Extra alpha-multiplication with Fusion 16

In Fusion 16, and the 2019.5 Mocha Pro plugin (on Linux), Mocha Pro seems to be doing an alpha multiplication on the insert channel when doing the insert. I’ve looked through both the plugin and in the Mocha UI, but didn’t find any place where I could turn this off. The plugin does have a “Multiply by Mask” setting, but that is already off (and doesn’t seem to have any effect on a rendered “Insert: Composite”.

I can make it work by putting an AlphaDivide node before the insert, but it was a little surprising that I couldn’t turn off the alpha multiplication within the Mocha Pro plugin itself. At least with Fusion, premultiplied images seems to be a bit more of the norm.


Interesting, it may be that we have optimized it for Nuke, which doesn’t always process Alphas the same way. Let me check in with @martinb and see.

Hi David,

Alpha is processed by default, but you can change this in the output settings in the Mocha interface under the mask settings:

If this isn’t what you mean, let me know the exact setup and I can see if there is an alternative.

This doesn’t seem to affect my issue. My problem is with the insert input to the Mocha Pro plugin. The plugin seems to always be doing an alpha multiply on that input, which since Fusion tends to already pre alpha multiply everything, results in a dark ring around my insert image. I can fix this by adding an “AlphaDivide” node right before the “Insert” input in the Mocha Pro plugin, but it’d be nicer to just have a way of turning off the extra alpha multiplication within Mocha Pro.

Hi there,

Fusion doesn’t multiply if you don’t tell it to do so. A lot of matte nodes indeed have a “pre-divide-post-multiply” option to tick, or a post multiply only but you should be able - in Fusion - to cancel any default premultiplication.
For example, I prefer adding an alpha divide before a key node than letting the node option do it itself, but I’m ok with the key node (or actually the matte control I always add) to post-multiply, but not both dividing and multiplying. This to say you must have the option to do it as it suits you.

Now, if you write about the plug-in only, I cannot give a hint because I’ve chosen to use Mocha in standalone only.
Anyway this thread is interesting and at this point, if I understood well what Martin suggested “should” work because it doesn’t force the multiplication and let the host deal with it…