Line2D animation

To get the marching ant effect you need to set the Fill->Texture Mode property of Line2D to Tile.

This shader also only applies the flat colors to the line, so if you have set anything to Fill->Gradient or Fill->Texture, these properties will be ignored.

You can customise these values:

Width and spacing of the tiles (Frequency)
how far the “spikes” / “tiles” go into the line (Amplitude)
Speed of the animation
Direction (-1 or 1, anything in between will also affect the speed)
Colors for the tiles
Opacity

Shader code

shader_type canvas_item;

// Parameters for the sine wave
uniform float frequency = 1.5;
uniform float amplitude = 1000.0;
uniform float speed = 10.0;
uniform float direction: hint_range(-1.0, 1.0) = 1.0;

// Uniforms for colors
uniform vec4 color1: source_color = vec4(1.0, 1.0, 1.0, 1.0);
uniform vec4 color2: source_color = vec4(0.0, 0.0, 0.0, 1.0);
uniform float opacity: hint_range(0.0, 1.0) = 1.0;

void fragment() {
    vec2 uv = FRAGCOORD.xy;
    float sine_wave = sin(UV.x * frequency + TIME * speed * direction) * amplitude;
    uv.y += sine_wave;
    float t = sin(UV.x * frequency + TIME * speed * direction);
    vec4 final_color = t > 0.0 ? color1 : color2;
    final_color *= vec4(1.0, 1.0, 1.0, opacity);
    COLOR = final_color;
}