An AEP file is normally used as an After Effects project definition that contains the instructions for building your composition rather than a finished movie, including timelines, multiple layer types, animation data like motion controls, effect parameters, masks, mattes, and 3D components such as cameras and lights, while referencing external media files to stay compact even if the project uses gigabytes of footage.
Because AEP files don’t embed footage, After Effects can throw "offline media" warnings if you relocate or rename the assets or copy only the AEP to another computer without its media, making Collect Files—or manual gathering of all referenced items—the safest way to move a project, and if an AEP won’t open in AE, details such as where it originated, what’s stored beside it, Windows’ "Opens with," or a quick text-editor look can reveal if it’s a standard AE file or something from another software vendor.
When an AEP seems to go "broken" on a different PC, the cause is almost always that it functions as a reference-based blueprint instead of a self-contained package, with After Effects saving absolute file paths to video, images, audio, and proxy files, and when the project lands on a machine where those paths don’t match due to new drive letters, folder differences, or absent assets, AE loads the project but reports Missing/Offline Media until you reconnect the files.
Even when all media exists, a project may seem incorrect if the destination PC doesn’t include the right fonts, making text substitute, or doesn’t have needed third-party plugins, disabling certain effects, or if the After Effects version is older and incompatible with newer features, and the consistent fix is using Collect Files or manually copying the folder structure, then relinking so that once fonts, plugins, and paths match, the project typically returns to normal instantly.
An AEP file is basically a compact internal database that can represent a full motion-graphics project without the storage weight of footage, containing comp attributes like resolution, frame rate, length, nesting, and background, all
timeline layers and their transforms such as placement values, scale, rotation, opacity, blending, track mattes, parenting, timing, plus animation instructions including keyframes, easing curves, motion blur, markers, expressions, effect parameters, and mask/roto data like contour shapes, feather, expansion, and animated points.
If you enable 3D features, the AEP keeps your cameras, lights, 3D-layer properties, and render-related settings, plus organizational details like bins, label colors, footage interpretations, and sometimes proxies, but it usually leaves out the actual media—your MP4s, MOVs, images, and WAVs stay on disk—so the file mainly stores the instructions for how everything works and the locations of your source files, which is why moving or renaming footage triggers missing-media prompts until you relink For more information regarding
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