good evening I'm thinking about the spark VFX from Burnout Revenge on the Xbox 360

I wish to high five whatever tech artist was responsible for this trick; just unbelievably good

Fundamentally it's just The Akira Taillights Thing: Particles move in world space, but are rendered as trails connecting their last few SCREEN-SPACE positions

The concept is simple but there are a lot of weird practicalities to work out

effectively every particle consists of:
- a world-space position, which is usually animated on a simple ballistic trajectory
- a "chain" of the last few screen-space positions, which are connected by ribbons when the particle is rendered

But! Screenspace positions are 2D, but somehow perspective still works: trails closer to the camera get fatter

which means there's some kind of Cool Tricks going on

Oh yeah, in addition to perspective, occlusion works too! Particle trails can still go behind solid objects, which means the chain vertices definitely have 3D positions, somehow

this is one of those problems where a "correct" solution doesn't exist because the terms are contradictory (chain verts must effectively be both 3D and 2D at the same time), so somebody has to make up a Cool Trick that Mostly Works

the most fun category of problem

I *think* what they're doing here is storing chain verts in 3D, but in *clip space*, and then projecting BACK into world space before DRAWING the ribbons

i.e. each vert has a 2D viewport pos + a Z depth into the frustum

Thus verts are pinned in screenspace but can vary distance

the Z value would get more and more wrong on each frame as the camera moves

you could conceivably try to mitigate this with more tricks (subtract Z delta from prev camera pos, maybe)

but it might work fine enough already? particularly since chains are only like 4 frames long

Too hard to find a video clip for this, but: A neat LoD trick is it sometimes stores the chain values for a CLUSTER of sparks instead of each one individually. Apparently drawing tons of ribbons wasn't a bottleneck, but they needed to reduce the memory cost of all the chains

(the visual result of this is that sometimes blobs of distant sparks will all have parallel trails. It works great; I never even noticed it until going over videos frame by frame today lol)

(also I think past a certain distance it stops tracking trails and falls back to traditional stretched-billboard particles. Not 100% sure about this though)

Two particularly cool things here:
- The way the camera shake turns the trails into squiggles
- The bit at about 0:14 where the camera is suddenly knocked backwards, causing the trails to reverse direction as the particles are now moving away from the screen instead of towards it

Side note:
I've been grabbing most of these clips from this youtube playlist:

so, uh, credit to this dude with one of the most jaw-dropping title cards I have ever seen

Anyway I really want to borrow this effect some day but, interestingly, it's really only effective in a game where the camera undergoes REALLY large, abrupt movements

It works perfectly in Burnout bc the camera constantly cuts away to show crashes, but most games aren't Burnout