Premultuplication weirdness

[Anonymous]

Hi there,

just a question about some premultiplication weirdness I have noticed lately in action.
Basically, when using some premultiplied media in action, I use to check the 'Div' button in the media list.
But it appears that, when the surface is set to 'Linear', there’s still some blackness remaining over the edges.
By switching to 'Anisotropic' ot 'Nearest', then Flame seems to properly handle the premultiplication.
Any other surface filter type gives edges issues.
Now I found that, when managing the premultiplication directly at the surface level by setting 'Premultiplied' under the blend mode,
everything looks as it should, whatever filter type you choose.

Now is the 'Div' button some kind of legacy from previous Flame versions that should not be used anymore ?

Thx

[ManChicken]

Div actually un-premultiplies.  But then when you scale/rotate you'll get funky edges because it's doing subpixel manipulations and creating other odd edges.

I've come to the determination that Action prefers premultiplied sources unless you're just comping with no transforms.  But, you need to switch the object to Premultiplied for its Surface (you know where all the transfer modes are like Add, Screen, etc.. it's around there.  Sorry, I'm not in front of my machine right now to be more usefully specific 🙂

[Anonymous]

Yep, it sounds like that's the correct way to do.
Thx a lot.

[Anonymous]

If you are doing any color work to a fill, you want to work on it straight, if you are doing any blur work to a fill you want to work on it multiplied*.  Blur your fill and matte with the same values and divide them after blurring.  Keep your fills straight unless you need them to be otherwise, and return them to straight right after.  Be thankful for the comp node.

Action will behave better with straight fills, and you should avoid using any of the layer blurs, CCs or keyers.  The state of your fill will need to be different depending on the action you wish to perform.  Blurs won't blur correctly unless you have "div" ticked and a multiplied fill.  CC's won't CC correctly unless you have a straight fill and "div" unticked.  Anything that requires both blurring and color correcting will break. 

Keeping your CC's, burs and keys loose in batch will allow you to correctly multiply and divide as your image requires.

*the terms "premultipled" and "unpremultiplied" are an assault on the English language, using one and two unnecessary prefixes respectively.  We don't  call upstream blurs "preblurs" and upstream color corrects "pre-color corrects".  It's silly to give a basic operation special and semi-magical status with the prefixes.

[Anonymous]

To your original question:

the 'divide' checkbox and toggling an image node to "premultiplied" use the same math. 

It looks like the order of operations when a layer blur is involved causes an issue with the checkbox but not the toggle.  So yeah, use the toggle.

There were certain situations where a non-100% zoom would cause perceived edges, but these were display-based and would render out correctly.