feat(api): on-th-fly thumbnail generation for any page

komga/src/main/kotlin/org/gotson/komga/application/service/BookLifecycle.kt

@@ -62,10 +62,20 @@ class BookLifecycle(
     MediaNotReadyException::class,
     IndexOutOfBoundsException::class
   )
-  fun getBookPage(book: Book, number: Int, convertTo: ImageType? = null): BookPageContent {
+  fun getBookPage(book: Book, number: Int, convertTo: ImageType? = null, resizeTo: Int? = null): BookPageContent {
     val pageContent = bookAnalyzer.getPageContent(book, number)
     val pageMediaType = book.media.pages[number - 1].mediaType
 
+    if (resizeTo != null) {
+      val targetFormat = ImageType.JPEG
+      val convertedPage = try {
+        imageConverter.resizeImage(pageContent, targetFormat.imageIOFormat, resizeTo)
+      } catch (e: Exception) {
+        logger.error(e) { "Resize page #$number of book $book to $resizeTo: failed" }
+        throw e
+      }
+      return BookPageContent(number, convertedPage, targetFormat.mediaType)
+    } else {
       convertTo?.let {
         val msg = "Convert page #$number of book $book from $pageMediaType to ${it.mediaType}"
         if (!imageConverter.supportedReadMediaTypes.contains(pageMediaType)) {
@@ -91,4 +101,5 @@ class BookLifecycle(
 
       return BookPageContent(number, pageContent, pageMediaType)
     }
+  }
 }

komga/src/main/kotlin/org/gotson/komga/infrastructure/image/ImageConverter.kt

@@ -1,6 +1,7 @@
 package org.gotson.komga.infrastructure.image
 
 import mu.KotlinLogging
+import net.coobird.thumbnailator.Thumbnails
 import org.springframework.stereotype.Service
 import java.io.ByteArrayOutputStream
 import javax.imageio.ImageIO
@@ -28,4 +29,13 @@ class ImageConverter {
       ImageIO.write(image, format, it)
       it.toByteArray()
     }
+
+  fun resizeImage(imageBytes: ByteArray, format: String, size: Int): ByteArray =
+    ByteArrayOutputStream().use {
+      Thumbnails.of(imageBytes.inputStream())
+        .size(size, size)
+        .outputFormat(format)
+        .toOutputStream(it)
+      it.toByteArray()
+    }
 }

komga/src/main/kotlin/org/gotson/komga/interfaces/rest/BookController.kt

@@ -241,6 +241,40 @@ class BookController(
       }
     } ?: throw ResponseStatusException(HttpStatus.NOT_FOUND)
 
+  @GetMapping("api/v1/books/{bookId}/pages/{pageNumber}/thumbnail")
+  fun getBookPageThumbnail(
+    @AuthenticationPrincipal principal: KomgaPrincipal,
+    request: WebRequest,
+    @PathVariable bookId: Long,
+    @PathVariable pageNumber: Int
+  ): ResponseEntity<ByteArray> =
+    bookRepository.findByIdOrNull((bookId))?.let { book ->
+      if (request.checkNotModified(getBookLastModified(book))) {
+        return@let ResponseEntity
+          .status(HttpStatus.NOT_MODIFIED)
+          .setNotModified(book)
+          .body(ByteArray(0))
+      }
+      if (!principal.user.canAccessBook(book)) throw ResponseStatusException(HttpStatus.UNAUTHORIZED)
+      try {
+        val pageContent = bookLifecycle.getBookPage(book, pageNumber, resizeTo = 300)
+
+        ResponseEntity.ok()
+          .contentType(getMediaTypeOrDefault(pageContent.mediaType))
+          .setNotModified(book)
+          .body(pageContent.content)
+      } catch (ex: IndexOutOfBoundsException) {
+        throw ResponseStatusException(HttpStatus.BAD_REQUEST, "Page number does not exist")
+      } catch (ex: ImageConversionException) {
+        throw ResponseStatusException(HttpStatus.NOT_FOUND, ex.message)
+      } catch (ex: MediaNotReadyException) {
+        throw ResponseStatusException(HttpStatus.NOT_FOUND, "Book analysis failed")
+      } catch (ex: NoSuchFileException) {
+        logger.warn(ex) { "File not found: $book" }
+        throw ResponseStatusException(HttpStatus.NOT_FOUND, "File not found, it may have moved")
+      }
+    } ?: throw ResponseStatusException(HttpStatus.NOT_FOUND)
+
   @PostMapping("api/v1/books/{bookId}/analyze")
   @PreAuthorize("hasRole('ROLE_ADMIN')")
   @ResponseStatus(HttpStatus.ACCEPTED)