Expressions access to media replacement comps

Sorry to bother you but I saw you posted a potential workaround but unfortunately the file doesn't seem to be accessible anymore. Would I be possible for you to post it again?

I experimented some more and came up with a preset for detecting the bounds on a single frame that can be applied to a single layer, no duplicates needed. The preset can be downloaded here.

Here is a project file with examples of the static and dynamic bounding box detection.

I'm demonstrating this using comps with nested text and shape layers here rather than include footage, but the method is relying entirely on the alpha channels of the resulting comps to measure the bounds.

The very brief version is that for each of the four sides of the bounding box I'm using Minimax and Transform effects to isolate the portion of the comp that is on the empty-alpha side of that boundary. Then I'm using a unidirectional box blur to average the alpha values of the pixels across that divide. That alpha value then represents the ratio of the number of pixels on each side of the boundary. Sample that value with sampleImage, and then the location of the boundary can be calculated through very basic arithmetic. Do this for each of the four boundaries (top, bottom, left, right) and you have the bounding box.

@Aaron Cobb this is awesome info!! 

Thank you for sharing. I would love an example project if you don't mind 🙏. 

Cheers,

Dacia

Okay, I've worked out a method for getting the rectangular bounds of the alpha channel of an arbitrary shape (or array of shapes) with only four sampleImage() calls per frame measured (one call for each edge, top/bottom/left/right. If you don't need the bounds to be dynamic you can get away with only sampling four times on a single frame and then you are done. I've confirmed it works in a MOGRT in Premiere, including on footage that has been transformed using the media replacement controls in the Premiere EG panel, and while I haven't benchmarked it against other methods (namely because this is the first method I've tried) I'm pretty confident this can't be done in less than four calls, so I suspect it's about as fast a method as is possible here.

If anyone is interested I can try to do a writeup and maybe post some .ffx files tomorrow.

In my experiments sampleImage() was unable to access the RGBA values of the replacement footage. It still returned values from the original template footage. Is there something I am missing?

Edit: Okay, I see now that I was wrong about this. sampleImage DOES work if you structure the comp network right.

Even if the media replacement itself can't adapt its properties to the new media, wouldn't it be possible to access the contents of the layer inside the media replacement comp, given either direct API access to the replacement comp or just a reliable method of predicting the media replacement comp's name?

In case I'm not making myself clear, my presumption is that the behavior when replacing media through the EG panel in Premiere is the same as the behavior exhibited when replacing media through the Essential Properties in After Effects. What happens in AE is a new folder is created in the project called "Media Replacement Comps", and inside that folder a comp is created as a container for the replacement footage. The renderer then, when traversing the network of comps, substitutes references to the original footage with this replacment comp.

If my supposition is correct, then all I should need to access the attributes of the new footage is the ability to construct a valid reference to this media replacement comp. My original hope had been that the source property of the layer that is being replaced would point to the new comp, much the way other EG-fed properties send updated values to expressions that query them, but this doesn't seem to be the case. So that leaves referencing the comps by name. Currently, the name of the media replacement comp isn't predictable without knowing the name of the footage in advance, because the name is generated in part based on the name of the new footage. That, as far as I can tell, is the main obstacle to what I'm trying to accomplish, not the method AE is using to do the media replacment per se.

Does this make sense?

Per @DaciaSaenz's response, marking this thread as Acknowledged.

- John, After Effects Engineering Team