From 9556ae51f262470b1fbb5fd9af2d8caeb3c1fa64 Mon Sep 17 00:00:00 2001
From: Gauthier Roebroeck <gauthier.roebroeck@gmail.com>
Date: Fri, 8 May 2020 15:43:04 +0800
Subject: [PATCH] fix: ignore alpha channel on image conversion

if image to convert targets a format that doesn't support transparency, but contains alpha channel, then the alpha channel will be ignore. Non-opaque images may produce visual artifacts.

related to #153
---
 .../infrastructure/image/ImageConverter.kt    | 35 +++++++++++++++++--
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/komga/src/main/kotlin/org/gotson/komga/infrastructure/image/ImageConverter.kt b/komga/src/main/kotlin/org/gotson/komga/infrastructure/image/ImageConverter.kt
index 7ad987d33..f04594848 100644
--- a/komga/src/main/kotlin/org/gotson/komga/infrastructure/image/ImageConverter.kt
+++ b/komga/src/main/kotlin/org/gotson/komga/infrastructure/image/ImageConverter.kt
@@ -3,6 +3,8 @@ package org.gotson.komga.infrastructure.image
 import mu.KotlinLogging
 import net.coobird.thumbnailator.Thumbnails
 import org.springframework.stereotype.Service
+import java.awt.Color
+import java.awt.image.BufferedImage
 import java.io.ByteArrayOutputStream
 import javax.imageio.ImageIO
 
@@ -23,11 +25,25 @@ class ImageConverter {
     logger.info { "Supported write mediaTypes: $supportedWriteMediaTypes" }
   }
 
+  private val supportsTransparency = listOf("png")
+
   fun convertImage(imageBytes: ByteArray, format: String): ByteArray =
-    ByteArrayOutputStream().use {
+    ByteArrayOutputStream().use { baos ->
       val image = ImageIO.read(imageBytes.inputStream())
-      ImageIO.write(image, format, it)
-      it.toByteArray()
+
+      val result = if (!supportsTransparency.contains(format) && containsAlphaChannel(image)) {
+        if (containsTransparency(image)) logger.info { "Image contains alpha channel but is not opaque, visual artifacts may appear" }
+        else logger.info { "Image contains alpha channel but is opaque, conversion should not generate any visual artifacts" }
+        BufferedImage(image.width, image.height, BufferedImage.TYPE_INT_RGB).also {
+          it.createGraphics().drawImage(image, 0, 0, Color.WHITE, null)
+        }
+      } else {
+        image
+      }
+
+      ImageIO.write(result, format, baos)
+
+      baos.toByteArray()
     }
 
   fun resizeImage(imageBytes: ByteArray, format: String, size: Int): ByteArray =
@@ -38,4 +54,17 @@ class ImageConverter {
         .toOutputStream(it)
       it.toByteArray()
     }
+
+  private fun containsAlphaChannel(image: BufferedImage): Boolean =
+    image.colorModel.hasAlpha()
+
+  private fun containsTransparency(image: BufferedImage): Boolean {
+    for (x in 0 until image.width) {
+      for (y in 0 until image.height) {
+        val pixel = image.getRGB(x, y)
+        if (pixel shr 24 == 0x00) return true
+      }
+    }
+    return false
+  }
 }