Getting the alpha to be effected by an opacity map

[max montana]

hi , i work on a scene where i have a chain-link fence in the foreground.
i use a png map to control opacity and the rendering is just fine.
but i cannot make it work the alpha to be effected by the opacity map, means that the transparent parts of the fence are ignored in the mmre.

[jonahhawk]

use the refraction channel instead of opacity. Check the box Affect Alpha.

[max montana]

tried it and get the same result as before. a blank white mask without the transparent parts. what am i missing?

[percydaman]

looking at your material editor screenshot, you still haven't checked the affect alpha box.

[max montana]

what do you mean? the affect alpha box is checked in the refraction section of the material editor. do you mean another one?

[rivoli]

I'm not sure I follow, how are you supposed to get an alpha of the fence if there are other solid objects behind it? unless they're alpha contribution is set to 0. am I missing something?
and you may be better off setting the ior of the fence mat to 1, if you're going to use refraction instead of opacity. might avoid weird results.

[max montana]

i want to get a multi matte render element (mmre) of a material with an opacity map(the chain-link fence).

i got the information that v-ray sp2 doesn`t support opacity maps, therefore the mmre can`t make a propper result. the solution is that v-ray sp3 now works with opacity maps.

[rivoli]

yes, of course. I knew there was something I wasn't quite grasping. anyway, opacity and multimatte now work perfectly here. can we have a peek of what your element looks like?

[max montana]

sure, the black dots are some leafs which are in front of the fence

[rivoli]

weird. have you tried going back to opacity leaving the refraction slot alone, and switch from png to any other format without alpha channel? like an actual black and white mask of your pattern?

[max montana]

my actual workaround ist to make all mmre with the fence hidden. then i hide everything out of the fence. the alpha then is correct. but to get correct masks i have to limit the mask of the fence to the visible areas, and then i have to substract the fence-mask from all other masks where the fence is in front of. works, but it`s very timeconsuming - and annoying when i have to make a correction of the 3d model, then i have to do it all again
the solution is to install sp3

[rivoli]

I thought you were on sp3, and couldn't understand why it wasn't working in your case. but yes, upgrading will solve the problem.

[jonahhawk]

To be clear to anyone else reading this; The Affect Alpha check box in the refraction works as intended, even in previous SPs.