dim-image-element.js

class DimImageElement extends Polymer.Element {
  async setConfig(config) {
    this.dim_entity = config.dim_entity;
    this.img = document.createElement('hui-image-element')
    this.img.setConfig(config);
    this.appendChild(this.img)
  }

  set hass(hass) {
    this.img.hass = hass;
    this.img.updateComplete.then(() => {
    if(this.img.shadowRoot) {
      let value;
      let state = hass.states[this.dim_entity]
      if(this.dim_entity.startsWith("light.")) {
        value = (state.state === 'on') ? state.attributes['brightness']/2.55 : 0;
      } else {
        value = state.state;
      }
      this.img.shadowRoot.querySelector("hui-image").$.image.style.filter = `opacity(${value}%)`;
    }
    });
  }
}

customElements.define('dim-image-element', DimImageElement);