Icon themes workgroup

Summary

App icons' sizes and decisions are a mess. Let's make things clearer.

Current state

The following problems have been raised: https://bugzilla.gnome.org/show_bug.cgi?id=764222

New icon themes

Pixel-perfection

If the icons are bitmaps, the icons have to be redrawn, pixel-perfect, for every needed size.

If the icons are vectorial, the icons have to be drawn pixel-perfect for at least the smaller needed size. As it has been noted, scales by multiples keep the icon pixel-perfect, and non-multiple scales are a little blurry but still acceptable (when the original was not already blurry, hence pixel-perfect) on nowadays screens. Jimmac says:

22:09 < jimmac> hinting gets looser as we go highpi
22:09 < jimmac> ie "it gets fuzzy" but fewer people care. people focus on the shapes
22:10 < jimmac> Jehan: for symbolic we made the decision not to do size variants
22:10 < jimmac> and went for one size fits all™
22:10 < jimmac> i'd go 16x16 everywhere, yes

Conclusion: there may be alternate versions for very big icons, like 128x128, adding details in particular. But smaller sets of icons are often OK from a single version.

Preferred style

The new GIMP preferred style is Symbolic, in particular light icons on dark background, which is the expected style for graphics applications nowadays: toned down for creation needs. We should avoid visual pollution to make the canvas the focus point and not "shine pretty pixels around" (paraphrasing Jimmac).

A color vectorial icon theme is still kept as a fallback for people who prefer color icons. The old bitmap icon theme is kept as "Legacy" but not a priority anymore.

Preferred sizes

GTK+2 `GtkIconSize` used to propose various intermediate sizes: 16, 18, 20, 24… On the other hand GTK+3 is settling for multiples of 16 and 24. Since many widgets use `GtkIconSize` to set their icon sizes, and not numerical sizes in pixel (sometimes workarounds exist, sometimes no, it seems), it is preferred that most our icons follow this pattern as well, in order to prepare GIMP 3.

Where to have icons?

Once again, Jimmac was noting that GIMP uses a lot of icons, with sometimes not-perfect metaphors. It may be interesting to wonder more when icons have to be done or not. Let's note that some DE (like GNOME) are not showing by default some of the icons, like in menus.

Spacing, margins

Our margins should also increase when we increase icon sizes to have breathing space.

Implementation note: houz was noting that GTK+ that sizes when given numerically in pixels are not scaled on HiDPI and that Darktable uses a DT_PIXEL_APPLY_DPI macro in its source to hardcode scaling. This should be tested.

Icons

Toolbox icons

Toolbox icons should be proposed in sizes 16x16 and 24x24. When vectorial, creating only 16x16 can be enough (up to the designer if one wants to do more).

Note: legacy icon theme was 16x16 and 22x22. It has been changed to follow GTK+3 direction, as noted above. If an icon theme designer wants to keep propose a 22x22 version, the index.theme should have the [22x22/apps] section with Type=Threshold to make sure these icons are taken in priority when GIMP requests the 24x24 size.