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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.
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.
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
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.
Tablet drivers offer a UX to see and modify pen pressure curves. Below you'll see how to access this UX in various tablet drivers.
Open the HuionTablet app
Click on Digital Pen
Click on Pressure Sensitivity Adjustment
Launch Wacom Tablet Properties
Under Tool, select a Pen
Select Mapping
The Tip Feel slider setting controls the pressure curve. So you can change the pressure curve, with this slider though you cannot see it.
SOME Wacom tablets offer a Customize button under the Tip Feel setting
If you click on Customize, then the Pen Feel Details dialog will launch
In the Pen Feels Details, you can see and modify the pressure curve
Only some Wacom tablets have a pressure curve in Wacom Center.
For older tablets, the Wacom Center will send you back to the Wacom Tablet Properties app
For newer tablets such as the Intuos Pro 2025 series, the Wacom Center has built in UX for pressure curves
If you are using one of these newer tablets ...
Open Wacom Center
Select the Tablet and the Pen
The Tip Sensitivity slider setting controls the pressure curve. So you can change the pressure curve, with this slider though you cannot see it.
Click on Advanced and you can see a visualization of the Tip Sensitivity and edit the pressure curve.
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.
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 labelled as "Output" is the output logical pressure
This particular curve bends down a little. But many other shapes are possible. Each shape has their uses.
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.
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.
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
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.
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]
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.
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.)
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)
Resources
Devid Revoy - Calibrating stylus pressure (for Krita) https://www.davidrevoy.com/article182/calibrating-wacom-stylus-pressure-on-krita
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.
