Tap pressure strength detection using accelerometer

Question

Yesterday, in the presentation about the new Garageband for iPad 2, Apple demoed an interesting feature: The detection of the tap pressure by using the accelerometer. (See the drums section on the Garageband page.)

I'm wondering how that's supposed to work if the iPad lays flat on the table. No movement, no measurable acceleration, no?

Answer 1

Some good answers. Here's some working code. I implemented this as a subclass of UIGestureRecognizer so that you can just drop it in and attach it to a UIView or UIButton. Once triggered, it will have the "pressure" set to a float between 0.0f and 2.0f. You can optionally set the minimum and maximum pressures required to recognize. Enjoy.

//
//  CPBPressureTouchGestureRecognizer.h
//  PressureSensitiveButton
//
//  Created by Anthony Picciano on 3/21/11.
//  Copyright 2011 Anthony Picciano. All rights reserved.
//
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions
//  are met:
//  1. Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//  2. Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in the
//     documentation and/or other materials provided with the distribution.
//  
//  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
//  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
//  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
//  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
//  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
//  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
//  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
//  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
//  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//

#import <UIKit/UIKit.h>

#define CPBPressureNone         0.0f
#define CPBPressureLight        0.1f
#define CPBPressureMedium       0.3f
#define CPBPressureHard         0.8f
#define CPBPressureInfinite     2.0f

@interface CPBPressureTouchGestureRecognizer : UIGestureRecognizer <UIAccelerometerDelegate> {
    @public
    float pressure;
    float minimumPressureRequired;
    float maximumPressureRequired;

    @private
    float pressureValues[30];
    uint currentPressureValueIndex;
    uint setNextPressureValue;
}

@property (readonly, assign) float pressure;
@property (readwrite, assign) float minimumPressureRequired;
@property (readwrite, assign) float maximumPressureRequired;

@end


//
//  CPBPressureTouchGestureRecognizer.h
//  PressureSensitiveButton
//
//  Created by Anthony Picciano on 3/21/11.
//  Copyright 2011 Anthony Picciano. All rights reserved.
//
//  Redistribution and use in source and binary forms, with or without
//  modification, are permitted provided that the following conditions
//  are met:
//  1. Redistributions of source code must retain the above copyright
//     notice, this list of conditions and the following disclaimer.
//  2. Redistributions in binary form must reproduce the above copyright
//     notice, this list of conditions and the following disclaimer in the
//     documentation and/or other materials provided with the distribution.
//  
//  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
//  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
//  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
//  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
//  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
//  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
//  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
//  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
//  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
//  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//

#import <UIKit/UIGestureRecognizerSubclass.h>
#import "CPBPressureTouchGestureRecognizer.h"

#define kUpdateFrequency            60.0f
#define KNumberOfPressureSamples    3

@interface CPBPressureTouchGestureRecognizer (private)
- (void)setup;
@end

@implementation CPBPressureTouchGestureRecognizer
@synthesize pressure, minimumPressureRequired, maximumPressureRequired;

- (id)initWithTarget:(id)target action:(SEL)action {
    self = [super initWithTarget:target action:action];
    if (self != nil) {
       [self setup]; 
    }
    return self;
}

- (id)init {
    self = [super init];
    if (self != nil) {
        [self setup];
    }
    return self;
}

- (void)setup {
    minimumPressureRequired = CPBPressureNone;
    maximumPressureRequired = CPBPressureInfinite;
    pressure = CPBPressureNone;

    [[UIAccelerometer sharedAccelerometer] setUpdateInterval:1.0f / kUpdateFrequency];
    [[UIAccelerometer sharedAccelerometer] setDelegate:self];
}

#pragma -
#pragma UIAccelerometerDelegate methods

-(void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
    int sz = (sizeof pressureValues) / (sizeof pressureValues[0]);

    // set current pressure value
    pressureValues[currentPressureValueIndex%sz] = acceleration.z;

    if (setNextPressureValue > 0) {

        // calculate average pressure
        float total = 0.0f;
        for (int loop=0; loop<sz; loop++) total += pressureValues[loop]; 
        float average = total / sz;

        // start with most recent past pressure sample
        if (setNextPressureValue == KNumberOfPressureSamples) {
            float mostRecent = pressureValues[(currentPressureValueIndex-1)%sz];
            pressure = fabsf(average - mostRecent);
        }

        // caluculate pressure as difference between average and current acceleration
        float diff = fabsf(average - acceleration.z);
        if (pressure < diff) pressure = diff;
        setNextPressureValue--;

        if (setNextPressureValue == 0) {
            if (pressure >= minimumPressureRequired && pressure <= maximumPressureRequired)
                self.state = UIGestureRecognizerStateRecognized;
            else
                self.state = UIGestureRecognizerStateFailed;
        }
    }

    currentPressureValueIndex++;
}

#pragma -
#pragma UIGestureRecognizer subclass methods

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    setNextPressureValue = KNumberOfPressureSamples;
    self.state = UIGestureRecognizerStatePossible;
}

- (void)touchesCancelled:(NSSet *)touches withEvent:(UIEvent *)event {
    self.state = UIGestureRecognizerStateFailed;
}

- (void)reset {
    pressure = CPBPressureNone;
    setNextPressureValue = 0;
    currentPressureValueIndex = 0;
}

@end