add webp support

src/main/java/org/teacon/slides/cache/ImageCache.java

@@ -73,7 +73,7 @@ public CompletableFuture<byte[]> getResource(@Nonnull URI location, boolean onli
             final HttpCacheContext context = HttpCacheContext.create();
             try (CloseableHttpResponse response = createResponse(location, context, online)) {
                 try {
-                    return IOUtils.toByteArray(response.getEntity().getContent());
+                    return WebpToPng.webpToPng(IOUtils.toByteArray(response.getEntity().getContent()));
                 } catch (IOException e) {
                     if (online) {
                         Slideshow.LOGGER.warn("Failed to read bytes from remote source.", e);

src/main/java/org/teacon/slides/cache/WebpToPng.java

@@ -0,0 +1,48 @@
+package org.teacon.slides.cache;
+
+import org.teacon.slides.Slideshow;
+
+import javax.imageio.ImageIO;
+import javax.imageio.ImageReader;
+import javax.imageio.stream.MemoryCacheImageInputStream;
+import javax.imageio.stream.MemoryCacheImageOutputStream;
+import java.awt.image.BufferedImage;
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.util.Iterator;
+
+public final class WebpToPng {
+    private static boolean isWebP(byte[] webp) {
+        if (webp.length < 12) {
+            return false;
+        }
+        boolean isRiff = webp[0] == 'R' && webp[1] == 'I' && webp[2] == 'F' && webp[3] == 'F';
+        boolean isWebp = webp[8] == 'W' && webp[9] == 'E' && webp[10] == 'B' && webp[11] == 'P';
+        return isRiff && isWebp;
+    }
+
+    public static byte[] webpToPng(byte[] webp) {
+        try {
+            if (!isWebP(webp)) {
+                return webp;
+            }
+            Iterator<ImageReader> imageReader = ImageIO.getImageReadersByFormatName("webp");
+            if (!imageReader.hasNext()) {
+                return webp;
+            }
+            ImageReader webpReader = imageReader.next();
+            try (ByteArrayInputStream bais = new ByteArrayInputStream(webp); MemoryCacheImageInputStream inputStream = new MemoryCacheImageInputStream(bais)) {
+                webpReader.setInput(inputStream);
+                BufferedImage bufferedImage = webpReader.read(0);
+                try (ByteArrayOutputStream baos = new ByteArrayOutputStream(); MemoryCacheImageOutputStream outputStream = new MemoryCacheImageOutputStream(baos)) {
+                    ImageIO.write(bufferedImage, "png", outputStream);
+                    outputStream.flush();
+                    return baos.toByteArray();
+                }
+            }
+        } catch (Exception e) {
+            Slideshow.LOGGER.warn("Failed to convert webp to PNG", e);
+            return webp;
+        }
+    }
+}