Blending of Result with Original Image

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 affects this final picture.

Filter Forge supports the usual convention accepted 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.) The easiest approach to support this convention would be to simply prohibit a filter from modifying pixels outside the selection (or, more precisely, use the selection as a mask for the filter's output.) However, blocking the filters from accessing and modifying pixels outside the selection would prevent the filter authors from creating effects that spread outside the selection, such as fire, smoke and glow.

On the other hand, end-users expect the filter output to be masked by the selection. Therefore, the question is how to allow complex filters that modify the image outside the selection but at the same time to make the majority of filters limited by the selection.

To deal with this problem, Filter Forge tries to 'guess' what kind of filter the author had in mind, and automatically chooses the best blending method from the three methods described below. This automatic choice can be overridden in the Overrides dialog.

Result overwrites Original (Selection is ignored)

This method is chosen when a filter uses the Selection component. Filter Forge assumes that the filter needs to modify pixels outside the selection and does not mask the filter output with the selection bitmap – the filter output simply overwrites the original image ignoring the selection. Authors of filters that use the Selection component should perform the selection masking manually by adding appropriate components. You can force the filter to use this blending method by selecting a corresponding option in the Overrides dialog.

Result is mixed with Original (Selection serves as a mask)

This method is chosen when a filter uses the Image component but no Selection component. Filter Forge considers the filter to be an image-based effect such as a distortion, and 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. You can force the filter to use this blending method by selecting a corresponding option in the Overrides dialog.

Result is placed over Original (Selection serves as a mask for Result)

This method is chosen when a filter uses no Image or Selection components. Filter Forge considers the filter to be a texture generator, and 'places' the filter output 'on top' of 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. You can force the filter to use this blending method by selecting a corresponding option in the Overrides dialog.