Filter is used for lighting (marking) of borders and increasing the sharpness. Its application marks the breaks of brightnesses levels at the image and suppresses areas with weak changes of brightnesses.
The essence of this transformation is in building of the filter mask and its imposition on the image. Mask is a two-dimensional massive (mainly it is 3X3), whose elements values are defined by the result of transformation. In each point of the image the response of the filter is counted as the sum of the elements of filter mask multiplied by respective values of pixels in the area under the mask.
For Laplace transformation the building of the filter mask is carried out with the use of the second derivative.
Response of the filter in this transformation does not depend on direction of heterogeneity at the image, that means, it is invariant to the rotation of the image.
Transformation influences the overfall of the brightness (for example, at a staircase overfall) in all directions, and selects isolated points, thin lines, and acute angles of objects.
Size parameter defines the area of transformation effect.
Adding original image to the Laplace one restores the general range of change of brightnesses at the image, and the Laplace strengthens contrasts in places of brightnesses breaks.
Please find more information on settings and the use of filter in the step by step instruction Tutorial on the use of Laplace filter.