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oops, sorry for the late reply, I forgot all about this thread, sorry :-)
Each pixel in your image has an intensity value somewhere between 0 and 255 (0 = black and 255 = white). ppmcentre sets a threshhold to distinguish between "background" pixels and "planet" pixels, any pixel that's darker than the threshhold is a background pixel, and any pixel thats lighter then the threshhold is considered a "planet" pixel.
ppmcentre finds the centre of your image by averaging together all the "planet" pixels and finding the centre.
You can change the threshhold that ppmcentre uses by adding the "-threshhold=X" commandline parameter when you run ppmcentre, and set X to the threshhold value that you want to use.
Now, to see the values in your own data you have to load a typical frame into an image editing program (like the GIMP, or photoshop), convert the image to greyscale and then move the mouse over the pixels and see what values they have.
Then you can decide for yourself whether the default value that ppmcentre uses (30) is good enough.
cheers, Bird
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