Alembic export is duplicating a frame every two frames when exporting fluid sims at high starting frame #98310

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B · 2022-05-22T04:04:25+02:00

B commented

2022-05-22 04:04:25 +02:00

System Information
Operating system: Windows 11
Graphics card: NVIDIA GeForce RTX 3070 TI

Blender Version
Broken: 3.1.2
Worked: Does not work on the other version/machine I was using (2.93.1, Windows 10, NVIDIA GeForce GTX 1650)

Short description of error
When exporting fluid sims as an alembic (using either Flip fluids or blender's built in simulator) when the starting frame is large (At least 7403) the resulting alembic will have a duplicated frame ever 2 frames.

(The simulation on the left, the alembic on the right)

(The alembic by itself)
It's very strange. Every two frames, one frame is duplicated.

This behavior is not present when exporting and importing alembics when the simulation was run from a smaller starting frame. Here is the same simulation, simulated from 0000-0040:

The alembic is identical to the simulation.

Exact steps for others to reproduce the error
NOTE: All alembics seen here and in the attachments were exported with the default settings, the only thing checked was "selected objects" next to "only"

I've included a zip that has two blend files (one for the sim at 0000-0040, one with the sim at 7403-7443). The zip also contains the alembics seen in the gifs above. To replicate this, create a cube, make that the fluid domain. Create a UV sphere, make that a fluid flow object. Make sure the domain is set to "fluid" and set the sim start to 7403 and the sim end to 7443. Once the simulation is baked, select the fluid mesh. Then, export the fluid mesh as an alembic. Check "selected objects" next to "only" (and make sure the frame range is set to 7403-7443). Then, import the created alembic.

I have tried this on multiple test fluid simulations, using both the Blender built in fluid sim and the FLIP fluids add-on and I experience the same result (duplicate frames)

(This is the FLIP Fluids sim I discovered this issue on)

And you can also do the above in Blender 2.93.1 (that's the only other version I've tested this one).

I'm hoping I'm just not overlooking something, but I can't understand why alembic exporting wouldn't be consistent from frames at high frame ranges. So, it seems to me this is unintended behavior.

EDIT: Not sure if the .zip file attached, I'll attach the .blends and alembics separately.
fluid test 7403-7443.blend

fluid test 0000-0040.blend

fluid test 0000-0040 alembic.abc

fluid test 7403-7443 alembic.abc

EDIT 2: Changing the frame offset also results in inconsistent behavior. For example, making it so both alembics start to play at frame 0 results in identical alembics (even though one was baked and exported from 7403-7443)

Changing the other alembic's frame start time to match the one that starts at 7403 makes it exhibit the frame dropping behavior.

Additionally, I've found a work around(?). The alembic on the left's frame offset is 7402 and the alembic on the right's frame offset is -1.5. Both have "vertex interpolation" turned off. So by using non-integer or other frame offsets, the alembic works as expected and appear to be identical.

But I haven't tried it on anything but these two alembics.

**System Information** Operating system: Windows 11 Graphics card: NVIDIA GeForce RTX 3070 TI **Blender Version** Broken: 3.1.2 Worked: **Does not work** on the other version/machine I was using (2.93.1, Windows 10, NVIDIA GeForce GTX 1650) **Short description of error** When exporting fluid sims as an alembic (using either Flip fluids or blender's built in simulator) when the starting frame is large (At least 7403) the resulting alembic will have a duplicated frame ever 2 frames. ![both with offset position.gif](https://archive.blender.org/developer/F13097010/both_with_offset_position.gif) (The simulation on the left, the alembic on the right) ![7403-7443 better example.gif](https://archive.blender.org/developer/F13097005/7403-7443_better_example.gif) (The alembic by itself) It's very strange. Every two frames, one frame is duplicated. This behavior is not present when exporting and importing alembics when the simulation was run from a smaller starting frame. Here is the same simulation, simulated from 0000-0040: ![both with offset position 0000-0040.gif](https://archive.blender.org/developer/F13097015/both_with_offset_position_0000-0040.gif) The alembic is identical to the simulation. ![0000-0040 example.gif](https://archive.blender.org/developer/F13097019/0000-0040_example.gif) **Exact steps for others to reproduce the error** **NOTE: All alembics seen here and in the attachments were exported with the default settings, the only thing checked was "selected objects" next to "only"** I've included a zip that has two blend files (one for the sim at 0000-0040, one with the sim at 7403-7443). The zip also contains the alembics seen in the gifs above. To replicate this, create a cube, make that the fluid domain. Create a UV sphere, make that a fluid flow object. Make sure the domain is set to "fluid" and set the sim start to 7403 and the sim end to 7443. Once the simulation is baked, select the fluid mesh. Then, export the fluid mesh as an alembic. Check "selected objects" next to "only" (and make sure the frame range is set to 7403-7443). Then, import the created alembic. I have tried this on multiple test fluid simulations, using both the Blender built in fluid sim and the FLIP fluids add-on and I experience the same result (duplicate frames) ![bad alembic slower.gif](https://archive.blender.org/developer/F13097043/bad_alembic_slower.gif) (This is the FLIP Fluids sim I discovered this issue on) And you can also do the above in Blender 2.93.1 (that's the only other version I've tested this one). I'm hoping I'm just not overlooking something, but I can't understand why alembic exporting wouldn't be consistent from frames at high frame ranges. So, it seems to me this is unintended behavior. EDIT: Not sure if the .zip file attached, I'll attach the .blends and alembics separately. [fluid test 7403-7443.blend](https://archive.blender.org/developer/F13097048/fluid_test_7403-7443.blend) [fluid test 0000-0040.blend](https://archive.blender.org/developer/F13097047/fluid_test_0000-0040.blend) [fluid test 0000-0040 alembic.abc](https://archive.blender.org/developer/F13097050/fluid_test_0000-0040_alembic.abc) [fluid test 7403-7443 alembic.abc](https://archive.blender.org/developer/F13097049/fluid_test_7403-7443_alembic.abc) EDIT 2: Changing the frame offset also results in inconsistent behavior. For example, making it so both alembics start to play at frame 0 results in identical alembics (even though one was baked and exported from 7403-7443) ![changed start time to match 0000.gif](https://archive.blender.org/developer/F13097077/changed_start_time_to_match_0000.gif) Changing the other alembic's frame start time to match the one that starts at 7403 makes it exhibit the frame dropping behavior. ![changed start time to match 7403.gif](https://archive.blender.org/developer/F13097080/changed_start_time_to_match_7403.gif) Additionally, **I've found a work around(?).** The alembic on the left's frame offset is 7402 and the alembic on the right's frame offset is -1.5. Both have "vertex interpolation" turned off. So by using non-integer or other frame offsets, the alembic works as expected and appear to be identical. ![possible fix.gif](https://archive.blender.org/developer/F13097089/possible_fix.gif) But I haven't tried it on anything but these two alembics.

B commented

2022-05-22 04:04:25 +02:00

Added subscriber: @velocirection

Evan Wilson commented

2022-05-23 02:22:25 +02:00

Added subscriber: @EAW

Evan Wilson commented

2022-05-23 02:22:25 +02:00

Changed status from 'Needs Triage' to: 'Resolved'

Evan Wilson closed this issue

2022-05-23 02:22:25 +02:00

Evan Wilson self-assigned this 2022-05-23 02:22:25 +02:00

Evan Wilson commented

2022-05-23 02:22:25 +02:00

Thank you for the report.

I reproduced this bug in the latest 3.1.2 official release, but not the latest 3.2 and 3.3 daily build hashes bf352df27a and fff8f969de, so it appears the bug has been fixed already, likely by f3a475a767

Please try the latest daily build: https://builder.blender.org/download/

If the problem persists, please let us know so we can re-open the report. Don't forget to mention the specific version you tested again.

Thank you for the report. I reproduced this bug in the latest 3.1.2 official release, but not the latest 3.2 and 3.3 daily build hashes bf352df27a and fff8f969de, so it appears the bug has been fixed already, likely by f3a475a767 Please try the latest daily build: https://builder.blender.org/download/ If the problem persists, please let us know so we can re-open the report. Don't forget to mention the specific version you tested again.

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Alembic export is duplicating a frame every two frames when exporting fluid sims at high starting frame #98310