The following code prints HTML with responsive images support. It's ready to use and defines three levels of resolution: 400px, 800px and original image.

Adapt and start using the following code in Kotlin (we use Picturefill as polyfill):

object KotlinRethumbResponsiveExample {

    @Throws(Exception::class)
    @JvmStatic fun main(args: Array<String>) {
        val imageUrl = "http://images.rethumb.com/image_coimbra_999x999.jpg"
        val smallWidth = 400 // Image width for small resolutions (less than 400px).
        val largeWidth = 800 // Image width for medium resolutions (less than 800px).
        val imageWidth = 999 // Image original width (used for resolutions larger than 800px).

        println("<script src=\"https://cdnjs.cloudflare.com/ajax/libs/picturefill/2.3.1/picturefill.min.js\"></script>\n" +
                "\n" +
                "<img src=\"http://api.rethumb.com/v1/width/" + smallWidth + "/" + imageUrl + "\"\n" +
                "\n" +
                " srcset=\"http://api.rethumb.com/v1/width/" + smallWidth + "/" + imageUrl + " " + smallWidth + "w,\n" +
                " http://api.rethumb.com/v1/width/" + largeWidth + "/" + imageUrl + " " + largeWidth + "w,\n" +
                " " + imageUrl + " " + imageWidth + "w\"\n" +
                "\n" +
                " sizes=\"100vw\" />")
    }
}

Use the following commands to get started:

$ git clone https://github.com/rethumb/rethumb-kotlin-examples.git
$ cd rethumb-kotlin-examples
$ kotlinc responsive-images.kt -include-runtime -d responsive-images.jar
$ java -jar responsive-images.jar