(0.18) layer transparency?

[ownlyme]

i had this script:

variation.trunk={layers={{
filename = "__robot_tree_farm__/graphics/dugup2.png",
priority = "extra-high",
width = 51,
height = 38,
scale = 0.5,
frame_count = (variation.trunk.frame_count or 1),
tint= {r=0.7-0.5*i/(TREE_LEVELS/10),g=0.7-0.5*i/(TREE_LEVELS/10),b=0.7-0.5*i/(TREE_LEVELS/10),a=0.7-0.5*i/(TREE_LEVELS/10)},
shift = {0,0.05},

},variation.trunk}}

which produced a fading layer of dug up earth as the tree growed in 0.17
but in 0.18 it just makes the sprite darker.
any ideas?