Opacity ( alpha channel ) get clamp bit rate

[mori]

HI ALL

i got issue when i render spline ( smoke )

with arnold

when i render spine with opacity map on aistandardsurface

my alpha channel look strange like alpha image are not smooth are low bit rate

anyone can advice me how to fix this issue ??

i try use same texture and render on Vray its render normally without problem

but this project request arnold render

thank you for your help