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Imp Fireball Jumping Over Map Geometry

edited 2012 Mar 13 in Technical Support
Not sure if this is Doomsday specific or if it happens in vanilla Doom but the Imp's fireball (and maybe other projectiles) moves over slightly elevated parts of a map instead of colliding with it. Once the fireball passes over the elevated geometry it moves back down and continues on its original course. A map to observe this would be map01 in the room with imps. Stand behind that thing on the ground that triggers the rocket launcher secret and watch the imp fireballs bounce over it and then back down before continuing. It doesn't just happen here but anywhere where a part of the map is only slightly higher than the fireball. I've seen this in older versions of Doomsday and assumed it would eventually be fixed. Now i'm wondering this is a known bug or if it isn't a bug at all.

*Oops! Didn't mean to post this in Tech Support*


  • No this isn't a bug. The fireball object is actually passing over that geometry (you can observe this by enabling the rend-dev-mobj-bbox 1 cvar), however because the sprite's offset would position it inside that geometry it is shifted upward (so that the bottom of the sprite is not occluded by said geometry).

    The id tech 1 software renderer would simply draw the sprite on top of the geometry in that situation. However Doomsday can't easily do the same what with the GL depth buffer et al.

    There are of course ways to emulate that behavior (depth peeling, for one). I expect we'll consider this when it comes time to design world renderer 2.0.
  • edited 2012 Mar 13
    Put simply, Dday can't display sprites partially embedded into the floor of the sector the mob it'self is in, only the ceiling and walls, so it shifts the sprite upward so that the entire sprite is above the floor.

    The actual hitbox of the mob is unaffected.

    In the case of the Imp fireball (and several other mobs), the sprite is bigger than the hitbox. This is why an Imp fireball that travels near the floor looks a bit like it is sliding along it; the sprite has touched the floor and is being shifted up by Dday, but the hitbox hasn't yet touched the floor.
  • Guess it isn't a problem with models. Looks like the previous build that made loading time increase also fixed a visual problem that made certain model projectiles not appear like they should, like the arachnatron plasma shot and caco energy ball.
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