I think the following is a bug but wanted to double check that I have not missed anything:
Motion 5.5 project
One layer with a png graphic, one layer of text.
One BG layer with a retangluar shape to give me a grey background.
During the animation I want to have the BG go semi transparent (but png and text do not go transparent).
So I keyframe the opacity of BG shape From 100 to 50.
Set project to Transparent BG, select Color+Alpha & ProRes 4444 in the export settings.
Here is the problem:
When the BG shape covers the whole view from the beginning Motion doesn't generate (or save) the alpha channel.
It only generates/saves an alpha channel when the BG shape IS NOT covering the whole view at the beginning of the composition. If the BG shape moves and starts to cover the whole view after, the transparency set via the opacity setting is correctly represented in the alpha channel. So it's not a problem that a partially opaque BG won't get incorporated in the alpha channel at all, but I just don't want the BG to be fully transparent at any stage of the animation.
I know there are other ways to do this via layers in FCP, I just wanted to confirm that I don't miss a setting in Motion 5.5 of some sort.
I'd have to look at your project, but it should work as you want it to. There are several settings to check. Try putting a blank layer at the very bottom of your layers stack, just leave it there empty, see if that helps.
Oh, and be sure you're set to high quality with Dynamic turned off in the Render menu (top right of Canvas) and your export settings are set for high quality.
Thanks Ben, yes project BG is set to transparent. And I also tried to add an empty bottom layer, which didn't change anything.
You can test it yourself by creating a project with one layer of text, one layer with a rectangular shape below. Export with Color+Alpha, Prores 4444. When the shape covers the whole view no alpha channel is created.
"When the shape covers the whole view no alpha channel is created."
That makes perfect sense, that if the shape is covering the entire canvas, there's no space for an alpha channel to exist. There must be blank space in the frame somewhere for an alpha channel to matter.
Did your test, and I got an alpha channel just fine when the shape covers only some of the image frame, no alpha when the shape encompasses the entire image frame, just as it should. Actually, there's an alpha channel in both, but in the second export, where the shape covers the entire image frame space, you can't see it, because the shape is covering it.
So I'm not sure what you mean by "When the shape covers the whole view no alpha channel is created." How would you see an alpha channel?
When I ask Motion to create an export containing Color + Alpha and a codec that supports an alpha channel the resulting movie should contain an alpha channel no matter if there is any part of the image transparent or not. Because otherwise you are not able to (as in my case) make a background shape semi transparent if the shape covers the whole view. Motion can do this without a problem if you convert the same project into a FCP title so I assume it’s a bug for a normal Motion project. If not it’s illogical to me at least.
Apples to oranges, they're not the same thing. Publishing a Title temple to FCP, you're just adjusting the Opacity. That's not an alpha channel.
An alpha channel works on greyscale. So if I take a white box, make it 40% opacity, export that as a ProRes with alpha, that's baked in with an alpha channel. I can place that in an FCP timeline, and it will be semi-transparent. But I can't adjust that original opacity any higher that what it baked in. I.E. I can't ever go back to 100% opacity like I could when using a Title template. That 40% transparency is baked in and unadjustable (of course I can use Opacity to make it more transparent, but not more).
A Title template, you're simply keyframing opacity, that's not an alpha channel. You could publish that Opacity setting and be able to fade it up or down as you want all day long.
I just tested all of this, and it works fine. Followed your original post step by step, and it works as expected, opacity acts as you would expect. So it's not Motion, as I can do it just fine.