It's been a few weeks since we last worked in the Photoshop Filter Factory. If you haven't read the first two parts of this series, it would probably be advisable for you to do so. In Part 1, we look at the basics of Filter Factory, along with a few examples of how to get some basic effects working by manipulating color channels. In Part 2, we used ResEdit to customize the interfaces of the Filter we created. Now we're going to go a little deeper into what Filter Factory can do to help you create your own Photoshop filters.

If you remember your terminology, you'll recall that we left off with two functions and four operators. Operators are things like multiplication, division, addition and subtraction. Functions we covered included "src" for evaluating and manipulating pixel positions and "rnd" for creating noise effects. Today we're going to take a look at a few new variables, several new functions and one conditional statement.

Don't worry. Just as in Part 1, we're not going to be doing any real programming. Just playing with some numbers and learning to mind our parenthetical expressions.

Convolution
The first thing we're going to do here is a little function that can generate a whole lot of effects, from blurs to more psychedelic effects. This is a function that looks at a pixel then looks at its surrounding pixels and does something to the whole lot of them. The way you write out a convolve expression is like this:

cnv(topleft,topcenter,topright,middleleft,middlecenter,middleright,bottomleft,bottomcenter,bottomright,divisor)

That is to say, you assign a value for each pixel surrounding the center pixel, and the values are divides by the divisor. First let's just try a simple application of this principle by blurring the image. This is accomplished like by placing the following expression in each of the R, G and B channels: cnv(0,1,0,1,4,1,0,1,0,8). (Remember, you can copy and paste expressions from here to the Filter Factory, so you don't have to type things manually.)

That wasn't so tough. So how could we take this function a step further? Well, we don't want to put our sliders to waste, do we? So let's just substitute slider values for regular, old numbers. We only have eight sliders, though, so we'll have to repeat them for now. So try: cnv(ctl(0),ctl(1),ctl(2),ctl(3),ctl(3),ctl(3),ctl(4),ctl(5),ctl(6),ctl(7)) and play around with the sliders. Notice that the divisor (ctl(7)) has the most impact on the way the other sliders behave. This is because any values you select in the first six sliders are added together and divided by the last.

So that's pretty fancy. But we can also bring in one of the functions we learned in Part 1 to make the thing do some even funkier stuff. (And I do mean funk-ay.) The function we used in Part 1 was "src" to tell Filter Factory what to do with the individual pixels and their positions in each channel. So now let's try wrapping this src function around our cnv function. That is to say, with a source function, we have src(x,y,channel). (Channel is represented as a number 0 through three for red, green, blue and alpha, respectively.) Instead of x and y, let's use the convolve expression. It will basically be cnv(expression,expression,channel). Try this one, remembering to separate each expression with a comma and remembering to put the correct channel number at the end:

R: src(cnv(ctl(0),ctl(0),ctl(0),ctl(0),ctl(0),ctl(0),ctl(0),ctl(0),ctl(0),ctl(6)), cnv(ctl(1),ctl(1),ctl(1),ctl(1),ctl(1),ctl(1),ctl(1),ctl(1),ctl(1),ctl(7)),0)

G: src(cnv(ctl(2),ctl(2),ctl(2),ctl(2),ctl(2),ctl(2),ctl(2),ctl(2),ctl(2),ctl(6)), cnv(ctl(3),ctl(3),ctl(3),ctl(3),ctl(3),ctl(3),ctl(3),ctl(3),ctl(3),ctl(7)),1)

B: src(cnv(ctl(4),ctl(4),ctl(4),ctl(4),ctl(4),ctl(4),ctl(4),ctl(4),ctl(4),ctl(6)), cnv(ctl(5),ctl(5),ctl(5),ctl(5),ctl(5),ctl(5),ctl(5),ctl(5),ctl(5),ctl(7)),2)

Here's what it might look like, depending upon your slider settings:

The original image (top) and two convolve
filters using the expressions above.

Again, ctl(6) and ctl(7), otherwise known as sliders 7 and 8, will have the most impact on the final result, since the other sliders are added up and divided by them. Tweak around with it for some psychedelic effects.