How to preview the filter in the original image

Hi,
I am writing a filter plugin to photoshop cs2.
as all plugins, I want to add the "Preview" option.
I want to see the filter effect in the original image when the filter dialog is open. (When I press the OK button and the dialog is closed, I can see the filter effect on the original image, but I want to see it also when the dialog is open),

Any one know how to do that? How to see the effect in the image, when the dialog is open?

Thanks, Inbal