Unfortunately, this is just a hack. It wouldn't work with e.g. cylindrical or spherical shapes, the outline wouldn't move with the camera. But having proper working outline like this isn't easy even in modern engines: http://www.michalorz...around-objects/
Frob in a separate glsl program is a great idea, I bet it would be easier to write a proper frob outline, or just use other custom solutions with such system.
Edit: At this point I'm thinking about what I used so far, so a cubemap but with this subtle slow pulse (alpha animated via simple table), so it's draws player's attention in most lighting conditions. Unfortunately cubemaps look dimmed in strong lighting. A blend add stage will look better, but it can't encompass the whole model in even manner, it needs some contrast. Stuff I did above needs custom alpha texture for every interactive object, so it's a no-go in a long run. A GLSL program that can use model geometry to do this, as in the link above, would be the best solution.
Edited by Judith, 04 January 2019 - 04:03 AM.