Make ShadowMaterial softer

I create shadow from ShadowMaterial. Because I need a transparent plane.

this.plane = new Mesh(new PlaneGeometry(20, 20),
    new ShadowMaterial({
        opacity: 0.1,
        color: 0xff0000
this.plane.receiveShadow = true

and I get this

But I want to make shadow softer like this

스크린샷 2021-02-26 오전 10.28.16


Is the only way to get soft shadow is following example?
It is little bit complex to create all renderTarget etc… and make blurShadow.

In documentation said,

parameters - (optional) an object with one or more properties defining the material’s appearance. Any property of the material (including any property inherited from Material) can be passed in here.

Is there any parameters that I can use for shadow softner?

ps. In my situation, I don’t need dynamic light/shadow. So if there is no way to get ShadowMaterial softer, I would consider using png shadow texture to MeshBasicMaterial…