Class methods VS Class static functions VS Simple functions - Performance-wise?

There is absolutely no difference between classic old C functions and static methods of classes. The difference is only aesthetic. If you have multiple C functions that have certain relation between them, you can:

• group them into a class;
• place them into a namespace;

The difference will again be aesthetic. Most likely this will improve readability.

In case if these C functions share some static data, it would make sense (if possible) to define this data as private static data members of a class. In this case variant with the class would be preferable over the variant with namespace.

I would discourage you from creating a dummy instance. This will be misleading to the reader of the source code.

Creating an instance for every bitmap is possible and can even be favorable. Especially if you call methods on this instance several times in a typical scenario.

At CPU level, there is only one kind of function, and it very much ressemble the C kind. You could craft your own, but...

As it turns out, C++ being built with efficiency in mind maps most functions directly to call instructions:

• a namespace level function is like a regular C function
• a static method is like a namespace level function (from a call point of view)
• a non-static method is very similar to a static method, except an implicit this parameter is passed on top of the other parameters (one pointer)

All those 3 have the exact same kind of performance.

On the other hand, virtual methods have a slight overhead. There was a C++ technical report on performance which estimated the overhead compared to a non-virtual method between 10% and 15% (from memory) for empty functions. Meaning that for any function with meat inside (ie, doing real work), the overhead itself is close to getting lost in the noise. The real cost comes from the inhibition of inlining unless the virtual call can be deduced at compile-time.