have one image negate the collision detection of another image - lua

Question

i have a water image that kills the user sprite on collision, taking 1 from the lives variable and re-spawns the sprite to the start. i have another image of a raft that constantly spawn new images that move across the screen. im trying to get it so the user can move onto the raft, that is over the water image, and not get re-spawned back to the start.
--sets the function for the death sequence.
function waterCollide(event)
frog.x = display.contentWidth/2
frog.y = 504
isOnRaft = 0
lives = lives - 1
showlives.text = "Lives: "..lives,230,-36,native.systemFont,25
lose()
end
--sets water
water = display.newRect(0,0,320,150)
water.x = display.contentWidth*0.5
water.y = 144
water.alpha = 0
physics.addBody(water,"static", {isSensor = true})
water:addEventListener("collision", function ()timer.performWithDelay(50,waterCollide)end)
--Set log position and movement
local mRandom = math.random
local raft = {"Raft1" ,"Raft2"}
local objectTag = 0
local object = {}
function spawnlogleft()
objectTag = objectTag + 1
local objIdx = mRandom(#raft)
local objName = raft[objIdx]
object[objectTag] = display.newImage(objName..".png")
object[objectTag].x = -96
object[objectTag].y = 216
object[objectTag].name = objectTag
transition.to(object[objectTag], {time = 10000, x = 416, onComplete = function(obj) obj:removeSelf(); obj = nil; end})
physics.addBody( object[objectTag], "static",{isSensor = true})
end
spawnlogleft()
timer.performWithDelay(3500,spawnlogleft,0)
--user sprite details
frog = display.newSprite(frogSheet, sequenceData)
frog.x = display.contentWidth/2
frog.y = 504
physics.addBody( frog, "dynamic", physicsData:get("FrogSheetData"))
frog.isFixedRotation = true
the user sprite is an animation of three frames and a time of 300. any help would be appreciated. and i will provide more details if needed.
thank you

Answer 1

Corona has these neat things called CollisionFilters. They show you how to use them in /Physics/CollisionFilter.
You basically set bits to make certain objects un-collideable with other objects. Check this main.lua example out:
local physics = require("physics")
physics.start()
physics.setScale( 60 )
display.setStatusBar( display.HiddenStatusBar )
local bkg = display.newImage( "stripes.png" )
borderCollisionFilter = { categoryBits = 1, maskBits = 6 } -- collides with (4 & 2) only
borderBodyElement = { friction=0.4, bounce=0.8, filter=borderCollisionFilter }
local borderTop = display.newRect( 0, 0, 320, 1 )
borderTop:setFillColor( 0, 0, 0, 0) -- make invisible
physics.addBody( borderTop, "static", borderBodyElement )
local borderBottom = display.newRect( 0, 479, 320, 1 )
borderBottom:setFillColor( 0, 0, 0, 0) -- make invisible
physics.addBody( borderBottom, "static", borderBodyElement )
local borderLeft = display.newRect( 0, 1, 1, 480 )
borderLeft:setFillColor( 0, 0, 0, 0) -- make invisible
physics.addBody( borderLeft, "static", borderBodyElement )
local borderRight = display.newRect( 319, 1, 1, 480 )
borderRight:setFillColor( 0, 0, 0, 0) -- make invisible
physics.addBody( borderRight, "static", borderBodyElement )
local red = {}
local blue = {}
local redCollisionFilter = { categoryBits = 2, maskBits = 3 } -- collides with (2 & 1) only
local blueCollisionFilter = { categoryBits = 4, maskBits = 5 } -- collides with (4 & 1) only
local redBody = { density=0.2, friction=0, bounce=0.95, radius=43.0, filter=redCollisionFilter }
local blueBody = { density=0.2, friction=0, bounce=0.95, radius=43.0, filter=blueCollisionFilter }
for i = 1,4 do
red[i] = display.newImage( "red_balloon.png", (80*i)-60, 50 + math.random(20) )
physics.addBody( red[i], redBody )
red[i].isFixedRotation = true
blue[i] = display.newImage( "blue_balloon.png", (80*i)-60, 250 + math.random(20) )
physics.addBody( blue[i], blueBody )
blue[i].isFixedRotation = true
end