Understanding SkImageFilter, SkBlender, SkColorFilter and SkMaskFilter setting in SkPaint

[Lucida]

Hi, there. 

When drawing an object, we can apply these kinds of operations for this paint.There must be some essential differences between these operations.

Here are some questions I want to figure out:

What's the priority of the operations? 

Are they othognal or one can override another?

It seems that SkImageFilter could combinate others into one single operations?

Also, is SkShader object a more general equivalent of all?

Which one has a notable effect on perfermance?

How they are implemented under the hood? (from OpenGL level)

Thanks.