Multi-layered icons in Breeze Icons?

[David Hurka]

Hi there,

I’m thinking about ways for some icons which can show different colors. Use 
case is as follows: In Okular, Simone is working on icons for color chooser 
buttons. There are three such buttons, so they need a) an icon to distinguish 
them and b) a color to show the current color. In Okular, I am working on 
icons for color mode selection. There are color modes of kind
“change color to ...”, so the color modes need a) icons to distinguish them 
and b) a color to show which color will be used.

The current way is to fetch an icon with QIcon::fromTheme(), fetch one pixmap 
from that icon, paint a colored rectangle on the pixmap, and create a QIcon 
from that pixmap. The disadvantage is that the color rectangle has no nice 
shape which follows the icon geometry.

My idea is to add multi-layered icons to the icon theme, which consist of 
multiple SVG files. E. g.:
change-paper-color.svg
change-paper-color_overlay.svg
The overlay would be pure black as a fixed color.

Then we can make a QIconEngine which fetches the icon base from
change-paper-color.svg trough QIcon::fromTheme(), so the icon is recolored 
though the Breeze style. Then it draws an overlay, which it gets from
change-paper-color_overlay.svg trough QIcon::fromTheme(), so the color shape 
fits nicely to the icon base. If the overlay icon is not available, the 
QIconEngine would fall back to boring rectangles.

Would it be possible (allowed) to add such multi-layer icons to the Breeze 
icon set? Currently we need max two overlays per icon. Other applications, 
which don’t implement such a QIconEngine, could still use the base icon.

And I will have to check whether it is possible at all to implement such a 
QIconEngine.

Cheers, David