Animation: At the boundary frame between non-muted NLA strips, evaluate the first frame of the later strip instead of the last frame of the earlier. #113487

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Paolo Amadini wants to merge 1 commits from pamadini/blender:nla-adjacent-strips into main

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Problem
When a strip immediately follows another on the same track, the first frame of the second strip should be evaluated, instead of the last frame of the first strip. The former is also how looping strips are currently handled.

Source of the Problem
This is a historic issue and not the result of intentional design. It is caused by the loop in anim_sys.cc (formerly anim_sys.c) stopping as soon as it finds a strip in range.

Solution
In nlastrips_ctime_get_strip, if we are exactly at the last frame of the clip, we look ahead to see if a non-muted strip immediately follows and use it instead.

The code is simplified because strips can't overlap, so we don't have to check whether we actually are at the last frame. We also don't need to care about the NLASTRIP_FLAG_NO_TIME_MAP case because when the flag is set there is only one strip in the list provided to the function, as animsys_create_tweak_strip is always followed by nlastrips_ctime_get_strip_single.

Test File
The CubeAction runs from frames 1 to 4, with the X Location being the same as the frame number.
Starting at frame 1 there are two consecutive instances, CubeAction and CubeAction.001.
Starting at frame 11 there is a single instance that is looped twice, CubeAction.002.

Expected result: At frame 4 the cube should be at X Location "1", which is the same as the looping case at frame 14.
Actual result before the patch: At frame 4 the cube is at X Location "4".

**Problem** When a strip immediately follows another on the same track, the first frame of the second strip should be evaluated, instead of the last frame of the first strip. The former is also how looping strips are currently handled. **Source of the Problem** This is a historic issue and not the result of intentional design. It is caused by the loop in `anim_sys.cc` (formerly `anim_sys.c`) stopping as soon as it finds a strip in range. **Solution** In `nlastrips_ctime_get_strip`, if we are exactly at the last frame of the clip, we look ahead to see if a non-muted strip immediately follows and use it instead. The code is simplified because strips can't overlap, so we don't have to check whether we actually are at the last frame. We also don't need to care about the `NLASTRIP_FLAG_NO_TIME_MAP` case because when the flag is set there is only one strip in the list provided to the function, as `animsys_create_tweak_strip` is always followed by `nlastrips_ctime_get_strip_single`. **Test File** The CubeAction runs from frames 1 to 4, with the X Location being the same as the frame number. Starting at frame 1 there are two consecutive instances, CubeAction and CubeAction.001. Starting at frame 11 there is a single instance that is looped twice, CubeAction.002. **Expected result:** At frame 4 the cube should be at X Location "1", which is the same as the looping case at frame 14. **Actual result before the patch:** At frame 4 the cube is at X Location "4".
Paolo Amadini added 1 commit 2023-10-10 13:12:52 +02:00
4a686008c7 Animation: At the boundary frame between non-muted NLA strips, evaluate the
first frame of the later strip instead of the last frame of the earlier.
Iliya Katushenock added this to the Animation & Rigging project 2023-10-10 13:13:11 +02:00
Paolo Amadini requested review from Nathan Vegdahl 2023-10-10 13:13:23 +02:00
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git fetch -u nla-adjacent-strips:pamadini-nla-adjacent-strips
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Reference: blender/blender#113487
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