Engineering Blotty Brushes in Photoshop

In order to make my library of Photoshop brush presets for digitally inking blotty line art, I had to dive deep into the subject of Photoshop brush tips and presets. I learned a great deal, much of which was difficult to cull from various obscure sources on the web, so I thought I'd share here some of the information I learned along the way.

I'll start by examining some of the underlying principles of how to build up a brush preset in Photoshop. I'll then set about the task of applying that knowledge to creating the effects I'm after: namely, simulating the blotty line work that results from working with physical ink pen nibs and crow quill pens.

If you already well-aquainted with the finer points of brush presets in PS, feel free to skip directly to the article where I build up the presets themselves.

I have created a little sample composition to show how one might use these presets. It is admittedly rather hastily made, but I'm not an illustrator, so please bear with me; I created it to show how one can combine different brushes from the resulting library for various purposes and effects in a single image.

References

Before we get started, a few references. The Andy Warhol Museum in Pittsburgh shows how to achieve Warhol's original effect in physical media. Also, check out an interview with Ed Fotheringham, a contemporary illustrator who mentions that he uses his own custom Photoshop brushes to achieve his own similar result, which is explicitly based on Warhol's technique and style. Also, see this blog post by another artist mentioning employing the same approach in PS.

So far as I'm aware, no one else on the web specifically shows the technical details of how actually to achieve this effect digitally, however, so I had to piece all the information together myself experimentally, employing as best I could what I've learned about Photoshop internals from Deke McClelland's amazing training resources (which I highly recommend).

Brushes in Photoshop

All brushes in Photoshop, whether they appear to create a continuous line or not, work by the repeated application of individual discrete daubs of digital paint. Unlike in Illustrator, there are no vectors specifying a well-defined region of brushed area. As with almost everything in Photoshop (the exceptions being unrasterized text layers, paths, and vector masks), every time you brush something in the software, you're just recoloring individual pixels to varying degrees in the underlying bitmap associated with the active layer.

We can demonstrate this by placing a standard round brush under some experimental scrutiny.

At a spacing of 25% (100% hardness, 30px size), the line appears continuous and smooth.

Yet when we increase the spacing to 50%, we can see clearly how these strokes are actually composed.

At a spacing of 100%, each individual discrete daub of paint is positioned precisely next to the other, without overlapping.

For each daub, Photoshop renders out a raster image, which is simply a copy of the base brush tip (which is itself just a grayscale raster image). When you're gliding along brushing out lines with a low spacing, these individual daubs visually blend together to the extent that their discrete nature is imperceptible. However, under the hood, their composition is entirely of discrete little images copied over and over.

As a little aside before we move on, let us also note that we can change the roundness of the daubs (30% roundness at 350% spacing below).

We can also change their angle (90 degrees below)

Various combinations of these simple options permit the creation of calligraphic line contours. The following is a spacing of 25% and a roundness 10%. You can imagine this being composed of lots of tiny overlapping flattened circles as in the above more widely-spaced examples.

Compare this to the same options at a roundness of 100% (i.e., no flattening of the composite circles).

Discrete daubs to our advantage

When we're using a stroke that is as regular as the simple hard round tip, large spacing values look artificial and take away from the illusion of natural brushing, creating an effect that is almost always undesirable.

However, the effect created by more widely spaced daubs does provide us with a side-effect that we can use to our advantage when creating the appearance of blotty ink lines. If we regard the discrete-daub nature of Photoshop brushes as a source of raw "ink blots" (rather than merely visual artifacts of a setting pushed too far) we suddenly have the raw material for an infinite well of ink blots.

In their simplest form, however, these regular round circles don't look at all like anything made by a human hand or pen. Our first task, then, is to find a way to introduce some randomness and spontaneity into our blots.

The Shape Dynamics Options Sub-panel

The Shape Dynamics options determine how the shape and size of ink daubs vary within a stroke of the brush.

An important prerequisite bit of vocabulary here is jitter, which is Adobe's term for random variation introduced into the values of parameters of the brush tip daubs as they are laid down in the course of a brush stroke (including parameters affecting the individual daubs themselves and their spacing and count within the overall line.)

Size Jitter

Size Jitter specifies what amount of variation is to be introduced into the size of the individual daubs that are laid down, and what its source is to be.

We can choose a totally random source, a user input source, of some combination of the two.

Still working with a round hard brush of 30px and spacing of 100%, here is what our base brush looks like without any dynamics introduced.

Here is if we leave Size Jitter at 0% but select Pen Pressurefrom the Control option drop-down.

Here is what things look like if we set Size Jitter to 100% but select Off from the Control option drop-down.

The size of the daubs will vary randomly, irrespective of tablet pressure.

Below is if we leave set Size Jitter to 100% and select Pen Pressure from the Control option drop-down.

This thumbnail is representing light pressure on both ends and heavy in the middle, so we can see that we get random variation in daub size, but influenced to some extent by pen pressure.

Roundness Jitter

This specifies what amount of variation is to be introduced into the roundness of the individual daubs that are laid down, and what its source is to be.

Still working with a round hard brush of 30px and spacing of 100%, here is our base brush

Control set to Pen Pressure, 0% Roundness Jitter

Control set to Off, 100% Roundness Jitter

Control set to Pen Pressure, 100% Roundness Jitter

The Scattering Options Sub-panel

This panel controls the number and distribution of the daubs within the stroke.

Scatter value

How far the scattering takes the daubs away from their axis (or both axes) of scatter.

100%

200%

300%

Note that the Both Axes checkbox (which is on by default) causes scattering in both directions (when unchecked, the single-axis scatter employed is the one perpendicular to the stroke path).

100%, Both Axes off

Count

Specifies the number of paint daubs laid down at each spacing interval.

Count 4, Spacing 50%, Scatter 200%

Count 4, Spacing 100%, Scatter 200%

Count 4, Spacing 200%, Scatter 200%

Count 16, Spacing 100%, Scatter 200%

Note that with Scatter set to 0, the Count value basically becomes irrelevant, because whatever number of daubs are specified in the Count value will all be laid on directly top of each other for each interval, looking the same as if there were only 1.

Scatter 0%, Count 2

Scatter 100%, Count 2

Count jitter

Allow us to introduce randomness into the Count value, similar to all the jitter scenarios described above.

Scatter 200%, Count 2, Count Jitter 0%

Scatter 200%, Count 2, Count Jitter 100%

Dual Brush

This is a fascinating (and slightly complicated to grasp) option. In the simplest sense, it takes two brush tips and combines them to make a single brush tip.

More precisely, and assuming you're using the (most common) Multiply mode, the dual brush option takes the brush tip that was set in the original Brush Tip Shape and then uses its outline as a mask in which to paint a second brush tip's line: only those areas where the two brush tips intersect will color the pixels below them.

So, using a Multiply blend mode and our original brush tip shape (still 30px, Spacing 100%), which looks like this:

If we add to this a 25px soft round brush tip as our Dual Brush tip, we can see that the smooth soft line painted by that Dual Brush tip is being masked away in the areas where it paints beyond the (effective) mask of the original parent brush tip.

Mode

The dual brush Mode option is rather perplexing. As Deke McClelland puts it in his PS One-on-One Mastery Course, "All I'm going to tell you about Mode: Mode makes no sense," which is to say that all of the traditional concepts around blend modes are inverted or muddled. For example, Multiply and Darken end up lightening, and what is normally a lightening blend mode, Color Dodge, has a darkening effect. We also have the modes Height and Linear Height, neither of which exist as part of the normal PS blend modes. I've done a lot of research and poking around the web (and experimentation on my own) looking for implementation details of how the two brushes tips interact with the Mode option when using a Dual Brush, but I have yet to work out the details. If you happen to stumble across any leads, please contact me and I'll happily post the details here. In the meantime, it's pretty much a matter of cycling through the options and seeing what looks good with your particular combination of other settings. I'll continue to look for more rigorous details of the underlying engineering and will update this document if I ever encounter any.

At any rate, here are the same settings as above but withHard Mix applied

Obviously, some sort of threhold value is being applied and pretty much all of both brush strokes are coming through, with a bit of rough edges (as we would expect from a Hard Mix under normal blending).

Size, Spacing, Scatter, and Count

Controls how these parameters are set for the Dual Brush tip stroke.

The Transfer Options Sub-panel

These options determine how opacity and flow change (or don't) over the course of a stroke.

Opacity Jitter

Here is at 50%

Or we can set the control to Pen Pressure (et al)

Flow

Same goes for flow.

Here is 100% jitter

And Pen Pressure control

Summary

Those are the important options for us to understand in order to build a set of blotty brushes, so let's get to it.