Apply a blur. The algorithm is "box blur".
Rectangle blur is the average of the brightness of the pixels in the "range" represented by two parameters, xblur and yblur. For example, if xblur = 10 yblur = 2, the average brightness of the pixels in the rectangular range -10 to 10 in the horizontal direction and -2 to 2 in the vertical direction around the target pixel is calculated, It is the brightness of that pixel.
The area of the range can be calculated by (xblur × 2 + 1) × (yblur × 2 + 1). In the current version, if the area is less than 256, the algorithm will be faster than if the area is more than 256.
If the Layer.face property is dfAlpha, it uses a special algorithm for alpha compositing, which is a bit slower. For dfAddAlpha and other drawing methods, it is a faster algorithm.