I am trying to design the architecture of a system that I will be implementing in C++, and I was wondering if people could think of a good approach, or critique the approach that I have designed so far.

First of all, the general problem is an image processing pipeline. It contains several stages, and the goal is to design a highly modular solution, so that any of the stages can be easily swapped out and replaced with a piece of custom code (so that the user can have a speed increase if s/he knows that a certain stage is constrained in a certain way in his or her problem).

The current thinking is something like this:

struct output; /*Contains the output values from the pipeline.*/

class input_routines{
    public:
    virtual foo stage1(...){...}
    virtual bar stage2(...){...}
    virtual qux stage3(...){...}
    ...
}

output pipeline(input_routines stages);

This would allow people to subclass input_routines and override whichever stage they wanted. That said, I've worked in systems like this before, and I find the subclassing and the default stuff tends to get messy, and can be difficult to use, so I'm not giddy about writing one myself. I was also thinking about a more STLish approach, where the different stages (there are 6 or 7) would be defaulted template parameters.

Can anyone offer a critique of the pattern above, thoughts on the template approach, or any other architecture that comes to mind?