File Action - Interactive Changes
Inside the Bitmap drawing command, the "Interplate:" option affects the Quartz drawing command that draws the bitmap. This will not change any of the pixels, just how it is drawn to the screen.
Until you drag the "Swirl" variable into the drawing window, no calculation has been performed. Only when you need the variable will the bitmap in "Image" be read from the file, sparsified, saved as a tiff file, acted on by mogrify as explained in the previous tutorial page and the output read into memory as a bitmap. All of this is done in a separate thread, so the user interface will not stutter.
When you hit the "Sparsify" button, the temporary files in "Bitmap file" and "Swirl action" will be removed and since you are drawing the "Swirl" bitmap the bitmap will be computed again, except the "Image" variable will be passed straight to the "Bitmap file" action unchanged.
When a file action takes more than a couple of seconds to finish you see an entry for it in the "Processes" window. Here you can force-quit the application that has been launched. This will cause the "Swirl action" computatinal module to return whatever is currently stored in Picture.tiff inside the temporary folder.