A Quantum of ImageMagick

ImageMagick defines Quantum as the base type to represent color. To make the framework flexible, a constant named MAGICKCORE_QUANTUM_DEPTH (or its deprecated alias, QuantumDepth) determines the number of bits in a Quantum. It gets a value among 8, 16, 32, and 64. An higher depth gives better precision, and it is paid in term of heavier resource consumption.

In the current ImageMagick version, MAGICKCORE_QUANTUM_DEPTH is defaulted to 16, you should see it in the magick-type.h include file. This lead also to a type definition for Quantum as unsigned short.

In this context, QuantumRange, the highest value that could be assigned to a Quantum variable (it has a deprecated alias, too, MaxRGB), is defined as a symbolic constant in this way:

#define QuantumRange  ((Quantum) 65535)

When working in C++ through the Magick++ library, this could lead to an unfriendly behavior.

Say that we need a Quantum variable, and we want to initialize it to QuantumRange. You are probably bound to write a line of code like this:

Magick::Quantum quantum = QuantumRange;

Alas, this could not work. As we have seen, the value of QuantumRange is casted to Quantum in its definition. This works fine when developing in C, but when we are writing code through Magick++, we need to qualify each Magick type with its proper namespace.
The compiler needs to know where to look for the Quantum definition, otherwise we'll get a compile error like "'Quantum' was not declared in this scope".

We have at least a couple of alternative solutions to this issue:

using namespace Magick;

// ...
Magick::Quantum quantum = QuantumRange;

All the code between the using directive and the end of file would have access to the the Magick namespace. A bit dirty, isn't it?

Magick::Quantum quantum;
{
  using namespace Magick;
  quantum = QuantumRange;
}

Split declaration and definition of the variable, and perform the second in a tiny scope where we use the namespace directive. Cleaner, but more verbose, too.