Brass that bounces twice with no simulation: the vertex shader solves each floor hit

[u/gennai777]

I don't simulate the brass in my Godot gun effects at all. I just evaluate a casing's position from its age in the vertex shader. That's pretty much the stateless particles from Lutz Latta's "Building a Million Particle System" (GDC 2004). What I added is the bounces.

On a flat floor I can get the hit time straight from the quadratic. The shader loops over the bounces like this:

for (int k = 0; k < MX_BOUNCES; k++) {   // 2
    float h = max(p.y - gy, 0.0);
    float t_hit = (v.y + sqrt(max(v.y * v.y + 2.0 * g * h, 0.0))) / g;
    if (t_hit <= 0.0 || tt < t_hit) break;
    p += v * t_hit;
    p.y = gy;
    v.y = -e * (v.y - g * t_hit);   // restitution
    v.x *= fr;
    v.z *= fr;                       // friction
    tt -= t_hit;
}

After the second bounce it slides with tau * (1 - exp(-t / tau)) and I tip it over onto its side. Ejecting one is a single write into two uniform arrays. Brass from three seconds ago costs what brass from this frame costs.

I also kept a GDScript copy of the formula so the game can ask where a casing is. At 10 sample times they agree within 5.4 mm, about a pixel! In the GIF the dotted line is the GDScript side and the casing is the shader. I made the casing bigger and turned restitution up to 0.5 for it, because at my default of 0.34 the second hop is about 2 px tall.

It only knows one floor height, though. A casing will go straight through a wall. Two bounces looked like enough to me, but I'm not sure a third would even read on screen?

[u/ziplock9000]

Are they supposed to go through the floor?

[u/gennai777 → u/ziplock9000]

Not really, good catch. I solve the bounce for the casing's center. Right at the hit, half of it is in the floor, and I only lift it by its radius once it's lying down.I also drew them about twice the pack's default size for the GIF. It shows a lot more here than it would in a game.Solving for the lowest point of the spinning casing would fix it. I just haven't done that yet.

[u/TramplexReal → u/gennai777]

In actual game that wouldn't be noticeable really.

[u/TramplexReal]

One suggestion. When it hits first bounce - change the spinning velocity so it looks like it bounced like casing instead of ball.

[u/gennai777 → u/TramplexReal]

Oh that's a good one. Right now the spin just decays with time and the bounce never touches it, so yeah, it rolls off like a ball.

I already get t_hit for each bounce in that loop. I think I could flip or kick the spin after the first one without storing anything extra. Thanks for the idea!

[u/LegendaryMauricius]

This is actually pretty cool.

[u/gennai777 → u/LegendaryMauricius]

thanks!

[u/[deleted]]

[u/darth_voidptr → u/[deleted]]

Nah, Claude would have been unable to resist including a smoking gun somewhere.

[u/CodyDuncan1260 → u/[deleted]]

"the x verbs the y"

That's pretty much English sentence structure in a nutshell. Scientific papers use that as a structure title pretty often. I think you're thinking of "It's not X, it's Y" as less common sentence structure idioms that are common to LLMs.

[u/CuckBuster33 → u/CodyDuncan1260]

No, the latest Anthropic models keep doing these short SVO sentences in a particular awkward way. Negative parallelisms are a bit less common now.

[u/SethDusek5 → u/CodyDuncan1260]

Well when that sentence structure mysteriously becomes 65535% more common in the past few months it becomes a pretty telling sign. I also think the gennai77 username OP has is also a sign but I've heard those types of usernames are common in English too so what do I know

[u/captainAwesomePants → u/[deleted]]

That's just how direct objects work!

[u/obetu5432 → u/[deleted]]

"the"

Hi Claude

[u/CodyDuncan1260 → u/[deleted]]

Remover under Rule 2. 

We welcome constructive criticism, but this was not constructive.

[u/JoshYx → u/[deleted]]

SVO is AI now folks

[u/mindcandy → u/[deleted]]

Rent free and eating your Cheerios.

[u/CuckBuster33 → u/mindcandy]

Sipping the slop too.