LineTraceMultiByChannel - Overlap until Hit - How To?

Elvince · May 20, 2016, 2:49pm

Hi,

Is there a equivalent of LineTraceMultiByChannel but where the overlap list stop when a “block” occured?

My goal is to retrieve the list of overlap between a point A and B and stop the process once a Block happen.

As of today, LineTraceMultiByChannel will list all Overlaping object and my “block” object but some Overlap item are behind the Block Object.
The only way to use this list is to sort / compare the “time” link to each HitResult and check that it happenned before the “block”.
In term of efficiency, I bet there is an alternative way.

Any proposal?

Thanks,

ExtraLifeMatt · May 20, 2016, 3:45pm

LineTraceSingleByChannel should be what you want. It returns the first blocking hit only (if found).

Elvince · May 23, 2016, 10:22am

It’s not what I’m looking for.
I need the list of Overlap until the Blocking hit.
I may do 2 traces or sort the LineTraceMultiByChannel by time, but I was wondering if something more efficient was already implemented.

thanks,

ExtraLifeMatt · May 23, 2016, 1:54pm

According to the comments in the code, the blocking hit will always be the last element in the array returned by LineTraceMultiByChannel:

 *  Trace a ray against the world using a specific channel and return overlapping hits and then first blocking hit
 *  Results are sorted, so a blocking hit (if found) will be the last element of the array
 *  Only the single closest blocking result will be generated, no tests will be done after that

So, if you aren’t seeing that behavior that would be a bug.

Elvince · May 23, 2016, 2:03pm

This is what I thought initially, but if you read carefully, it never say that the Overlapping Hit will be constraint by the blocking hit.
So you have ALL the overlapping hit in your trace distance and also the position of the first blocking hit but there is no relation between Overlapping hits and Blocking hit. That’s confuse me at first glance.

ExtraLifeMatt · May 23, 2016, 2:26pm

Yea, I wouldn’t expect there to be information relative to the blocking hit vs the overlapping shapes. If you need the overlapping shapes sorted, then you just have to do that manually it seems - which isn’t the end of the world. I’d definitely lean towards just sorting the list than doing a 2nd raycast. If you use the Array::Sort method with a custom predicate - it’ll be quick (it automatically does a QuickSort if the element count is above 8, otherwise it defaults to BubbleSort).

pickersZ · May 24, 2016, 12:59am

Why dont you try sorting the results by distance? Surely the “only” blocking hit should be the furthest?

https://docs.unrealengine.com/latest/INT/API/Runtime/Engine/Engine/FHitResult/index.html

Elvince · May 24, 2016, 7:49am

SOrting by distance / time is not an issue.

I was just asking the community if there was something built-in that is optimized for this type of request. Just because, if you are doing a long distance trace, you can have to sort hundred objects while your blocking hit is near to you.

Thanks for your answer and I will implement the sort on time in my code!