Raycast

duke2017

Is there any examples of using a raycast? How return parameter of "the fixture hit by the ray"?

wall( 100, 100, 50, 50 )
function rayfunc(event)
???
end
ray = self.world:rayCast(0, 0, 200, 200, rayfunc)

jack0088

changing the Alpha?

duke2017

Alpha?) No.. I'm talking about b2.World:rayCast(x1, y1, x2, y2, listener, data)
I need to know which object was crossing ray. how this parameter is considered?
sorry for my english)

ar2rsawseen

the parameter listener is the callback function which accepts 6 or 7 parameters.
First one is the fixture hit by the ray

duke2017

yes, I understand that, but how exactly do I callback this the first parameter?

function rayfunc(event)
if event."First parameter" == box then
......
elseif."First parameter" == crate then
....
end
end
ray = self.world:rayCast(0, 0, 200, 200, rayfunc)


I do not understand how to read its "first parameter"..

ar2rsawseen

Ok.

So, for example, you have a variable world where you have world instance and some kind of box2d object ball. You want to project raycast on x axis through all the screen and get fixture if there is one

local x, y = ball:getPosition() local screenWidth = application:getContentWidth()   local raycastCallback function(fixture, hitX, hitY, vectX, vectY, fration) --so if this function is called, it means we hit some kind of object --and it's fixture is stored in first variable we named "fixture" --so we can for example get body local body = fixture:getBody()   end   --now we add callback function for projected raycast --Parameters: --object x coordinate --object y coordinate --projection vector on x axis --projection vector on y axis --callback function world:rayCast(x, y, x+screenWidth, y, raycastCallback)

duke2017

Oh, sorry for my stupidity)) Thank you very much for the detailed explanation! What would I do without you)

ar2rsawseen

@duke2017 thats completely ok, we're all there to learn

jdbc

I am trying to use world:rayCasting and after I called to rayCastCallback function, but it seems fixtures are not sorted.

I mean I defined the function:



list = {}

function raycastCallback (fixture, hitX, hitY, vectX, vectY, fration)
local body = fixture:getBody()
list[#list + 1] = body
end


but list is not sorted in the sense of the raycast. Is it correct?

ar2rsawseen

@jdbc it should return the first fixture it hits by the ray cast, and then depending on your return value

• return no value or -1: ignore this fixture and continue
• return 0: terminate the ray cast
• return fraction: clip the ray to this point
• return 1: don't clip the ray and continue

either proceed to next fixture or terminate, etc

jdbc

I use rayCastCallback function to check collision between a fixed body and a set of bodies on the left, returning 1.

A list of collision points is built but this list is not ordered by axis X (sense of raycast from fixed body to the left side), it seems to be randomized.

Anyway I can ordered this list by axis X but I believe raycast will do it the job.

ar2rsawseen

Well it should be ordered by the increasing distance from your starting point to the raycasting direction, which might not always be increasing with the x axis values (depending on your direction).

if its not, maybe there is some other problem

jdbc

I execute something like:


local left = -2000

-- Left hits
self.world:rayCast( v:getX(), v:getY(), v:getX() + left, v:getY(), self.raycastCallback, self)



where v is a sprite object and self.list is the bodies list and contains all bodies in the raycast trajectory but sometimes not ordered.

I suppose it is a behaviour of box2d engine.