rendering masks

[romullus]

I'm trying to render some masks for post processing, but have a little problem. Here's what i do: switch both GI solvers to none, turn off all light sources, assign CoronaMtl with black diffuse to all objects that should render black, assign CoronaLightMtl with "emit light" off and BW texture in Color slot to objects that needs to be masked. Set Colour Mapping as follows: exp - 0, high compr - 1, contr - 1, WB - 6500. Hit render and expect whites to be precise white, but that's not exact case. See attachment. What i'm doing wrong?

[Ondra]

There is slight error introduced because the gamma correction is only approximate to keep it fast. Usually it is enough as the error is only 0.5% in the worst case (for comparison, error from using 16bit floats is around 0.1% in this case). Do you have some problems with it? If so, you use some CoronaMask element, there is no mapping, so you will get exact results.

[romullus]

No, it's not a problem, i just thought, maybe i'm missing something. Thanks for explanation.