Introduction

The null pressure curve is the most important pressure curve - exactly because it does nothing. We can apply the null pressure curve to any pressure profile and it will not shift the profile in any direction.

Definition

The definition of a null profile is a pressure curve function that takes the input logical pressure and returns that very same value as the output logical pressure. In order words f(p) = p.

If you take the range of input logical pressures which range from zero to one and plot them on the X axis of a chart. And then you apply the null pressure curve to those values and plot the result of the function on the Y axis - then you get a straight line at 45° that goes from the lower left corner to the upper right corner of the chart.

Anytime you see a chart like this it clearly identifies that null pressure curve. And you can be sure that it does not do anything to a pressure response.

And the chart above you can see there's been no change because the orange line which represents the pressure response after applying the curve has exactly the same shape as the original pressure response.

null pressure curves are extremely useful.

We encounter pressure curves in tablet drivers and in creative applications. These kinds of applications often use the null pressure curve as the default value for any pressure curves they have. This isn't always true but it is a very common thing to see.

If you're ever trying to solve some problem with the pressure of your pen the knowing about the null pressure curve is useful. Because it might be that somehow your pressure curve was modified and is affecting your pen. So a very common troubleshooting tip is to make sure that your pressure curve has been reset back to the null pressure curve. This way you can be sure it is not affecting what is going on

Not all pressure curve shapes can are possible in all cases.

• Drivers usually offer only simple pressure curves

• Applications are generall very flexible

Overview

Typically, a pressure curve function takes input logical pressure values in the range of [0,1] and maps it to values in an output range of [0,1]. This means that the full output range is used.

Some curves can limit their output range to achieve better control over brush strokes.

Visual interpretation

The easy way to tell that a pressure cuve constrains the output range is to notice that the curve shape does not reach to the bottom or to the top of the pressure curve graph. Two examples are below.

Even though the shapes look a little diffrent, they both effectively take the input logical pressure values between [0,1] and map that to an output region close to [0.3, 0.6]

Impact on brush strokes

Imagine the user's brush setting is 100px and the brush is set to change its size in response to pressure. Then suppose the user draws a stroke that goes from the IAF value to the MAX physical pressure.

The stroke size is computer like this:

pressure = apply_curve( pressure )
brush_size = max( 1, 100 * pressure )

• With a null pressure curve - The stroke width will go from a size of 1px to 100px.

• With a the curves shown above - the stroke width will go from 30px to 60 pz. So the widht of the stroke does not vary as much.

Use cases

• Helps give you more consistent brush strokes while still allowing some variability

• By avoiding the lower end of output logical prerssure, you can have your strokes start off a little thicker than normal. (Though there are other ways some apps have of accomplishing this goal.)

Here are two examples of curves that ignore input.

IGNORE BOTTOM - ignores the lower end of logical pressure

IGNORE TOP - ignores the upper end of logical pressure

IGNORE BOTTOM

Effectively this increases the perceived IAF of the pen.

This is useful in several cases:

• Your pen might be drawing while hovering and this is one way that might help that problem

• Some people don't want the pen to draw when in contact with the tablet, the only want to use it to point. They can ignore pressure to help make this happen.

IGNORE TOP

Some people have issues with their hands and with pens that have a very high pressure range they can never get to the full maximum pressure. They can use IGNORE TOP to effectively lower the maximum physical pressure.

Ultimately a pressure curve is a mathematical function that takes input logical pressure (p) and returns an output logical pressure (p’)

that is it maps logical pressure to logical pressure. The logical pressure comes in as a value between zero and one, and the output is a logical pressure between zero and one. The specifics of the mapping of the input to the output are completely arbitrary and we can make pressure curves do whatever we want.

In reality we wouldn't just have one pressure curve function with a single input logical pressure parameter like that. More typically we'd have a pressure curve function that accepts multiple parameters. By tweaking these additional parameters we can control what the pressure curve is actually doing in a dynamic way.

Before you begin

Before you read this section it would be good for you to read: pen pressure response. This document won't make a lot of sense without you understanding clearly what a pen pressure response is.

Overview

A pressure curve is essentially a little bit of math that transforms a pressure response to another pressure response. In more everyday terms a pressure curve creates a new pressure behavior for the pen.

Most often, we see the results of this transformation shown to us visually in driver or application UI.

For example in the Wacom Tablet Properties app it looks like this:

The X axis labelled as "Pen pressure" is the logical input pressure

The Y axis lavelled as "Output" is hte output logical pressure

This particular curve bends down a little. But many other shapes are possible. Each shape has their uses.

Pressure curve vs pressure response

It is important to distinguish the pressure curve from the pressure response because they describe different things completely.

A pressure response describes the pressure behavior of a pen - and this is a description of physically happens in the world. It relates physical pen pressure to logical pen pressure.

A pressure curve is an abstract mathematical entity that transforms logical pressure values. A pressure curve is completely arbitrary and does not describe a relationship in the physical world. It is a purely logical construct.

Popular coverage of pressure curves is highly misleading

So often in documents and YouTube videos you might encounter people describe the pressure curve as the pressure behavior of the pen. This is completely inaccurate. The pressure curve describes how the pressure behavior (the pressure response) is being modified. You cannot look at a pressure curve and understand the pressure behavior of your pen. The only way for you to understand the pressure behavior of pen is to physically measure it with the scale and start mapping physical pressure values to logical pressure values.

Pressure curve shapes

There are a variety of pressure curve shapes - each of which can solve some problem or achieve some visual effect.

To see which drivers and apps support witch shapes see this: Curve support in applications

Details on specific shapes

• Null pressure curve

• Pressure curves that constrain the output logical pressure range

• Pressure curves that ignore input

Misc notes: Wacom pressure deadzone

Some of Wacom's Pro Pens are so sensitive, Wacom by default configures pen pressure curve in the driver to have a small "deadzone" at the lowe end of pressure. This means the very lowest pressure readings are ignored.

Here is a screenshot showing that pressure deadzone in the default pressure curve the Wacom driver uses for the Wacom Pro Pen 2 (KP-504E)

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