Update README

Add Spring Solver info and make other small changes.

README.md

@@ -5,7 +5,9 @@ INTUAnimationEngine makes it easy to build advanced custom animations on iOS.
 
 INTUAnimationEngine provides a friendly interface to drive custom animations using a CADisplayLink, inspired by the UIView block-based animation API. It enables interactive animations (normally driven by user input, such as a pan or pinch gesture) to run automatically over a given duration. It can also be used to get a callback every frame of an animation.
 
-INTUAnimationEngine includes an extensive library of easing functions that can be used to customize animation timing. A complete library of interpolation functions is also included to animate any type of value or property, including those that are not animatable by Core Animation.
+INTUAnimationEngine includes an extensive library of easing functions that can be used to customize animation timing, as well as a complete library of interpolation functions to animate any type of value or property including those that are not animatable by Core Animation.
+
+The project also includes a standalone spring physics library to simulate damped harmonic motion. This is used under the hood to power a spring animation API on INTUAnimationEngine that allows full control over the damping, stiffness, and mass parameters. Since the spring solver is a completely independent and generic library implemented in pure C, it can be used on its own for many other applications apart from animation.
 
 ## Installation
 *INTUAnimationEngine requires iOS 5.0 or later.*
@@ -21,25 +23,25 @@ INTUAnimationEngine includes an extensive library of easing functions that can b
 
 **Manually from GitHub**
 
-1.	Download all the files in the [Source directory](Source).
+1.	Download all the files in the [INTUAnimationEngine directory](INTUAnimationEngine).
 2.	Add all the files to your Xcode project (drag and drop is easiest).
 3.	Import the `INTUAnimationEngine.h` header.
 
 ## Usage
 The primary difference between INTUAnimationEngine and the UIView animation methods is how the `animations` block works. With the UIView methods, the `animations` block is only executed once, and the changes made to views within this block represent the new state at the end of the animation.
 
-With INTUAnimationEngine, the `animations` block is called many times during the animation (once per frame), and each time it is called, your code should update the state of views based upon the current value of the `percentage` or `progress` passed into the block. Typically, you'll want to use one of the interpolation functions included in this library to help generate all the intermediate values between the start and end states for a given property.
+With INTUAnimationEngine, the `animations` block is executed many times during the animation (once per frame), and each time it is executed, your code inside the block should update the state of views based upon the current value of the `percentage` or `progress` passed into the block. Typically, you'll want to use one of the interpolation functions included in this library to help generate all the intermediate values between the start and end states for a given property.
 
 ### Animation Engine
-INTUAnimationEngine has a few API methods that can be used to start an animation.
+There are a few different API methods on INTUAnimationEngine that can be used to start an animation.
 
 #### Without Easing (Linear)
 	+ (INTUAnimationID)animateWithDuration:(NSTimeInterval)duration
 	                                 delay:(NSTimeInterval)delay
 	                            animations:(void (^)(CGFloat percentage))animations
 	                            completion:(void (^)(BOOL finished))completion;
 
-This method will start an animation that calls the `animations` block each frame of the animation, passing in a `percentage` value that represents the current percentage complete. The `completion` block will be executed when the animation completes, with the `finished` parameter indicating whether the animation was cancelled.
+This method will start an animation that calls the `animations` block each frame of the animation, passing in a `percentage` value that represents the animation's current percentage complete. The `completion` block will be executed when the animation completes, with the `finished` parameter indicating whether the animation was canceled.
 
 #### With Easing
 	+ (INTUAnimationID)animateWithDuration:(NSTimeInterval)duration
@@ -48,20 +50,36 @@ This method will start an animation that calls the `animations` block each frame
 	                            animations:(void (^)(CGFloat progress))animations
 	                            completion:(void (^)(BOOL finished))completion;
 
-This method will start an animation that calls the `animations` block each frame of the animation, passing in a `progress` value that represents the current progress of the animation (taking into account the easing function). The `easingFunction` can be any of the easing functions in [`INTUEasingFunctions.h`](Source/INTUEasingFunctions.h), or a block that defines a custom easing curve. The `completion` block will be executed when the animation completes, with the `finished` parameter indicating whether the animation was cancelled.
+This method will start an animation that calls the `animations` block each frame of the animation, passing in a `progress` value that represents the current progress of the animation (taking into account the easing function). The `easingFunction` can be any of the easing functions in [`INTUEasingFunctions.h`](INTUAnimationEngine/INTUEasingFunctions.h), or a block that defines a custom easing curve. The `completion` block will be executed when the animation completes, with the `finished` parameter indicating whether the animation was canceled.
 
 There is also another variant of the above method that takes an `options:` parameter, which is a mask of `INTUAnimationOptions`. This can be used to repeat or autoreverse animations.
 
+#### Using a Spring
+	+ (INTUAnimationID)animateWithDamping:(CGFloat)damping
+	                            stiffness:(CGFloat)stiffness
+	                                 mass:(CGFloat)mass
+	                                delay:(NSTimeInterval)delay
+	                           animations:(void (^)(CGFloat progress))animations
+	                           completion:(void (^)(BOOL finished))completion;
+
+This method will start a spring animation that calls the `animations` block each frame of the animation, passing in a `progress` value that represents the current progress of the animation. The animation will simulate the physics of a spring-mass system with the specified properties:
+
+* `damping` – The amount of friction. Must be greater than or equal to zero. If exactly zero, the harmonic motion will continue indefinitely. Typical range: `1.0` to `30.0`
+* `stiffness` – The stiffness of the spring. Must be greater than zero. Typical range: `1.0` to `500.0`
+* `mass` – The amount of mass being moved by the spring. Must be greater than zero. Typical range: `0.1` to `10.0`
+
+Note that the total duration of the animation is determined by simulating a spring-mass system with the above parameters until it reaches a resting state. The `completion` block will be executed when the animation completes, with the `finished` parameter indicating whether the animation was canceled.
+
 #### Canceling Animations
 	+ (void)cancelAnimationWithID:(INTUAnimationID)animationID;
 
-When starting an animation, you can store the returned animation ID, and pass it to the above method to cancel the animation before it completes. If the animation is cancelled, the completion block will execute with `finished` parameter equal to NO.
+When starting an animation, you can store the returned animation ID, and pass it to the above method to cancel the animation before it completes. If the animation is canceled, the completion block will execute with `finished` parameter equal to NO.
 
 ### Easing Functions
-[`INTUEasingFunctions.h`](Source/INTUEasingFunctions.h) is a library of standard easing functions. Here's a [handy cheat sheet](http://easings.net) that includes visualizations and animation demos for these functions.
+[`INTUEasingFunctions.h`](INTUAnimationEngine/INTUEasingFunctions.h) is a library of standard easing functions. Here's a [handy cheat sheet](http://easings.net) that includes visualizations and animation demos for these functions.
 
 ### Interpolation Functions
-[`INTUInterpolationFunctions.h`](Source/INTUInterpolationFunctions.h) is a library of interpolation functions.
+[`INTUInterpolationFunctions.h`](INTUAnimationEngine/INTUInterpolationFunctions.h) is a library of interpolation functions.
 
 #### Proximal Interpolation
 For discrete values (where linear interpolation does not make sense), there are two proxmial interpolation functions. For example:
@@ -104,10 +122,13 @@ You can concatenate transforms to combine them:
 ##### UIColor / CGColor
 When interpolating between two colors, both colors must be in the same color space (grayscale, RGB, or HSB). Interpolating between colors in the HSB color space will generally yield better visual results than the RGB color space.
 
-	[UIColor colorWithWhite:1.0 alpha:1.0] // grayscale color space; white
-	[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:1.0] // RGB color space, white
+	[UIColor colorWithWhite:1.0 alpha:1.0] // Grayscale color space; white
+	[UIColor colorWithRed:1.0 green:1.0 blue:1.0 alpha:1.0] // RGB color space; white
 	[UIColor colorWithHue:0.0 saturation:0.0 brightness:1.0 alpha:1.0] // HSB color space; white
 
+### Spring Solver
+The [SpringSolver directory](INTUAnimationEngine/SpringSolver) in the project contains a spring physics library to simulate damped harmonic motion, based on the spring solver that powers Facebook's [Pop](https://github.com/facebook/pop). The INTUAnimationEngine spring solver has been extensively refactored for simplicity and performance, and as a fully independent pure C library is highly portable to any platform and can be leveraged for other use cases beyond animation.
+
 ## Example Project
 An [example project](AnimationEngineExample) is provided. It requires Xcode 6 and iOS 6.0 or later.
 
@@ -117,7 +138,7 @@ Please [open an issue here on GitHub](https://github.com/intuit/AnimationEngine/
 Pull requests are welcome and encouraged! There are no official guidelines, but please try to be consistent with the existing code style.
 
 ## License
-INTUAnimationEngine is provided under the MIT license.
+INTUAnimationEngine is provided under the [MIT license](LICENSE). The spring solver library within the project is provided under a [BSD license](LICENSE-SpringSolver).
 
 # INTU on GitHub
 Check out more [iOS and OS X open source projects from Intuit](https://github.com/search?utf8=✓&q=user%3Aintuit+language%3Aobjective-c&type=Repositories&ref=searchresults)!