InterpolationTest.gltf fails to load, crashes with "Inconsistent animation sampling"

[stephengold]

InterpolationTest.gltf has been included in the glTF-Sample-Assets test suite since its inception.
https://github.com/KhronosGroup/glTF-Sample-Assets/tree/main/Models/InterpolationTest

When I try to load it using jme3-plugins, I get warnings followed by a crash:

Mar 05, 2025 10:33:28 AM com.jme3.scene.plugins.gltf.GltfLoader readAnimation
WARNING: JME only supports linear interpolation for animations
Mar 05, 2025 10:33:28 AM com.jme3.scene.plugins.gltf.GltfLoader readAnimation
WARNING: JME only supports linear interpolation for animations
com.jme3.asset.AssetLoadException: An error occurred loading InterpolationTest/glTF/InterpolationTest.gltf
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:181)
	at com.jme3.scene.plugins.gltf.GltfLoader.load(GltfLoader.java:107)
	at com.jme3.asset.DesktopAssetManager.loadLocatedAsset(DesktopAssetManager.java:274)
	at com.jme3.asset.DesktopAssetManager.loadAsset(DesktopAssetManager.java:390)
	at com.jme3.asset.DesktopAssetManager.loadModel(DesktopAssetManager.java:441)
	at com.github.stephengold.wrench.test.CompareLoaders.loadModel(CompareLoaders.java:889)
	at com.github.stephengold.wrench.test.CompareLoaders.loadModel(CompareLoaders.java:855)
	at com.github.stephengold.wrench.test.CompareLoaders.onAction(CompareLoaders.java:496)
	at jme3utilities.ui.DefaultInputMode.onAction(DefaultInputMode.java:186)
	at com.jme3.input.InputManager.invokeActions(InputManager.java:174)
	at com.jme3.input.InputManager.onKeyEventQueued(InputManager.java:474)
	at com.jme3.input.InputManager.processQueue(InputManager.java:871)
	at com.jme3.input.InputManager.update(InputManager.java:923)
	at com.jme3.app.LegacyApplication.update(LegacyApplication.java:810)
	at com.jme3.app.SimpleApplication.update(SimpleApplication.java:249)
	at com.jme3.system.lwjgl.LwjglWindow.runLoop(LwjglWindow.java:707)
	at com.jme3.system.lwjgl.LwjglWindow.run(LwjglWindow.java:797)
	at java.base/java.lang.Thread.run(Thread.java:840)

Caused by: com.jme3.asset.AssetLoadException: Inconsistent animation sampling 
	at com.jme3.scene.plugins.gltf.TrackData.checkTimesConsistency(TrackData.java:191)
	at com.jme3.scene.plugins.gltf.TrackData.update(TrackData.java:175)
	at com.jme3.scene.plugins.gltf.GltfLoader.readAnimation(GltfLoader.java:906)
	at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:162)
	... 17 more

I discovered issue while testing JME v3.8.0-alpha4. However, the behavior hasn't changed since at least JME v3.2.4-stable, so I don't consider this a regression.

[mondogo24]

I think I have found the source of this issue.

In TrackData::checkTimesConsistency

jmonkeyengine/jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/TrackData.java

Lines 187 to 193 in ae1715f

	public void checkTimesConsistency() {
	if ((translations != null && times.length != translations.length)
	|| (rotations != null && times.length != rotations.length)
	|| (scales != null && times.length != scales.length)) {
	throw new AssetLoadException("Inconsistent animation sampling ");
	}
	}

, just before the moment of crash, scales.lenght returns 15 while times.lenght returns 5. Because of it, the if(...) statement success and triggers the exception.

Tracking this issue lead me to this line

jmonkeyengine/jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/GltfLoader.java

Lines 1068 to 1069 in ae1715f

	Vector3f[] scales = readAccessorData(dataIndex, vector3fArrayPopulator);
	trackData.scales = scales;

in GltfLoader.java (just before trackData.scales is set). Then, in readAccessorData, when dataIndex is 9 the count readed value from the gltf file is 15. I checked the InterpolationTest.gltf at accessors index 9 (https://github.com/KhronosGroup/glTF-Sample-Assets/blob/5109ab2a499c5a2c784b86e460fa491d52256e25/Models/InterpolationTest/glTF/InterpolationTest.gltf#L96-L102) and you can verify that count is set with 15.

I used System.out.println for tracking the numbers and this is what I got:

GltfLoader.readAnimation dataIndex: 8
GltfLoader.readAccessorData type: VEC3
GltfLoader.readAccessorData count: 5
GltfLoader.readAnimation scales.lenght: 5
GltfLoader.readAnimation dataIndex: 8
GltfLoader.readAccessorData type: VEC3
GltfLoader.readAccessorData count: 5
GltfLoader.readAnimation scales.lenght: 5
abr 01, 2026 5:47:41 P. M. com.jme3.scene.plugins.gltf.GltfLoader readAnimation
WARNING: JME only supports linear interpolation for animations
GltfLoader.readAnimation dataIndex: 9
GltfLoader.readAccessorData type: VEC3
GltfLoader.readAccessorData count: 15
GltfLoader.readAnimation scales.lenght: 15
TrackData.checkTimesConsistency times.length: 5
TrackData.checkTimesConsistency scales.lenght: 15
abr 01, 2026 5:47:41 P. M. com.jme3.app.LegacyApplication handleError
SEVERE: Uncaught exception thrown in Thread[jME3 Main,6,main]
com.jme3.asset.AssetLoadException: An error occurred loading Models/InterpolationTest/glTF/InterpolationTest.gltf
        at com.jme3.scene.plugins.gltf.GltfLoader.loadFromStream(GltfLoader.java:233)

Since I didn't know if consider it as something to fix in jmonkeyengine or something external. I thought about posting this here.

[javagl]

I have not (yet) looked at the details, but just stumbled over this (and maybe will have a closer look later). But from the description, I'd say that the crux is the output

WARNING: JME only supports linear interpolation for animations

When an animation uses a non-linear interpolation, then each "time" entry has three corresponding entries in the "values", namely, the value itself, as well as the input- and output tangents. (I recently talked about this around KhronosGroup/glTF-Sample-Assets#272 (comment) which shows the 'rotation' values for this asset - one can see the 15 lines there, and the same will be the case for the 'scale' values)