Android Kotlin: How to erase drawing from a Canvas

This code demonstrates how to create a specific image with an erased circular area on an Android app using Kotlin. It achieves this by creating a bitmap, drawing a rectangle on it, and then erasing a circular region in the center. Finally, the resulting image is displayed on an ImageView.

Code Breakdown:

  1. MainActivity.kt:

    • This file defines the main Activity class responsible for the app's functionality.
    • In the onCreate method, it retrieves the reference to the ImageView from the activity layout using its ID (imageView).
    • It then calls the eraseDrawing function to generate the image with the erased area and sets it on the ImageView using setImageBitmap.

  2. eraseDrawing() function:

    • This function creates a new bitmap with a specific size (1500x850 pixels) and desired configuration (ARGB_8888).
    • A canvas is created from the bitmap. The canvas is then filled with a light blue background color.
    • A rectangle is drawn on the canvas with a dark blue color, most likely representing the base drawing.
    • A Paint object is created specifically for erasing. It enables anti-aliasing for smoother edges and sets the xfermode property to PorterDuff.Mode.CLEAR. This transparency mode ensures pixels drawn with this paint are cleared to fully transparent (0 alpha).
    • Finally, a circle is drawn in the center of the canvas using the erasing paint. This erases the previously drawn rectangle within the circle's area.
    • The function returns the resulting bitmap with the erased circle.

  3. activity_main.xml:

    • This file defines the layout of the main activity screen.
    • It uses a ConstraintLayout as the root element and includes an ImageView with the ID imageView. The ImageView will display the image generated by the eraseDrawing function.

Summary

This code snippet showcases a basic approach to erasing a specific area on a canvas in an Android application using Kotlin. The eraseDrawing function demonstrates how to create a bitmap, draw on it, and then selectively erase a portion using a PorterDuffXfermode with the CLEAR mode. This technique offers a way to create custom images with specific transparency effects.


MainActivity.kt 

package com.cfsuman.kotlintutorials

import android.app.Activity
import android.graphics.*
import android.os.Bundle
import android.widget.ImageView
import kotlin.math.min


class MainActivity : Activity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        // get the widgets reference from XML layout
        val imageView = findViewById<ImageView>(R.id.imageView)


        // show drawing on image view
        imageView.setImageBitmap(eraseDrawing())
    }
}



// function to erase / clear drawing from canvas
fun eraseDrawing():Bitmap?{
    val bitmap = Bitmap.createBitmap(
        1500,
        850,
        Bitmap.Config.ARGB_8888
    )


    // canvas for drawing
    val canvas = Canvas(bitmap).apply {
        drawColor(Color.parseColor("#A2A2D0"))
    }


    // draw a rectangle on canvas
    canvas.drawRect(
        100F,
        100F,
        canvas.width - 100F,
        canvas.height - 100F,
        Paint().apply { color = Color.parseColor("#333399") }
    )


    // paint to erase / clear drawing from canvas
    val paint = Paint().apply {
        isAntiAlias = true

        // destination pixels covered by
        // the source are cleared to 0
        xfermode = PorterDuffXfermode(PorterDuff.Mode.CLEAR)
    }


    // draw circle to erase / clear circular
    // area drawing from canvas
    canvas.drawCircle(
        canvas.width / 2F,
        canvas.height / 2F,
        min(canvas.width,canvas.height) / 2F - 25F,
        paint
    )

    return bitmap
}
activity_main.xml 

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/rootLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#DCDCDC"
    android:padding="24dp">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>
More android kotlin tutorials