicsMagick was compiled with {{Codeline|--with-quantum-depth <N>}}, which implements that limitation. The [ INSTALL-unix page] of GraphicsMagick documentation explains the rationale of this option.
There are several bug reports requesting package maintainers to compile GraphicsMagick with higher values. You should not create a new bug; instead look for open bugs for your system, and leave a comment there. For convenience, here are the links for the bugs reported against [ Debian], [ Ubuntu], and [ Fedora].

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