shader_type canvas_item;

// Use the captured screen texture from the BackBufferCopy
uniform sampler2D screen_texture : hint_screen_texture, filter_linear_mipmap;

uniform float threshold : hint_range(0.0, 2.0) = 0.5;
uniform float smoothness : hint_range(0.0, 1.0) = 0.02;
uniform vec4 fluid_color : source_color = vec4(0.2, 0.5, 1.0, 1.0);

void fragment() {
    // 1. Grab the pixel from the 'screen' behind this ColorRect
    vec4 screen_color = texture(screen_texture, SCREEN_UV);
    
    // 2. Read the brightness (density)
    float density = screen_color.r;
    
    // 3. Apply the threshold/cutoff
    float alpha = smoothstep(threshold, threshold + smoothness, density);
    
    // 4. Output the fluid
    COLOR = vec4(fluid_color.rgb, alpha * fluid_color.a);
}