how do i have one physics body negate the collision of another. - function

Question

i have two images on the screen that overlap called A and B. i have a third image called C that the user is able to move around the screen. the bigger one (A) of the two resets the C to the spawn point on collide. i need to know how to make it so that C can walk over image B and not be reset to the spawn point. as in image B overlaps image A and negates the collision function.
here is the code:
--Hides status bar from top of page
display.setStatusBar( display.HiddenStatusBar )
--Set a test background image
local backgroundimg = display.newImageRect("testbackground.png", 320,576)
backgroundimg.x = display.contentWidth*0.5
backgroundimg.y = display.contentHeight*0.5
--Set the position and amount of score and lives
local score = 0
local lives = 5
local showscore = display.newText("Score: "..score,0,-36,native.systemFont,25)
showscore:setTextColor(0, 0, 0)
local showlives = display.newText("Lives: "..lives,230,-36,native.systemFont,25)
showlives:setTextColor(0, 0, 0)
--Set physics for collisions, etc.
physics = require("physics")
physics.start()
physics.setGravity(0,0)
--set water
local water = display.newImageRect("water.png",320,192)
water.x = display.contentWidth*0.5
water.y = 144
physics.addBody(water,"static")
water:addEventListener("collision", function()timer.performWithDelay(50,waterCollide)end)
function waterCollide(event)
lives = lives - 1
display.remove(frog)
frog = display.newImageRect("FrogTest.png",32,48)
frog.x = display.contentWidth*0.5
frog.y = 504
physics.addBody(frog, "dynamic")
frog.isFixedRotation = true
end
--Sets buttons images and positions
local forward = display.newImageRect("Forward Button.png",106,100)
forward.x = 160
forward.y = 478
local left = display.newImageRect("Left Button.png",106,100)
left.x = 53
left.y = 478
local right = display.newImageRect("Right Button.png",106,100)
right.x = 267
right.y = 478
--Set log position and movement
local log1 = display.newImageRect("log1.png", 96, 48)
log1.x = 32
log1.y = 226
physics.addBody(log1,"kinematic")
transition.to(log1, {time = 3500, x = 288})
--Set a frog sprite on the screen
frog = display.newImageRect("FrogTest.png",32,48)
frog.x = display.contentWidth*0.5
frog.y = 504
physics.addBody(frog, "dynamic",{density = 1.0, friction = 1, bounce = -1})
frog.isFixedRotation = true
--Sets motion variables
local motionX = 0
local motionY = 0
local speed = 4
--Moving forward
function forward:touch()
motionX = 0
motionY = -speed
end
forward:addEventListener("touch",forward)
--Moving Right
function right:touch()
motionX = speed
motionY = 0
end
right:addEventListener("touch",right)
--Moving left
function left:touch()
motionX = -speed
motionY = 0
end
left:addEventListener("touch",left)
--Moves Frog each time frame is called
function movefrog (event)
frog.x = frog.x + motionX
frog.y = frog.y + motionY
end
Runtime:addEventListener("enterFrame", movefrog)
--Stops frog from moving continuously
local function stop (event)
if event.phase == "ended" then
motionX = 0
motionY = 0
end
end
Runtime:addEventListener("touch", stop)
--Making sure the frog does not go off the screen
local function stopfrog (event)
if frog.x <= 16 then
frog.x = 16
end
if frog.x >= 304 then
frog.x = 304
end
end
Runtime:addEventListener("enterFrame", stopfrog)

Answer 1

You need to set a condition under the waterCollide function in order to negate the respawning.
Depending on how complex you'll need it to be, you could simply check if frog's position is within log1's boundaries, or perhaps you could have log1 and any future logs have their own collision event which sets a flag on collision to not trigger the respawn then clear the flag when collision with any log ends.
Here's an example:
local onLog = 0
function frogDie()
lives = lives - 1
display.remove(frog)
frog = display.newImageRect("FrogTest.png",32,48)
frog.x = display.contentWidth*0.5
frog.y = 504
physics.addBody(frog, "dynamic")
frog.isFixedRotation = true
end
function waterCollide(event)
if onLog < 1 then frogDie() end
end
function logCollide(event)
if event.phase == 'began' then
onLog = onLog + 1
else
onLog = onLog - 1
end
end
log1:addEventListener("collision", logCollide)
--log2:addEventListener("collision", logCollide)
Using a number to track whether the frog is on a log should be safer than a boolean, since logs might end up overlapping and clearing the flag before it can be reset properly on multiple collision passes.