Paint effects are effects used inside digital painting system, where the effect is limited to a small area of the source image via a masking image. Perhaps 'brush' would have been a better choice for the name of the context.
In this context, a plugin has the following mandated objects...
Any other input clips that are specified must be optional.
The masking images consists of pixels from 0 to the white point of the pixel depth. Where the mask is zero the effect should not occur, where the effect is whitepoint the effect should be 'full on', where it is grey the effect should blend with the source in some manner.
The masking image may be smaller than the source image, even if the effect states that it cannot support multi-resolution images.
The pixel preferences action is constrained in this context by the following,