This article describes how the image generated by components inside a filter is combined with the original image loaded into Filter Forge to form the final picture, and how the selection masks the final picture.
Filter Forge supports the accepted convention in bitmap editing software that requires all operations, including filters, to constrain their output to a user-selected area known as selection (usually visible as a dashed line around selected pixels.) However, some effects need to modify pixels outside the selection, such as fire, smoke, and glow effects.
To accommodate both these approaches, Filter Forge offers an automatic blending method that tries to guess what filter type the author had in mind. Additionally, you can select the blending method manually in the Overrides dialog:
This method is selected by default for new filters. It works as follows:
Note: in older versions of Filter Forge (before version 11.0), this method used to work as follows:
Use this method when your filter needs to modify pixels outside the selection, such as a fire, smoke or glow effect. When this method is used, the filter output simply overwrites the original image ignoring the selection. If necessary, the selection masking can be done by adding appropriate components.
Use this method when you're creating an image-based effect such as a distortion. When this method is used, the filter combines its output with the original image by linear interpolation (as in the Lerp component) using the selection as an interpolation coefficient, so that the filter output is restricted by the current selection.
This is an obsolete method maintained for backwards compatibility and historical reasons. When this method is used, the filter output is placed over the original image using the selection as a mask using the same method as in the Normal blending mode of the Blend component – the original image is 'seen through' the transparent areas in the filter output.
