A “.BYU” file is most frequently interpreted as a BYU mesh storing 3D vertices and indexed faces, which you can verify by opening it in Notepad: if the content is readable text with many XYZ-style rows, it’s the ASCII mesh; the file starts with a header of integers describing numbers of parts, vertices, faces, and indices, leading into blocks of vertex coordinates and polygon index lists (1-based), with the final index of each polygon written as a negative value—e.g., “10 11 12 -13”—to mark the end of that face.If opening the file in a text editor looks nothing like structured text, it may not be a BYU mesh, as some systems reuse the extension; checking the first bytes in a hex editor is more accurate—“PK,” “ftyp,” and “RIFF” indicate ZIP, MP4-family, or AVI/WAV containers masked as .byu, and renaming a copy for testing in 7-Zip or VLC can confirm this; if no standard signatures appear and the file lacks the familiar BYU layout, the correct way to open it is usually through the software that created it, and if you share early lines or hex bytes, I can identify it quickly.“Movie.BYU” is essentially the classic BYU mesh form built around two components: XYZ vertex coordinates and polygon faces defined by vertex indices with a negative final index indicating the face boundary, allowing the mesh’s structure to move between tools without additional metadata or overhead.What makes BYUanced BYU file handler .BYU a *surface-geometry interchange* format is largely what it intentionally omits: it usually contains no textures, materials, lighting, cameras, animation rigs, or scene hierarchies, focusing only on the mesh surface itself, which is ideal for scientific and engineering workflows where you just need a clean surface for viewing or simulation; so despite the name, “Movie.BYU” is really a classic mesh container whose structure—header, vertex list, and polygon list—lets tools easily read the number of parts, vertices, and faces before consuming blocks of XYZ coordinates that define the 3D shape.After the vertex block, the file switches to connectivity—the mapping that define how points form triangles, quads, or other polygons—using integer vertex indices (often 1-based), with each polygon’s sequence ending when the final index is written as a negative value, a hallmark BYU “stop” marker; some variants also group polygons into parts to keep components organized, and because the format is geometry-only, you won’t find textures, UVs, materials, lights, or cameras—just points plus connectivity forming a 3D shell.