Max Transparency Levels reached - alpha is cutout

[rcm_andy]

Hi there,

we have the following problem:

When the max transpareny level is reached and there are objects with mapped/none white opcacity in the path, the alpha-channel is set to the opacity value, regardless if there is a object which is opaque behind it. This leads to a cutout alpha channel which is some kind of wrong.

I did a test to illustrate:



A simple checker applied to the 1st/2nd plane opacity , and max transparecy levels to 1. In the second level the transpareny is stopped, the opacity map sets the color to zero.

The alpha channels looks as follows:



Second level opacity map cuts out alpha to black.

Any ideas to get the alpha to white if the max levels is reached ? This not usable for compositing!

I´ve attached the maya test scene as well.

Thanks,
Andy

[doppelgaengerdigital]

i don't know if ot's the same, but I worked with volume grid today in volumetric geometry mode (which is sliced volume into stripes). I have some kind of the same issue I guess. The volume is written to alpha at some point, although I don't know if it's the max transparency level.
Weird behaviour though.

[hermit.crab]

The opacity (and the max.transp. levels) is working as expected. You may try using some kind of a GLSL/OSL shader, which is transparent only until a certain point.

[rcm_andy]

Why is it an expected behaviour, that the alpha channel paints black on an object ?

[vlado]

Why is it an expected behaviour, that the alpha channel paints black on an object ?

That's the way the code is written at the moment. If the max transparent level is reached, the renderer assumes that the rest of the ray is also fully transparent, which is not an unreasonable assumption. Of course it may not be what you want. Increasing the max transparent levels generally helps.

Best regards,
Vlado

[rcm_andy]

Hi Vlado,

increasing the max levels is not an option in this situation with mass-scattering of grass to only get the alpha right. This would kill the rendertimes with no benefit for the image quality. It would be very helpfull to have an option like "exit color" where one can choose what happens when the max level is reached, in this case render alpha to opaque.

Best regards,
Andy

[frodopytlicek]

We also bumped to this issue. What can we do?