Instead of directly using the spritesheet, you can create
AtlasTexture is a special kind of texture that only references another one with an optional sub-rectangle.
So yes, you can use spritesheets with AnimatedSprite, but currently the UI isn't very friendly to do so:
You need to create one
AtlasTexture for each frame of your animation. Create from the inspector, set them to use the spritesheet and choose which sub-rectangle to use, then save as a file (
frame2.tres etc for example).
Then, you'll be able to use them in your
AnimatedSprite just like regular textures.
Another way is of course to manually cut your spritesheet in an image editor. It depends which alternative you prefer.
You can also just not use
AnimatedSprite and use
Sprite directly, by playing with the
frame properties which you can animate either by script or by
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