Hello, I need help...
I don't know how to make a sprite to become transparent only when the characters are behind it, not fully transparent. Like in this stage :


The blue sprite of [BG lamp] that i wanted transparent :

the code for the stage here :
Spoiler, click to toggle visibilty
[Info]
name = "Ray Palmer's Lab"
;by Marvelvsdcu 2010
[Camera]
startx = 0
starty = 0
boundleft = -177
boundright = 358
boundhigh = -45
boundlow = 0
verticalfollow = 0.3
tension = 50
[PlayerInfo]
p1startx = -70
p1starty = 0
p1startz = 0
p1facing = 1
p2startx = 70
p2starty = 0
p2startz = 0
p2facing = -1
leftbound = -374
rightbound = 579
topbound = 0
botbound = 0
[Scaling]
topz = 0
botz = 50
topscale = 1
botscale = 1.2
[Bound]
screenleft = 15
screenright = 15
[StageInfo]
zoffset = 210
autoturn = 1
resetBG = 0
[Shadow]
intensity = 90
color = 100,100,100
yscale = 0.35
reflect = 0
[Music]
bgmusic = sound/atom.mp3
bgvolume = 255
bgm.loop=1
end.time=999999999
[BGdef]
spr = stages/atomlab.sff
debugbg = 0
[BG background]
type = normal
spriteno = 1,0
mask = 0
start = 150, -70
delta = 0.8, 0.8
[BG lightning]
type = anim
actionno = 1
mask = 1
start = 225, -90
delta = 0.8, .8
[Begin Action 1]
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
3,5, 0,30, 1
3,4, 0,30, 1
Loopstart
3,0, -30,20, 1
3,2, 0,0, 1
3,1, 0,0, 1
3,3, 0,0, 1
3,1,0,0,1
3,2,0,0,1
3,0,-30,20,1
[BG comp ceiling]
type = normal
spriteno = 1,1
mask = 1
start = 145, -70
delta = 0.8, .8
[BG trans]
type = normal
spriteno = 2,0
mask = 1
start = 225, -90
delta = 0.8, .8
trans = add
[BG lab equipment]
type = normal
spriteno = 1,3
mask = 1
start = 225, -90
delta = 0.8, .8
[BG monitors]
type = normal
spriteno = 1,5
mask = 1
start = 145, -35
delta = 1, 1.1
[BG desk]
type = normal
spriteno = 1,2
mask = 1
layerno=0
start = 110, -95
delta = 1, 1.1
[BG lamp]
type = normal
spriteno = 1,4
mask = 1
layerno=1
start = 150, -70
delta = 1, 1.1
Could someone help me for this, please ? (cause that blue big lamp really get on my nerves
)