Output full alpha, without in-scene transparency

Hello BA community, pretty new user here. As such, I am not sure if this falls under ‘basics and interface’ or ‘materials and textures’, please recategorise as you see fit.

My goal is to create ‘stinger’ transitions with Blender. This is an A → B transition, where the change from scene A to scene B happens when scene A is fully ‘covered’ by the stinger content. See post #7 here for a basic example.

I have a simple stinger working in the form of a big arrow transiting a camera with ‘transparent’ set in Scene→Film properties.

What I would like to do for another transition, is use a mask (or similar) to uncover and re-cover a simple scene. The animation output would start with full transparency, progressively reveal the scene, then un-reveal it. However, using materials with transparency makes those materials transparent (naturally!); whereas I would like a material (or to achieve this in some other way) that has the effect full ‘A’ channel output when encountered.

I have searched for things like masks and ‘covered’ renders, but the ones I’ve found that might be of relevance apply to older Blender versions that have things – eg layers – removed or re-purposed.

Apologies for the imprecise terminology; I am happy to clarify or to create a mockup image sequence showing the effect I would like.

Thanks in advance :slight_smile:

As an example, ‘material’ (in organe) with example scene on left, desired output on right:

stinger_example_final2

This is a very simple ‘animation’ for the purposes of an example, but if it is possible to do this more complex animations could be done.

How are you doing this? Could you post that blend file?
I’m just wondering, because there are so many ways to do transition effects in Blender… most of them don’t even rely in materials…

Either the Wrong Way, or the Homer Simpson Way (wrong, but faster!) :smile: I’ve only been using Blender for a few days now so my approach is more “brute force” than anything elegant.

The above example was created by hand in GIMP, sorry! :blush:

However, I have manage to create a very basic transition using compositing; the thought of ‘replacing one colour with another’ in my post above made me think in that direction.

I have three scenes:

  • the actual scene (in this case Suzanne example)
  • the mask scene
  • a composition scene, to combine the two above

Some twiddling with compositor nodes got me:

But like I say, I can’t help but feel I am going at this ham-fisted. My original question is answered, as-is, but I am not sure how well this would work if the transition was more complex, as parts of the underlying scene get keyed out even with tight tolerances on the ‘chroma key’ node.

I am happy to post the blend of above, however, the file size is ~265MB (without packing external data). Is that normal? I subdivided Suzanne a couple of times to smooth her out which presumably inflated the number of nodes/verts/faces.

That’s a good start!! :ok_hand:
Now, in the mask scene, you can do a lot of things…
For example, use material ID’s, render passes, uv maps, criptomates, etc. And those will appear in the RenderLayers node. You can then be very creative mixing all your stuff. :slight_smile:

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Hi bertieb,

for your project a (animated) Holdout shader suggests itself.

The disadvantage (in this example) of an animated holdout shader is that you need a material index pass (since I worked with two videos), so you have to render with Cycles.
I also added keyframes to the holdout shader and the material index pass in this example.
But it always depends on what you want to do, simple things also work in Eevee

A little Test.

The principle.

Edit:

You can also generate BW masks for external postproduction.

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I uh, know some of those words! :smile: I’ll look into those things, thanks!

I’ll look into that. I’ve been using Eevee so far because I love the speed, and it seemed like the future direction of travel for Blender (?). I can switch back to Cycles to try out your technique, which looks great :slight_smile:

For the purposes of this I am just looking for a simple stinger transition that I can use with OBS, but I’ll keep that in mind for the future!

Cheers :slight_smile:

Now I first had to see what a Stinger Transition actually is.
As I understood it, it goes from transparency to solid to transparency.
Right?
If so, you can make normal keyframe animations.

Example: (I don’t know exactly if the parameters like resolution, framerate etc.) are correct.

Edit:
I see just that here on the website transparency is shown as white.
To save, right click on the video and select “Video save as…”.

Correct- the minimum of 1 frame full opacity is needed to change from scene A to B; ie the two are not mixed.

Looks good :slight_smile: Did you do that with a holdout + material index pass in Cycles as you described above?

I tried the composite chroma key approach and got something simple and imperfect, but usable:


(downscaled from 1920x1080)

The chroma approach works for this simple shape, but I suspect would be difficult to achieve with a more complex animated transition; for example, a smoky cloud that grows and fills the frame.

It renders appropriately here! Chromium 80, Arch Linux. Similarly, if you see red #ff0000 in the video above rest assured it is indeed full alpha! :smiley:

Thanks for the help so far! :slight_smile:

No, simple Keyframes in Eevee.

Blend:
http://pasteall.org/blend/index.php?id=52999

D’oh, of course! I was still fixated on the alpha aspects, but keying out and holdouts aren’t needed in in the simple case of a moving object and a transparent background.