How To Write Your Own Photoshop Filters
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For example, if the slider of Control 2 is at 255 and the slider of Control 3 is at 254, then the result is 1. What this simple number does depends on the formulas you create, which we will look at next. Just remember that you can move around the sliders freely while you're creating an effect, and this will allow you to preview what the filter will do. When you finally finish your filter, the position of the sliders at the end will become the default values in your final filter, though they can still be adjusted by the user later.

The sliders in FIlter Factory generate numbers between 0
and 255. These numbers are used to enter values interactively
into your formulas. (See below.)

So now we come to the portion of the tutorial where you learn how to make formulas. I'll try to make it as simple as possible so as not to alert the gnome in your head to any coding that's going on. After my explanation, you can take my formulas and change them around yourself to generate different results through a process of trial and error. Avery doesn't mind trial and error.

A bit formulaic
So, right now you're staring at four blank fields labeled R, G, B and A. As you might imagine, these represent red, green, blue and alpha, respectively. Please note that Filter Factory filters you create will only work on RGB or RGBa images. CMYK is definitely out.

So what do you do with these fields? You enter in some math. First, just to get acquainted with the fields, type in a single whole number into each one, something from 0 to 255, and watch the preview window. If you enter 0 in all the fields, your preview remains blank. Enter in higher values, and your RGBa channels start coming into view. You have just set the absolute values of your RGBa channels across your entire image.

Now take it a step farther. Instead of numbers, enter in ctl(0) into the field labeled R, ctl(1) into the field labeled G, ctl(2) into the field labeled B and ctl(3) into the field labeled A. You can now adjust your first four sliders to interactively adjust your RGBa channels. (You will have to adjust the fourth slider first before you'll be able to see changes from the other three sliders. This is because the fourth slider controls transparency, and, by default, the slider is set to 0, meaning completely transparent.

By entering in a slider number in your R, G, B and A entry fields,
you can control the levels of your red, blue, green and alpha
channels interactively. Sliders are referred to as ctl(0)
through ctl(7), which represent sliders 1 through 8.

Before we move on, let's just recap to make sure you've got it. In the field labeled R, we have placed the text "ctl(0)." This tells the Filter Factory to use the first slider (confusingly called Control 0) to adjust the value of our red channel (R) from 0 to 255. A value of 0 means no red, while a value of 255 means total red. Same for the other three channels. If you're lost now, this means Avery is up to his old shenanigans again, and there's no way you'll be able to get through the next part. So go back up to the beginning of this section and read it again, making sure you look at the Filter Factory interface to gain some point of reference.

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