Bizarre Redshift transparency and alpha change


Apparently in the latest version of Redshift they have moved the ability for transparent materials to apply to the alpha channel to a Legacy status and turned off by default. Transparent materials are just opaque unless you find it in the System>Legacy tab and turn it back on. Redshift is terrible at documentation and I haven’t found any reasoning behind it yet besides it looks like it was causing issues with EXR and AOV passes. So if you’re workflow doesn’t usually involve AOV passes, be aware of this. This is a bizarre thing to me as I’m sure a lot of peoples workflows involve transparency to some degree and comping.


Bummer… thanks for the heads-up! I really liked the old behaviour, just a ready-to-comp file with no extra effort needed. At least there’s still a buried option for this…
Regarding documentation: Yeah, I sometimes have the feeling the team is moving faster than it should :slight_smile: But since they are super supportive and focused on bug-fixing, I must admit, I enjoy the ride. Also, Redshift’s speed still feels magical every day :smiley:


Agreed. It still blows my mind that we can get this quality at these speeds.


Yep, I was a bit surprised to see this decision. Apparently having alpha transparency for refractive objects is non-standard in most of the higher end renderers, and it was also causing issues with Cryptomatte. I totally understand the principle - you can’t comp your render and get ‘refraction’ in the background plate.
Of course, in the real world you can get away with a lack of refraction a lot of the time - particularly if it’s a thin surface such as window glass.
I can see a number of folks have raised this on the Rs forum - so hopefully they’ll keep that ‘legacy’ option available for those of us who need to fake it from time to time.

PS: I haven’t fully checked it out, but If you have the IOR set to 1, or the ‘thin walled’ option on, I think you’ll get the ‘legacy effect’ without needing to enable the legacy option.


Yea, mike’s answer sort of nails the why.

I mean to say higher end renderer don’t allow this isn’t accurate in that light path expressions let write any alpha I want.

The defaults though are that refractive objects are solid alpha. If we need to refract a plate we typically lod it into render. If we are doing clever things in post we will typically render a refracted UV pass so we can distort the images that would refract and then usually add the specs only on top of that.

The advantage of light paths though is any aov can be aware of reflection and or refraction if desired. Normals aovs for instance, don’t, which sucks for feature based denoisers. So we right out a special normals aov where instead of normals on a window or mirror we see the normals of either the reflected or refracted geo. We also do reflected id masks so we can color correct an iris that is refracted behind a cornea lens.


Interesting info - thanks. I know the Redshift devs have light path expressions on their to-do list, but I didn’t know what they were used for. Now I do : )