clamped alpha with linear workflow

[ojk]

Hey guys,

I've rendered some fluid smoke with vray and merged it with a grey background and noticed some clamping problems.



The smoke is rendered linear 16 bit floating point exr with these color mapping settings:

type: Linear multiply
Dark multi.: 1
Bright multi.: 1
Gamma: 2.2
Affect Background: on
Subpixel mapping: off
Don't affect colors: on
Linear workflow: off
Clamp output: off

In compositing the clamping only occurs when background and smoke are merged in linear space. When I merge them in sRGB space it works fine.
I've also noticed that the clamping is caused by the alpha channel. The rgb channels are fine and smooth but when I'm pulling down the gamma of the alpha channel the clamping gets visible:



For comparison I've rendered the same frame with mental ray and got a smooth alpha channel:



I'm using vray 2.30.01 with phoenixFD shaders 2.0.0.
I've also attached the exr files. One rendered with Vray and the othe for comparison with mental ray:

alphaClamping.zip

I've also noticed the same problems with phoenixFD smoke renderings...
Do you have any ideas?