Brush Settings Panel

There are many, many brush settings in PD Pro, and with the ability to create your own, it will be helpful to have a good understanding of the brush settings panel.
Across the top are the three brush types, the AA pen, Internal brushes, and Custom brushes, covered in the section on brush types.  You can switch between brush types freely, and most features are supported for each type.

There are three tabs on the panel that encapsulate various categories of functions.

The main tab contains settings that directly effect the rendering of all types of brushes.

The Custom tab offers tools that are useful for working with custom brushes.

The PostFX tab contains a set of post process effects that can be used with custom brushes

 

The Main Tab

Anti-aliasing

  Anti-aliasing is a setting that applies smoothing to images when they are scaled, rotated, or transformed in some way.  The effect is to reduce 'jaggies' or stair steps in the image.

There are two levels of Anti-aliasing, however Low will often be sufficient.  In most cases, anti-aliasing is accomplished with bi-linear filtering, a technique the samples values anywhere in between pixels.

The downside of bi-linear filtering, is that it only uses one sample for every pixel, and in the case of downsizing an image, this can still lead to aliasing artifacts.

The High settings will apply multi-sampling, if the tool supports it.  Currently only internal brushes support the High setting.

 A brush is seen here first without, then with anti-aliasing as it is scaled to a smaller size.
Note that while the scaling of internal brushes can be anti-aliased, the placement of internal brushes is not anti-aliased.  The placement will always fall on pixel coordinates.  This might  make the internal brush look rough, or pixilated compared to the AA pen.  Again, sometimes this is desirable, and sometimes it isn't.  That's why you have a choice of different brush types.

The AA pen will give you smooth lines.
Tablet Support

If you have a tablet that supports pressure, you can take advantage of the effect by checking the pressure size button.  It is not on by default.  This setting is not a tool specific setting, so you can manage it independently as you switch between tools.

Pressure Opacity will use the pressure from the tablet to alter the opacity of the brush.

 Pressure will have slightly different effects on each brush type.  In the case of the AA Pen, the size of the brush is smoothly interpolated between samples, making for a vary smooth effect.

Internal brushes also use the pressure effect, however interpolation is not applied.  This can cause a slight 'pop' in the scale of the brush if the pressure changes drastically between samples.

By default, custom brushes are not allowed to change size or shape based on any of the brush dynamics on this tab.  This is because custom brushes are often used for moving images around like a stamp.  However, you can enable transformations with the 'Allow custom brush transforms' checkbox on the Custom tab.  Then custom brushes will behave just like internal brushes.
Brush Transformations

Scale by speed alters the size of the brush based on the speed that you paint.

Random position randomizes the placement of your brush.

Random size randomize the size of your brush

Random Angle randomizes the angle of your brush

Pen angle causes your brush to point in the direction that

If you do not have a tablet, scaling the brush based on speed will let you achieve some similar effects based on the speed that you move your mouse.

 

Examples of randomizing the placement of a brush.  You can use this to create a spatter effect, or to make the patterns created by your brush less predictable.
A brush with Random size applied.

A brush with Random angle applied.

 The Pen angle setting can be very useful for creating brush strokes that look like they flow along a path.
Color Transforms

Random hue randomizes the hue of the brush.

Random Sat randomizes the saturation of the brush

Random value randomizes the value of the brush

Bleed causes your paint color to blend with the colors on screen.

Dryout causes your paint to fade out, like you are running out of pigment.

 Random hue alters the hue component of your brush's color to achieve a variety of new colors.  Colors that are close in hue a called analogous colors, and they can produce very lovely images.  Lower values are preferred, unless you are going to a phycodelic effect.

Random saturation alters the saturation component of your brush's color, causing it to look more gray in places.

 

Random value alters the value component of your brush's color, with the effect of lightening or darkening it.

 

Note that these effects will work with every brush type, including custom brushes in either color or matte mode.  Use of each of these effects in combination can produce wonderful varieties of colors in your brushes.
Bleed is one of the most painterly effects on this tab.  It can achieve very realistic paint like results.  A high value is often required to get the best effect.  It should be combined with lower levels of opacity.

Dryout is another realistic effect that will make you feel like your working with actual paint.  It can be quite pleasing when used in combination with bleed.

The Custom tab

The Custom brush tab contains a number of tools that are useful for working with your custom brush.

This control lets you switch your custom brush between Color and Matte mode.  See the section on brush types for more information. Custom brushes can be either full color, or work like a regular single color brush. The modes are called Color (F1) and Matte(F2).

Clicking the 'allow custom brush transforms' lets you apply dynamic effects to a brush like random scale and random rotation from the main tab.

Now you can transform your custom brushes just like regular brushes.  Even full color brushes can have randomized hue, saturation, and value.

You may not always want to do this.  Sometimes you just want to copy an image from one place to another, so you can turn this feature of and off as you need it.

 Also see the section on brush types. 
Along the left edge of the panel are 4 tools that apply to custom brushes.  They are BrushFX , Patterns , the Brush Keyer , and the Brush Manager .
This controls will let you build a new airbrush.  The shape is created as a new custom brush.  The new brush will contain no imagery, just a transparency channel (key).  The brush is automatically switched to matte mode.  If you happen to switch to color mode, the brush will appear to be black because there is no imagery.

This control is useful for building airbrush's of custom sizes.  you can make one as large as you like, so you are not limited to the presets.  To use it, just click and drag in the control to set the new size.
The brush radius will be set for you with the above control, but if you want to, you can set it with the slider.

The bias control changes the shape of the airbrush.

Cell size and density control a cellular effect, which can be useful for simulating bristles in a brush, or for creating unique textures.

The invert checkbox reverses the effect of the cells.

 Bias changes the falloff from the center.  You can get a very hot center that falls off rapidly, then tapers off, or you can get a very bulbous falloff.

Cell size controls the size of cells used to create texture effects.  Small cells can be used for bristled, while larger cells start to resemble the cells in a leaf.

Cell density controls how many cells appear in the brush.  When there are a lot of cells, they begin to overlap and crowd each other.

The PostFX tab

The PostFX tab lets you apply post process effects to your brush strokes.

You brush can take on a whole new look with the post process filters.  These filters are applied after you have finished your brush stroke and let up on the mouse button or tablet.
The emboss effect give a 3d appearance to a brush. The 'Thick oils' preset is a good example of this effect.
The watercolor effects change the opacity of the brush, while giving a dark outline around their edges.  This is similar to the way surface tension causes paint to build up around the edges of watercolor paint, and dry more darkly. Examples of opaque and transparent watercolor effects apply to brush strokes.

If you do not get dark or bold enough the first time, press Shift-a to repeat the brush stroke.
The shadow effect drops a shadow underneath a custom brush.

Offset controls how far the shadow is cast.

Blur effects how soft and spread out the shadow is.

Opacity controls how dark the shadow is.

Examples of a brush dropping a shadow.
The model post effect uses the shape defined by the brush stroke to apply a single color (the primary or secondary color depending on which button you use) based on a mathematical apply mode.

 The modes here are the same used by the drawing modes.  This can be useful for applying colors based on those modes, but without the side effect of the color 'building up' like they do with the regular drawing tools.  They will be applied like they were a single color on a layer.

The one unique mode here, is the 'Paint on alpha' mode.  This tool will let you paint right into the alpha channel.  It is used be several presets, like 'blobby' under the paint on alpha menu.