diff --git a/apng_library/src/androidTest/java/oupson/apng/ApngEncoderInstrumentedTest.kt b/apng_library/src/androidTest/java/oupson/apng/ApngEncoderInstrumentedTest.kt
index 81b88b2..f7a4fcc 100644
--- a/apng_library/src/androidTest/java/oupson/apng/ApngEncoderInstrumentedTest.kt
+++ b/apng_library/src/androidTest/java/oupson/apng/ApngEncoderInstrumentedTest.kt
@@ -4,11 +4,17 @@ import android.content.Context
 import android.graphics.Bitmap
 import android.graphics.BitmapFactory
 import android.graphics.Color
+import android.graphics.drawable.AnimationDrawable
+import android.graphics.drawable.BitmapDrawable
 import androidx.test.platform.app.InstrumentationRegistry
 import junit.framework.TestCase.assertFalse
 import junit.framework.TestCase.assertTrue
 import org.junit.Test
+import oupson.apng.decoder.ApngDecoder
+import oupson.apng.encoder.ApngEncoder
 import oupson.apng.utils.Utils
+import java.io.ByteArrayInputStream
+import java.io.ByteArrayOutputStream
 
 
 class ApngEncoderInstrumentedTest {
@@ -38,7 +44,7 @@ class ApngEncoderInstrumentedTest {
     }
 
     @Test
-    fun containTransparency() {
+    fun testContainTransparency() {
         val context = InstrumentationRegistry.getInstrumentation().context
 
         val bunnyFrame1 = getFrame(context, "bunny/frame_apngframe01.png")
@@ -48,6 +54,118 @@ class ApngEncoderInstrumentedTest {
         assertTrue(Utils.containTransparency(ballFrame1))
     }
 
+    @Test
+    fun testOptimiseBall() {
+        val context = InstrumentationRegistry.getInstrumentation().context
+        val list = context.assets.list("ball")?.map { getFrame(context, "ball/$it") }!!
+
+        val optimisedOutputStream = ByteArrayOutputStream()
+        val optimisedEncoder = ApngEncoder(optimisedOutputStream, list[0].width, list[0].height, list.size)
+            .setOptimiseApng(true)
+
+        list.forEach {
+            optimisedEncoder.writeFrame(it)
+        }
+
+        optimisedEncoder.writeEnd()
+        optimisedOutputStream.close()
+
+        val bytes = optimisedOutputStream.toByteArray()
+        val optimisedInputStream = ByteArrayInputStream(bytes)
+
+        val optimisedApng =
+            ApngDecoder.decodeApng(context, optimisedInputStream) as AnimationDrawable
+
+        optimisedInputStream.close()
+
+
+
+        val nonOptimisedOutputStream = ByteArrayOutputStream()
+
+        val nonOptimisedEncoder = ApngEncoder(nonOptimisedOutputStream, list[0].width, list[0].height, list.size)
+            .setOptimiseApng(false)
+
+        list.forEach {
+            nonOptimisedEncoder.writeFrame(it)
+        }
+
+        nonOptimisedEncoder.writeEnd()
+
+        nonOptimisedOutputStream.close()
+
+        val nonOptimisedBytes = nonOptimisedOutputStream.toByteArray()
+        val nonOptimisedInputStream = ByteArrayInputStream(nonOptimisedBytes)
+
+        val nonOptimisedApng =
+            ApngDecoder.decodeApng(context, nonOptimisedInputStream) as AnimationDrawable
+        nonOptimisedInputStream.close()
+
+        for (i in 0 until optimisedApng.numberOfFrames) {
+            assertTrue(
+                isBitmapSimilar(
+                    (optimisedApng.getFrame(i) as BitmapDrawable).bitmap,
+                    (nonOptimisedApng.getFrame(i) as BitmapDrawable).bitmap
+                )
+            )
+        }
+    }
+
+    @Test
+    fun testOptimiseBunny() {
+        val context = InstrumentationRegistry.getInstrumentation().context
+        val list = context.assets.list("bunny")?.map { getFrame(context, "bunny/$it") }!!
+
+        val optimisedOutputStream = ByteArrayOutputStream()
+        val optimisedEncoder = ApngEncoder(optimisedOutputStream, list[0].width, list[0].height, list.size)
+            .setOptimiseApng(true)
+
+        list.forEach {
+            optimisedEncoder.writeFrame(it)
+        }
+
+        optimisedEncoder.writeEnd()
+        optimisedOutputStream.close()
+
+        val bytes = optimisedOutputStream.toByteArray()
+        val optimisedInputStream = ByteArrayInputStream(bytes)
+
+        val optimisedApng =
+            ApngDecoder.decodeApng(context, optimisedInputStream) as AnimationDrawable
+
+        optimisedInputStream.close()
+
+
+
+        val nonOptimisedOutputStream = ByteArrayOutputStream()
+
+        val nonOptimisedEncoder = ApngEncoder(nonOptimisedOutputStream, list[0].width, list[0].height, list.size)
+            .setOptimiseApng(false)
+
+        list.forEach {
+            nonOptimisedEncoder.writeFrame(it)
+        }
+
+        nonOptimisedEncoder.writeEnd()
+
+        nonOptimisedOutputStream.close()
+
+        val nonOptimisedBytes = nonOptimisedOutputStream.toByteArray()
+        val nonOptimisedInputStream = ByteArrayInputStream(nonOptimisedBytes)
+
+        val nonOptimisedApng =
+            ApngDecoder.decodeApng(context, nonOptimisedInputStream) as AnimationDrawable
+        nonOptimisedInputStream.close()
+
+        for (i in 0 until optimisedApng.numberOfFrames) {
+            assertTrue(
+                isBitmapSimilar(
+                    (optimisedApng.getFrame(i) as BitmapDrawable).bitmap,
+                    (nonOptimisedApng.getFrame(i) as BitmapDrawable).bitmap
+                )
+            )
+        }
+    }
+
     private fun isSimilar(buffer : Bitmap, frame : Bitmap, diff : Utils.Companion.DiffResult) : Boolean {
         val btm = buffer.copy(Bitmap.Config.ARGB_8888, true)
 
@@ -69,6 +187,17 @@ class ApngEncoderInstrumentedTest {
         return true
     }
 
+    fun isBitmapSimilar(btm1 : Bitmap, btm2 : Bitmap) : Boolean {
+        for (y in 0 until btm1.height) {
+            for (x in 0 until btm1.width) {
+                if (btm1.getPixel(x, y) != btm2.getPixel(x, y)) {
+                    return false
+                }
+            }
+        }
+        return true
+    }
+
     private fun getFrame(context: Context, name: String) : Bitmap {
         val inputStream = context.assets.open(name)
 
diff --git a/apng_library/src/main/java/oupson/apng/encoder/ApngEncoder.kt b/apng_library/src/main/java/oupson/apng/encoder/ApngEncoder.kt
index 75e593d..886c358 100644
--- a/apng_library/src/main/java/oupson/apng/encoder/ApngEncoder.kt
+++ b/apng_library/src/main/java/oupson/apng/encoder/ApngEncoder.kt
@@ -284,7 +284,13 @@ class ApngEncoder(
     ) {
         if (currentFrame == 0) {
             if (btm.width != width || btm.height != height)
-                throw InvalidFrameSizeException(btm.width, btm.height, width, height, currentFrame == 0)
+                throw InvalidFrameSizeException(
+                    btm.width,
+                    btm.height,
+                    width,
+                    height,
+                    currentFrame == 0
+                )
         }
 
         var frameBtm = btm
@@ -292,21 +298,21 @@ class ApngEncoder(
         var frameYOffsets = yOffsets
         var frameBlendOp = blendOp
 
-        if (currentFrame !=  0 || (currentFrame == 0 && firstFrameInAnim)) {
-                if (bitmapBuffer == null && optimise) {
-                    bitmapBuffer = btm.copy(btm.config, false)
-                } else if (optimise) {
-                    val diff = Utils.getDiffBitmap(bitmapBuffer!!, btm)
-                    frameBtm = diff.bitmap
-                    frameXOffsets = diff.offsetX
-                    frameYOffsets = diff.offsetY
-                    frameBlendOp = diff.blendOp
-                    bitmapBuffer = btm.copy(btm.config, false)
-                }
+        if (optimise && currentFrame != 0 || (currentFrame == 0 && firstFrameInAnim)) {
+            if (bitmapBuffer == null) {
+                bitmapBuffer = btm.copy(btm.config, false)
+            } else {
+                val diff = Utils.getDiffBitmap(bitmapBuffer!!, btm)
+                frameBtm = diff.bitmap
+                frameXOffsets = diff.offsetX
+                frameYOffsets = diff.offsetY
+                frameBlendOp = diff.blendOp
+                bitmapBuffer = btm.copy(btm.config, false)
             }
+        }
 
-        if (btm.width > width || btm.height > height)
-            throw InvalidFrameSizeException(btm.width, btm.height, width, height, currentFrame == 0)
+        if (frameBtm.width > width || frameBtm.height > height)
+            throw InvalidFrameSizeException(frameBtm.width, frameBtm.height, width, height, currentFrame == 0)
 
         if (firstFrameInAnim || currentFrame != 0)
             writeFCTL(frameBtm, delay, disposeOp, frameBlendOp, frameXOffsets, frameYOffsets)