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bool doingProxy
doingProxy is a built-in read-only Boolean variable, which is True if the filter is running, and False if the filter is applying the effect to the image in the host program.
With the doingProxy variable you can check if the filter is processing the proxy image ('preview image') or the original image (layer) in the host program.
Since it sometimes can be useful to let the filter process the preview image and the final image differently, the doingProxy variable can be used to separate these cases in your code. You might e.g. want to add guide lines in your preview image and not in the final image, or you might prefer to add a watermark to the final image and not in the preview (e.g. in case you create a demo version of your plugin).
if (doingProxy) {
// execute this if filter is running
}
else {
// execute this if filter is
// applying effect to host image
}