Android Kotlin: How to change alpha of a Bitmap

This code demonstrates how to adjust the transparency (alpha) of a Bitmap in an Android application written in Kotlin. It achieves this by creating a new Bitmap with a specific level of transparency and then displaying both the original and transparent versions in separate ImageViews.

The code includes three parts:

MainActivity.kt: This is the main activity class that handles the UI and logic.
assetsToBitmap: This is an extension function that retrieves a Bitmap from the app's assets folder.
setAlpha: This is another extension function that creates a new Bitmap with a specific alpha value (transparency level).

Breakdown of the Code

MainActivity.kt

The code first retrieves references to two ImageViews and two TextViews from the activity's layout using findViewById.
It then loads a bitmap from the assets folder using the assetsToBitmap extension function.
The bitmap?.apply block checks if the bitmap is loaded successfully and then performs the following actions:
- Sets the original bitmap to the first ImageView and updates the corresponding TextView with a label.
- Calls the setAlpha function on the original bitmap with an alpha value of 75 (semi-transparent).
- Sets the resulting transparent bitmap to the second ImageView and updates the corresponding TextView with a label and explanation of the alpha range (0-255).

assetsToBitmap

This function takes a filename as input and attempts to open the corresponding file from the assets folder.
If successful, it decodes the file into a Bitmap object and returns it.
In case of any errors during the process, it prints the error message and returns null.

setAlpha

This function takes a Bitmap object and an alpha value (between 0 and 255) as input.
It creates a new Bitmap with the same dimensions and configuration (ARGB_8888) as the original one.
It then creates a Paint object and sets its alpha value to the provided parameter.
Finally, it draws the original bitmap onto the new one using a Canvas object with the modified paint, effectively creating a copy with adjusted transparency.

Summary

This code snippet provides a clear and concise approach to adjusting the alpha of a Bitmap in Kotlin. It utilizes extension functions for cleaner code and offers informative descriptions within the TextView elements to enhance user understanding.

MainActivity.kt


package com.cfsuman.kotlintutorials

import android.app.Activity
import android.content.Context
import android.graphics.*
import android.os.Bundle
import android.widget.*
import java.io.IOException


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)
        val imageView2 = findViewById<ImageView>(R.id.imageView2)
        val textView = findViewById<TextView>(R.id.textView)
        val textView2 = findViewById<TextView>(R.id.textView2)


        // get the bitmap from assets folder
        val bitmap = assetsToBitmap("flower103.jpg")


        bitmap?.apply {
            // show original bitmap in first image view
            imageView.setImageBitmap(this)
            textView.text = "Original Bitmap"

            // show the transparent bitmap in image view
            // int: set the alpha component [0..255]
            // of the paint's color.
            // 0 is full transparent
            // 255 is full opaque
            imageView2.setImageBitmap(setAlpha(75))
            textView2.text = "Bitmap Alpha (75)"
            textView2.append("\n\nRange 0..255")
            textView2.append("\n0 is full transparent")
            textView2.append("\n255 is full opaque")
        }
    }
}



// extension function to get bitmap from assets
fun Context.assetsToBitmap(fileName:String):Bitmap?{
    return try {
        val stream = assets.open(fileName)
        BitmapFactory.decodeStream(stream)
    } catch (e: IOException) {
        e.printStackTrace()
        null
    }
}



// extension function to make transparent bitmap
fun Bitmap.setAlpha(alpha:Int):Bitmap?{
    val bitmap = Bitmap.createBitmap(
        width,height, Bitmap.Config.ARGB_8888
    )

    val paint = Paint()
    // int: set the alpha component [0..255]
    // of the paint's color.
    // 0 is full transparent
    // 255 is full opaque
    paint.alpha = alpha

    Canvas(bitmap).drawBitmap(
        this,0f,0f,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:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/rootLayout"
    android:background="#DCDCDC"
    android:padding="24dp">

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="0dp"
        android:layout_height="250dp"
        android:layout_marginTop="8dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="Original Bitmap"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="@+id/imageView"
        app:layout_constraintTop_toBottomOf="@+id/imageView" />

    <ImageView
        android:id="@+id/imageView2"
        android:layout_width="0dp"
        android:layout_height="250dp"
        android:layout_marginTop="32dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView" />

    <TextView
        android:id="@+id/textView2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:text="Transparent Bitmap"
        app:layout_constraintEnd_toEndOf="@+id/imageView2"
        app:layout_constraintStart_toStartOf="@+id/imageView2"
        app:layout_constraintTop_toBottomOf="@+id/imageView2" />

</androidx.constraintlayout.widget.ConstraintLayout>