Galactica/crates/render/shaders/include/animate.wgsl

// Pick a frame of a sprite animation from an instance.
fn animate(sprite_index: u32, age: f32, offset: f32) -> f32 {

	let len = global_sprites[sprite_index].frame_count;
	let rep = global_sprites[sprite_index].repeatmode;
	let fps = global_sprites[sprite_index].fps;
	var frame: f32 = 0.0;

	// Once
	if rep == u32(1) {

		frame = min(
			age / fps + offset,
			f32(len) - 1.0
		);

	// Reverse
	} else if rep == u32(2) {
		let x = age / fps + offset;
		let m = f32(len) * 2.0 - 1.0;
		// x fmod m
		frame = x - floor(x / m) * m;

		if frame >= f32(len) {
			frame = (
				f32(len) + f32(len) - 1.0
				// Split integer and fractional part so tweening works properly
				- floor(frame)
				+ fract(frame)
			);
		}

	// Repeat (default)
	} else {
		let x = age / fps + offset;
		let m = f32(len);
		frame = x - floor(x / m) * m;
	}
	
	return frame + f32(global_sprites[sprite_index].first_frame);
}