Core vs Extension policy

[sdebionne]

Related to the grand true merge from develop to master discussed in #667, I wonder if that could be an opportunity to discuss "core vs extension". For instance I wonder if "rasterization" should not be an extension?

[striezel]

What is an extension? What is core functionality? What is the difference between those two - besides the directory where they are located?

For example, I use (among others) GIL's capabilities to read and write JPEG images. While the code for that is in the I/O extension and thus considered an extension, to me that is a "core" functionality in the sense that it is vital for an image library to be able to read and write images. That is why I believe the distinction between core and extension is not really relevant from the perspective of the users of the library. In the end both core and extension are part of GIL, and that is what matters to library users in the first place.

[simmplecoder]

I believe being in extension directory has two purposes. The first one is build simplicity. I/O libraries require 3rd party dependencies to be used. This is clearly the case for PNG, JPEG and co.
The other usage is for sort of a preview/beta test. Even though I remember testing rasterization well enough that I can say "it will work in most cases including the usual ones", I still think there might be some needs to change interface, change preconditions, adapt other algorithms and what not. I believe that we give much stronger API stability guarantee to core.
This is only my two cents though.

[sdebionne]

What is an extension? What is core functionality?

Beside the points made by @simmplecoder, I would say the main difference might be the maintenance effort given to it. Core is the main building block. Core should be stable, flawless, while I would expect little glitches in the extensions. Does that mean it's a question of software quality? Probably not. There is a notion of "optional" in extension.

Just a couple of observations:

• The framework part of I/O is in Core, the plugins for each formats are extensions.
• Runtime images, also a "vital" feature IMO, is an extension since it can be built on top of Core.
• AFAIU processing primitives are part of Core, while more "specific" processing implementations are extensions. Maybe the book "Principles of Digital Image Processing" can help do the triage between the two?
• Histogram is an extension while Rasterization is part of Core
• The documentation TOC does not reflect the distinction between core and extension (e.g. Histogram is documented as Core)

[mloskot]

AFAIR, we included, with @simmplecoder, the rasterization as part of the image processing bag during the GSoC

I wonder if "rasterization" should not be an extension?

I agree it is a good idea to move it to extension.
It also is the right time to do it before it's released.

[mloskot]

Yes, @simmplecoder and @sdebionne explained the differences very well.
I'd also add that having separate core and extension spaces allows us to

• keep the core minimal, clean or agnostic to specific domains or use cases (see note on Boost.Geometry below).
• accept extensions with general purpose as well as domain/use case specific features, and keep the GIL structure clean

The notion of extensions in Boost.GIL is very similar, if not the same, as in Boost.Geometry. For example, it offers geometric/vector features for Geographic Information Systems (GIS) as an extension, so such domain-specific stuff does not mess the core functionality/structure.

[marco-langer]

Should this distinction between core and extension also be made in the examples directory, i.e. move all extension examples into the relevant subdirectory?

[mloskot]

I always considered examples/ as a bag for all sorts of examples. Some present single feature but some present use of many features, also across core/extension. I'm not sure we have so many examples to justify distributing them into multiple folders.
I'd keep all in examples/ for now. Otherwise, there would have to be examples/ and extensions/examples/ side by side. Is it worth it?