and then , I add ambient light with 0xffffff color, the intensity set to 1, it is default value, the scene become bright,
but the shadow 's color is nomore black, it becomes the ground color. and the ground color becoms more white, see below img
so, the second question is, why this happens ?
I tried the same thing in unity, and the result is the same
the last question is, how to make the scene a little bright, but do not influence the ground color and make the shadow color always be black color?
edit: I have posted all imgs in one page
update: I found another issue, the shaodw is not complete,
https://imgur.com/yFtOXBU this is the shadow in three.js,
https://imgur.com/dZwxwWZ this is shadow in unity,
it seems like the bottom surface of the box is not cast shadow , if I rotate box around x 180 degree, up side down, then the shadow look fine, so I suspect the bottom surface has something wrong, but I can’t tell what it is, maybe front /back face related ?