constant interpolation ignored in export, scale to zero introduces frame with weird negative scale

[TimKraemer]

Describe the bug Whenever I try to export this example scene (a cube with a linear translation animation and a constant scale animation from 1,1,1 to 0,0,0 at the end) the exporter is giving me two LINEAR interpolations:

{
    "asset":{
        "generator":"Khronos glTF Blender I/O v4.1.63",
        "version":"2.0"
    },
...
    "animations":[
        {
            "channels":[
                {
                    "sampler":0,
                    "target":{
                        "node":0,
                        "path":"translation"
                    }
                },
                {
                    "sampler":1,
                    "target":{
                        "node":0,
                        "path":"scale"
                    }
                }
            ],
            "name":"CubeAction",
            "samplers":[
                {
                    "input":4,
                    "interpolation":"LINEAR",
                    "output":5
                },
                {
                    "input":4,
                    "interpolation":"LINEAR",
                    "output":6
                }
            ]
        }
    ],
...
}

when I then manually change the second one to a STEP interpolation, it gives me a weird scale for one frame with x: -1.083, y: -1.083, z: -1.083 before it scales to zero.

To Reproduce

Steps to reproduce the behavior:

1. Download the example blender file
2. Export as gltf2.0 with Operator Presets (optional export als gltf to see the source)

Expected behavior Ideally linear interpolation should get exported as "interpolation":"LINEAR", while constant interpolation get's exported as "interpolation":"STEP" independent from each other. Since it's easy to write a script to do this manually, the second problem, that one frame leads to a weird scale of -1.083 should be prevented completely.

.blend file/ .gltf (mandatory) Example blender file and output gltf with already modified STEP interpolation: simple_test.zip

Version

• OS: tested Windows 11 and Linux Mint 21.3
• Blender Version: 4.1

Additional context related: #2137

[TimKraemer]

in case someone is interested to replace "interpolation":"LINEAR" with "interpolation":"STEP" for every scale animation, I'm using this python script to do so:

import json
import sys

def modify_gltf(input_path, output_path):
    # Load the GLTF file
    with open(input_path, 'r') as file:
        gltf = json.load(file)

    # Iterate over the animations
    for animation in gltf.get('animations', []):
        for channel in animation.get('channels', []):
            target = channel.get('target', {})
            # Check if the target path is "scale"
            if target.get('path') == 'scale':
                sampler_index = channel.get('sampler')
                if sampler_index is not None:
                    sampler = animation.get('samplers', [])[sampler_index]
                    # Change the interpolation to "STEP"
                    if sampler:
                        sampler['interpolation'] = 'STEP'

    # Save the modified GLTF file
    with open(output_path, 'w') as file:
        json.dump(gltf, file, indent=2)

    print(f"GLTF file has been modified and saved as '{output_path}'")

if __name__ == "__main__":
    if len(sys.argv) != 3:
        print("Usage: python modify_gltf.py <input_path> <output_path>")
    else:
        input_path = sys.argv[1]
        output_path = sys.argv[2]
        modify_gltf(input_path, output_path)

[julienduroure]

Hello,

• You are using "Scene" animation mode, that is a full bake mode, where we can't retrieve interpolation

• In your case, I suggest changing to "Action" for example.

  • If you still disable sampling
    • Some of your scale keyframe are not in CONSTANT interpolation, so sampling will be forced. You should set all your keyframe in CONSTANT mode, so export will use STEP
  • If you enable sampling, same thing. Correct STEP retrieving will need correct interpolation for each keyframe of your fcurve, so be sure that all keyframes are in CONSTANT. This is not the case for now

• Seems I can't reproduce your issue with negative scale when changing to STEP

[julienduroure]

More than a month without any answer, closing this ticket. Feel free to comment and provide the .blend file if you want me to have a look