4.13p1 Alembic crash importing file with empty first frame


I am getting an easily reproducible crash when importing alembic files where the first frame has no vertices (for example, the result of a fluid or particle simulation, where the number of vertices is varying for the duration of the file and often the first frame is empty).

This crash happens no matter the import settings. If you try to import the file as a static mesh, it will crash just the same as if trying it as a Geometry Cache, or selecting a different range of frames within the cache. Exporting a file with the same geometric complexity, same varying vertices, but removing the first frames (so the first frame is not empty), will work ok and will not crash.

I am willing to give links to some test files, just let me know where I should send them.

While I understand why this might be logical, I consider that many use cases where alembic might be used (such as results of simulations, like Real Flow) could feature empty first frames, and having the engine gracefully detect that case and work accordingly will remove an extra import step and allow for a better flow of user interaction.

Here is the crash report:

Assertion failed: (Index >= 0) & (Index < ArrayNum) [File:D:\Build++UE4+Release-4.13+Compile\Sync\Engine\Source\Runtime\Core\Public\Containers\Array.h] [Line: 833]
Array index out of bounds: 0 from an array of size 0

UE4Editor_Core!FDebug::AssertFailed() [d:\build++ue4+release-4.13+compile\sync\engine\source\runtime\core\private\misc\outputdevice.cpp:421]
UE4Editor_AlembicLibrary!AbcImporterUtilities::RetrieveTypedAbcData >,FVector>()
UE4Editor_Core!FRunnableThreadWin::Run() [d:\build++ue4+release-4.13+compile\sync\engine\source\runtime\core\private\windows\windowsrunnablethread.cpp:74]

Hi Diego,

Would you mind posting a test asset? I didn’t see the same result in one test asset I already had when I attempted this.

You can share the asset via a PM on the forums or you can post here and under the comment button you should see the option to select the option for “Viewable by Moderators Only” so that the staff will have access but general users will not see the link if sharing is a concern.

Thank you!


Hello, Tim!

Sure, actually I don’t mind sharing it public in this case, it’s just a simple test file.

Here is the file that isn’t working. There is absolutely no geometry on the first frame: Dropbox - File Deleted

And here is a version of the same file working, I just removed some of the initial frames:

If you need anything else let me know!

Thanks for the test asset! I’ve submitted a ticket for this to be investigated.

You can follow the status here: Unreal Engine Issues and Bug Tracker (UE-34515)