Understanding CGAffineTransform.identity in Swift 2.3

Introduction to Core Graphics and CGAffineTransform

Core Graphics is a graphics library used for creating 2D graphics on iOS, macOS, watchOS, and tvOS platforms. It provides a wide range of functionality for tasks such as drawing shapes, text, and images, as well as transforming graphics.

At the heart of Core Graphics lies the CGAffineTransform struct, which represents a 2x2 transformation matrix. This matrix can be used to scale, rotate, translate, or combine multiple transformations with each other.

CGAffineTransform.identity

One specific value in the CGAffineTransform enum is identity. The purpose of this constant is to represent a transformation that leaves the graphics unchanged.

When you use transform = CGAffineTransform.identity, you’re creating a new CGAffineTransform object with all its elements set to zero, which effectively doesn’t change anything. This is useful when you want to start with a blank slate or return to an original state without any transformations applied.

Understanding Scaling Transformations

Scaling transformations are used to increase or decrease the size of graphics. The general formula for scaling is:

x’ = x * scale y’ = y * scale

where (x, y) represents the original coordinates and (x’, y’) represent the new scaled coordinates after applying the transformation.

In Core Graphics, you can use CGAffineTransformScale to apply a scaling transformation. The method takes two arguments: the original matrix and the scale factor.

Scaling Using CGAffineTransform.identity

Now that we understand scaling transformations, let’s revisit the code snippet provided in the question:

var transform = CGAffineTransformIdentity
transform = transform.scaledBy(x: scale, y: scale)

Here’s what happens when you apply a scaling transformation to an identity matrix:

scaledBy(x:y:) multiplies the diagonal elements of the identity matrix by the provided scale factors.
The resulting matrix is then assigned back to transform.

This effectively scales the graphics by the specified factor, but with one important caveat: since we’re starting from an identity matrix, which already has a scaling factor of 1, this transformation will leave the graphics unchanged.

However, if you’re expecting to scale your graphics, you’ll want to use CGAffineTransformScale instead:

var transform = CGAffineTransformIdentity
transform = CGAffineTransformScale(transform, scale, scale)

Translation Transformations

Translation transformations are used to move graphics from one point in space to another. The general formula for translation is:

x’ = x + dx y’ = y + dy

where (x, y) represents the original coordinates and (x’, y’) represent the new translated coordinates after applying the transformation.

In Core Graphics, you can use CGAffineTransformTranslate to apply a translation transformation. The method takes two arguments: the original matrix and the translation vector.

Combining Scaling and Translation Transformations

When you want to scale your graphics and then move them around in space, you’ll need to combine these transformations. Here’s an example:

var transform = CGAffineTransformIdentity
transform = transform.scaledBy(x: scale, y: scale)
transform = transform.translatedBy(x: translation.x/scale, y: translation.y/scale)

However, as we discussed earlier, CGAffineTransform.identity doesn’t actually apply any scaling transformations. Instead, you should use CGAffineTransformScale to achieve this:

var transform = CGAffineTransformIdentity
transform = CGAffineTransformScale(transform, scale, scale)
transform = transform.translatedBy(x: translation.x/scale, y: translation.y/scale)

Conclusion

In conclusion, understanding the basics of CGAffineTransform.identity in Swift 2.3 is crucial for working with Core Graphics and performing transformations on graphics. By recognizing how scaling and translation work together, you can create powerful transformations that will enhance your graphics applications.

Example Use Cases

Creating a new UI component with a specific scale or rotation
Transforming images to fit within the screen boundaries
Moving graphics around the screen without changing their size

Last modified on 2023-05-29