Simple 抠图：
参考视频: https://www.youtube.com/watch?v=OlY4YFs3d9k
quick selection
左下角多选了一天背景，hold option key 来deselect。
点击refine Edge on top, output出来你选择的东西，可以选择output layer or mask.
然后点击保存，或者继续操作。 可以加一些background layer之类的。
如果用lasso tool也一样，只不过是自己完全手动selection。 选择完以后top bar也会出现一个select and mask的output出口，你可以在那里对你选中的东西搞一点事情。比如说给选中的人物上色，或者output成layer。
"To remove an area from a selection, hold down yourAlt(Win) /Option(Mac) key, which temporarily switches the Quick Selection Tool toSubtract from selectionmode (you could also select the "Subtract from selection" option in the Options Bar but you'd need to remember to switch it back to the "Add to selection" mode when you're done). The small plus sign in the center of the cursor will be replaced with a minus sign (-). Then, with Alt / Option still held down, click and drag inside the area you need to remove. I'll need to make my cursor smaller here by pressing the left bracket key a few times:"
补充一下，option key 必须得你先松开鼠标后再点。比如你一开始点着鼠标在select，发现多选了，这个时候要松开鼠标，然后再按下option，这个时候你的所有action都是在remove selection。
