Default Export Setting for Images w/Alpha

[rafael]

When you select "Image Sequence" as your format, and choose an image type that can have an alpha channel (PNG, TIFF, Etc.) Moho should automatically check "Do not premultiply alpha channel"

My thought is that It's exceedingly likely that if someone is exporting an image sequence of these types, they are compositing in another software. If that is true, the alpha shouldn't be premultiplied. This would save inexperienced animators from ending up with edge outline issues etc. Also, I wouldn't have to remember to check it each time.

What do y'all think?

[Greenlaw]

It really depends. For Moho exports for compositing elsewhere, I usually render Layer comps, and the default premultiplied setting seems to work fine...more specifically, I don't see matte lines in my Ae comps. But normally, I think you're right, that straight alpha may be preferable for compositing...but it may depend on the program you're compositing in or what you're compositing against.

Matte lines are the usual giveaway. If you see them, try another setting. Also, I think if you have soft or transparent edges (like fire,) you probably want straight alpha, not premultiplied. (Note: I might have that backwards...I never seem to remember.)

In any case, you can set/change Moho's export defaults to use what you want...

1. Open Export Animation or Moho Exporter
2. Click on Choose A Configuration To Apply and select Edit Configuration
3. Under Configuration choose Make Configuration
4. Name the new configuration (maybe something like PNG No Premult)
5. Click Ok.

Now you can choose the setting with your preferred options from the Choose menu.

If you want to always use the same configuration by default, select the configuration you want and then click Save As Default.

[Greenlaw]

Just to be clear, After Effects can accept either straight alpha or premultiplied, and it will try to automatically determine which type is used. This is probably why I never paid much attention to this when rendering from Moho for Ae. And when Ae can't figure it out, you can manually set which type it should be.

For other programs, you may only have one option, which is why it's not so simple to choose a single 'default' setting suitable for all users.

[rafael]

From my perspective, the software you'll be bringing the assets into is irrelevant.

When you premultiply your asset, you're changing the value of your semi-transparent pixels to include values from the background. If you're exporting the asset to composite it in AE etc., you want those pixels unpremultiplied, so that you can multiply them against the actual background you want the asset on.

For instance, if you premultiplied a character while working with a black background, you'd be introducing black pixels into the edges of your character. When you bring it into AE and slap it on a light background, you'd have a weird dark outline around the character. If you export that same asset straight, without premultiplication, the blending would happen in AE, and you'd have the various colors from the actual background in your characters semi-transparent edges.

I'm gonna run some tests when I have free time, and see if Moho actual does premultiply when you have the BG set to default...

[davoodice2]

When you select "Image Sequence" as your format, and choose an image type that can have an alpha channel (PNG, TIFF, Etc.) Moho should automatically check "Do not premultiply alpha channel"

My thought is that It's exceedingly likely that if someone is exporting an image sequence of these types, they are compositing in another software. If that is true, the alpha shouldn't be premultiplied. This would save inexperienced animators from ending up with edge outline issues etc. Also, I wouldn't have to remember to check it each time.

What do y'all think?

With default setting of png, there is not any outline. Everything is good. I did that always

[Greenlaw]

Same here. I'm just using the default premultiply setting and this works fine for me in Ae. It seems I may have had to change this setting once but I don't recall what that situation was. I'll re-examine my Moho-Ae workflow but I don't think there's been anything wrong with this. (At least nobody at work has complained my work yet...well, not much anyway.)

FWIW, way back when I was with The Box team at Rhythm & Hues, we always rendered floating point EXR from a 3D program, and I think we rendered our layers premultiplied and against black, but this was probably because we always comped additively in Fusion. In this mode, the black can't possibly appear and you still get the alpha channel's effect on the image area only. This make the edges exceptionally clean which no chance for matte lines.

But there will be exceptions of course. I just use what works until I run into a problem. For example, in 3D, I may have to use different settings for fine hair or certain volumetric effects. (Sorry for being vague and not showing examples...the solutions only come to me when confronted with a problem.)

As mentioned earlier, it depends on which program you composite with. For example, I read that Avid users will need straight alpha but Final cut users will prefer premultiplied.

[Greenlaw]

I thought about what I just wrote, and I think the key is to always render your passes on black...which is what I think Moho does by default, at least it does when I output using Layer Comps in Moho Exporter. If the image is premultipled against black, it should be fine for many compositing programs. But, as pointed out earlier, there will be some exceptions.

When I get a chance today, I'll check how my Moho renders are being imported to Ae and for other programs I use. This can certainly make a difference too.

[rafael]

As mentioned earlier, it depends on which program you composite with. For example, I read that Avid users will need straight alpha but Final cut users will prefer premultiplied.

The software should be able to use either, as it's the same process: multiplying RGB values against the alpha values. That said, not everyone understands what premultiplication is, so they ask for what they've used in the past, not realizing the possible implications to their image. In most scenarios the difference is likely negligible, but someone doing extensive compositing or receiving assets from another person can get into hot water if they don't know the difference and choose accordingly.

After doing some tests it seems to not really matter, as images rendered from Moho with or without that box checked come into AE identically. I'm assuming this is because the default BG isn't taken into consideration in the premultiplication.

As long as I can let go the need to check the box, I'm fine. :p

[Greenlaw]

Yeah, if it looks fine, I wouldn't worry about it.

As mentioned, it seemed like I ran into a problem a while back where transparent edges were involved, but it was a unusual comp I was working on. In that case, I changed the premultiply setting but I think I also had to manually change how Ae imported the footage because I used the Replace Footage command, and then my comp looked fine. It was a rare situation though, and I don't think I've had to do this since.

If it's an new import and you're not using Replace, Ae should adjust automatically.