How To Write Your Own Photoshop Filters
The basics of using Filter Factory

by David Nagel
Executive Producer
dnagel@digitalmedianet.com

So you want to write your own Photoshop filters, huh? Understandable. After all, there's a special little place inside all of us that yearns to be a programmer. Unfortunately, this special little place is inhabited by a twisted little gnome named Avery who hates programming code and does everything in his power to prevent the knowledge of such code from entering this special little place. Sure, it'll let in a little HTML every now and again. But C++? Forget about it. Avery don't cotton to no C++. (In case you're wondering, yes, there are ways to break down Avery's defenses and turn yourself into a real programmer, but the methods for doing so can be quite taxing on the user. These methods include living exclusively off Tina's™ bean & cheese burritos during college, driving a Gremlin and giggling at FORTRAN jokes. Seems a pretty high price to pay just to write code for other people to enjoy.)

For the rest of us, there's Filter Factory, a free plugin that ships with Adobe Photoshop. It's buried somewhere on your Photoshop CD. If you want to follow along with this, you'd better install it now. Filter Factory is a little tool that lets you write your very own Photoshop filters. It's not completely Avery-resistant—there is math involved—but it certainly helps.

For this opening tutorial, I'm going to show you how I created a few filters. I'll even let you download them, along with their settings, so that you can play with them and modify them in an effort to learn the process. (Incidentally, this tutorial is being done on a Mac. You can do the same things in the Windows version. But if you want filters that work on both Mac and Windows machines, you need to create the filters on their respective platforms. This will become more clear later.)

Filter Factory allows you to create your own Photoshop plugins. Click on the
image (or right here) to download the Digital Media Designer Pack 1,
which includes settings files for following along with the tutorial.
(Mac filters, Mac/Win settings files, 370 KB)

To get started, launch Photoshop, open up a document and convert the background to a layer; and select Filter > Synthetic > Filter Factory. Your document should contain an image so that you can see the effect of the filter. And it should contain an alpha channel. If it doesn't, you will not be able to create filters that manipulate the alpha channel.

Getting in control
The first thing you'll notice is an interface that doesn't seem to make any sense. Map 1? Control 4? What the.... This is easy. Forget Map 1, Map 2, etc. I don't know what they do, and I don't care. What you do need to care about are the controls associated with these Maps, eight in total, numbered 0 through 7. These sliders are what the end user will be able to adjust to control the effect of one of your filters. You will not necessarily be using them all.

Huh? The Filter Factory interface.

So what do these Controls do? By adjusting a given slider, you generate a number 0 through 255. These numbers are used in the formulas you will be writing to make this filter do something. In other words, you might have a formula that calculates Control 2 minus Control 3. The end result will always be different depending on the slider value of Control 2 and Control 3, and it's this end result that determines how an effect looks.

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