Kotlin Thursdays: Image Processing in Kotlin Part 3

Part 1: https://codetown.com/group/kotlin/forum/topics/kotlin-thursdays-ima...

Part 2: https://codetown.com/group/kotlin/forum/topics/kotlin-thursdays-ima...

Welcome to Kotlin Thursdays!  Last week, we figured out how to write primitive filters and apply them to our images with the or function. This week, we look at refactoring with higher-order functions.

Resources

Think of these resources as supplemental if you happen to be more curious.  We always encourage looking into documentation for things you use!

• Project Github Repository: https://github.com/Kotlin-Thursdays/Image-Processing-in-Kotlin
• Installing Java with Oracle:  https://www.oracle.com/technetwork/java/javase/downloads/jdk8-downl...
• Kotlin: https://kotlinlang.org/docs/reference/
• TornadoFX: https://edvin.gitbooks.io/tornadofx-guide/content/

Higher Order Functions

We could continue to write individual functions that feeds 2 images and a particular function, but in Kotlin, we have the ability to use a single function that accepts 2 images and a function with the help of higher order functions.  Below, you can see how similar our orFilter function and makeDuller function is.

In programming, programs may take data as parameters and pass those parameters into the function to return a different output or alter the behavior of a program.  Kotlin is a functional language, and one characteristic of a functional language is that functions are also able to treat functions as data. You can pass a function as a parameter, which is really powerful!

A higher-order function is a function that may take functions as parameters. You can pass a function with double-colon (::).  Double-colon (::) is used to get function (callable) reference in Kotlin.

Ruby facilitates higher order functions with yield, which involves passing a block to a method.

Like Ruby, Kotlin treats functions as first-class citizens, which is a pillar of functional programming. In Kotlin, the equivalent of block code is known as lambda functions, indicated by the pattern:

Instead of passing the object as an argument, you invoke the lambda variable as it if were an extension function.  Haskell also has higher order functions which can designate the kinds of parameters a function may take within a function.

Implementing applyFilter

In this case, we are going to work with a general function, as opposed to an extension function that is invoked with a qualifer.

The function we write will take a filter function and 2 pixelReaders.  Our function parameter, in particular, will only accept functions that take 2 Color parameters and returns a Color.  

So here, the input function that accepts the 2 parameters is the receiver type, the output Color receiver object.

Conclusion

I hope you all had fun learning a little bit about image processing! Keep exploring and creating new image filters and maybe even as a challenge, think about how you might implement an RGB system to create image filters for colors.  Until next time :)