I my experience promotion traits are absolutely necessary to implement genereric image processing algorithms. Consider the simple matter of adding two or more byte images. In order to avoid overflow, intermediate results should be stored in a short or an int. If the result is to be written back to a byte image, the destination accessor can take care of shifting the result back into the appropriate range or clipping at the range limits. Promotion traits control this behavior. Hardly any algorithm in VIGRA can do without them.
Regards Ulli
Ulli: We don't believe the type of the result could automatically be inferred simply from the types of arguments. The optimal type depends on your intent and the context. For example, VIGRA's promotion traits choose double as the promotion for int when dealing with floating point operations. Why not float? On many architectures float is faster, and precision may not be an issue in the context in which you use it. Why should int always be the result type of adding two ints? Depending on what you are doing int may overflow... Int for summing two shorts will not overflow, but what if you are building, say, the integral image and you want to sum the values of all pixels in your image? As you can see, the optimal type depends on a complex interaction between speed, capacity and desired precision and hard-coding it makes the algorithms less generic. This is why in GIL we allow the type of the result to be specified explicitly upon invoking the operation. Promotion traits can at best be useful for a reasonable default, but even then we feel that they belong in boost::type_traits and not in an image library, as the problem generalizes beyond images. Lubomir