CSQC Tracelines

[c0burn]

[frag.machine]

[toneddu2000]

Saw this post just now. Please frag.machine, can you please explain how to use trace_fraction? I never understood how to use it.
Is it the percentage of world hit by line(0.1 = 10%, 0.5 = 50%, etc.)?

Thanks in advance for your clarification

[Spike]

trace_fraction:
if you cast a ray from A to B, that ray you can hit planes C,D,E,F, etc. each plane has its own plane and normal.

distfromA = dotproduct(A, planenormal) - planedist;
distfromB = dotproduct(B, planenormal) - planedist;
fraction = distfromA / (distfromB-distfromA);
endpos = A + (B-A)*fraction;

if you're doing triangle-based collision, then there are 3 more dotproducts against endpos and the 3 planes that define the edges of the triangle.
brush collision does some traces against every single plane of the brush to figure out which plane it actually hit (furthest, so long as it wasn't clipped by another plane).
bsp collision provides an ordered tree of planes, with the results of plane checks determining which other leafs/planes need to be checked.

the actual trace that is returned by traceline is then the trace against the plane which resulted in the lowest fraction (where it actually hit the geometry that included that plane, and where the far side is considered solid with the near side not).
things get more messy when using traceboxes - the plane distances are expanded according to the size of the box. in the case of q1bsp these distances are precomputed, hence why only certain 'hull' sizes work correctly.

so yes, fraction is the (decimal)percentage of the distance travelled. 0 if it hit immediately, 50% if the wall was half of the distance away, and 1 if it travelled the full distance without hitting anything.

the startsolid flag is poorly defined. logic dictates that it can only be set when fraction is 1, which makes testing for it unreliable. often you'll want to ignore startsolid, because it can be better to allow traces through solids if you're already inside them as a way to attempt to counter any precision issues. alternatively you can do what nq player physics does, and just teleport the player back to their oldorigin if the player gets stuck.
the other contents flags are similarly unreliable, and reflect only the nearest impact. if you're pushing against a submodel while in water, if it detects and impact against that submodel then you won't be in water, but if you move away slightly then suddenly you will be. pointcontents is more reliable for this sort of thing.

[toneddu2000]

[frag.machine]

There is a lot of applications to trace_fraction, but I mostly use it for either proportional damage or because as I said is a more reliable collision test than checking trace_ent directly.

[toneddu2000]

[frag.machine]

If trace_fraction is less than 1.0 means it hit something (including the world).

if trace_ent == world either can be "I hit nothing, then I will return a null reference entity (trace_ent == world and trace_fraction == 1.0)
OR
"I hit the world, thus I'll return it, even though I normally return it when I hit nothing too (trace_ent == world but trace_fraction < 1.0).

traceline has a very particular behaviour regarding entities it may ignore. In DP and FTE its behaviour was extended (and additional buitins like tracebox are supported), making it even more complicated to understand.

I remember reading some stuff from FrikaC that helped me a lot. Check the .

EDIT: added wiki link.

[toneddu2000]

As far as I know ssqc and csqc don't know anything about each others (but they can share data, though). So, basically is as they would live in different dimensions. I usually do all the traceline computation in ssqc and then sendentity a vector and then I read that vector in csqc and make all the magic directly in csqc. Sorry cannot to help you more now but I'm replying from mobile and it's impossible for me to write code with phone..

[toneddu2000]

This is why I hate smartphones. I saw first post on my mobile and I thought c0burn's post it was the last one and I replied to it. Back to pc I found out it was the first one...grrrr