v4: a still is scaled from its first frame alone, a second image in its file made the copy flash — an avatar came out as a WebP of two frames looping at 40 ms each, the picture then a blank. A still's file may hold more than one image, a phone's JPEG carrying a gain map or a depth map after the photo, and ffmpeg reads each as a frame: only the JPEG and PNG copies were held to one, the WebP keeping them all as it does a GIF's. scalableImage (lib/upload.ts) now says whether the image is an animation, which a GIF alone is, and scaleImage (lib/ffmpeg.ts) writes one frame of anything else, in every format; the image proxy scales through the same call. The copies already made stay, a copy being current while it has its upload's date: uploads/scaled is to be deleted once on an instance that ran the previous version, everything in it being made again on first request.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/lib/ffmpeg.ts

@@ -200,28 +200,44 @@ const ORIENTATION_FILTERS: Record<number, string> = {
 /** What ffmpeg is told to write each {@link ScaledFormat} with. */
 const ENCODERS: Record<ScaledFormat, string[]> = {
   webp: ["-c:v", "libwebp", "-q:v", "80", "-loop", "0", "-f", "webp"],
-  // A still: the first frame of an animation.
-  jpg: ["-frames:v", "1", "-c:v", "mjpeg", "-q:v", "4", "-f", "image2"],
-  png: ["-frames:v", "1", "-c:v", "png", "-f", "image2"],
+  jpg: ["-c:v", "mjpeg", "-q:v", "4", "-f", "image2"],
+  png: ["-c:v", "png", "-f", "image2"],
 };
 
+/**
+ * What {@link scaleImage} is told of the image it scales, which
+ * `scalableImage` (lib/upload.ts) reads from its first bytes.
+ */
+export interface ImageTraits {
+  /** The EXIF orientation to turn the image by, 1 when it is upright. */
+  orientation: number;
+  /**
+   * Whether its frames are an animation, as a GIF's are. A still may hold
+   * more than one image too, a phone's JPEG carrying a gain map or a depth
+   * map after the photo, and ffmpeg reads each as a frame.
+   */
+  animated: boolean;
+}
+
 /**
  * Writes a copy of an image at most `width` wide, never wider than the image
  * itself, as a WebP unless `format` names another. WebP keeps a PNG's
  * transparency and a GIF's animation, at a fraction of either's bytes.
- * `orientation` is the EXIF orientation of a JPEG, as `scalableImage`
- * (lib/upload.ts) reads it.
  */
 export function scaleImage(
   srcPath: string,
   outPath: string,
   width: ImageWidth,
-  orientation = 1,
+  { orientation, animated }: ImageTraits,
   format: ScaledFormat = "webp",
 ): Promise<void> {
   const upright = ORIENTATION_FILTERS[orientation];
   // A JPEG's colour planes halve its height, which they need even.
   const height = format === "jpg" ? -2 : -1;
+  // Only an animation keeps its frames, and only as a WebP: of anything else
+  // the first is the image, and a copy of them all would flash through what
+  // follows it.
+  const frames = animated && format === "webp" ? [] : ["-frames:v", "1"];
   return runFfmpeg([
     "-noautorotate",
     "-format_whitelist",
@@ -231,6 +247,7 @@ export function scaleImage(
     "-vf",
     `${upright ? `${upright},` : ""}scale='min(${width},iw)':${height}`,
     "-an",
+    ...frames,
     ...ENCODERS[format],
   ], outPath);
 }
@@ -243,7 +260,7 @@ export function scaleImage(
 export async function scaleImageBytes(
   bytes: Uint8Array,
   width: ImageWidth,
-  orientation = 1,
+  traits: ImageTraits,
 ): Promise<Uint8Array | null> {
   if (!await ffmpegAvailable()) return null;
   const srcPath = `${SCALED_DIR}/tmp/${crypto.randomUUID()}`;
@@ -251,7 +268,7 @@ export async function scaleImageBytes(
   try {
     await Deno.mkdir(dirname(srcPath), { recursive: true });
     await Deno.writeFile(srcPath, bytes);
-    await scaleImage(srcPath, outPath, width, orientation);
+    await scaleImage(srcPath, outPath, width, traits);
     return await Deno.readFile(outPath);
   } catch {
     return null;

api/lib/upload.ts

@@ -14,6 +14,7 @@ import { MAX_IMAGE_SIZE_BYTES } from "../config.ts";
 import { type ImageWidth, parseImageWidth } from "../../shared/image.ts";
 import {
   ensureGenerated,
+  type ImageTraits,
   type ScaledFormat,
   scaledImagePath,
   scaleImage,
@@ -113,9 +114,7 @@ function jpegOrientation(data: Uint8Array): number {
 const IMAGE_HEAD_BYTES = 64 * 1024 + 32;
 
 /** Returned by {@link scalableImage}, for `scaleImage`. */
-export interface ScalableImage {
-  /** The EXIF orientation to turn the image by, 1 when it is upright. */
-  orientation: number;
+export interface ScalableImage extends ImageTraits {
   /**
    * The format of its copy for a reader without WebP: a JPEG, or a PNG for
    * an image that may be transparent, which a JPEG would fill with black.
@@ -133,16 +132,16 @@ export function scalableImage(head: Uint8Array): ScalableImage | null {
   // A file dump may be a BMP ("BM"), which no upload route here accepts.
   if (!mime) {
     return head[0] === 0x42 && head[1] === 0x4D
-      ? { orientation: 1, compat: "jpg" }
+      ? { orientation: 1, animated: false, compat: "jpg" }
       : null;
   }
   // "RIFF....WEBPVP8X", whose flags then say whether frames follow.
-  const animated = mime === "image/webp" && head[15] === 0x58 &&
+  const animatedWebp = mime === "image/webp" && head[15] === 0x58 &&
     (head[20] & 0x02) !== 0;
-  if (animated) return null;
+  if (animatedWebp) return null;
   return mime === "image/jpeg"
-    ? { orientation: jpegOrientation(head), compat: "jpg" }
-    : { orientation: 1, compat: "png" };
+    ? { orientation: jpegOrientation(head), animated: false, compat: "jpg" }
+    : { orientation: 1, animated: mime === "image/gif", compat: "png" };
 }
 
 /** Reads the first {@link IMAGE_HEAD_BYTES} of the file at `path`. */
@@ -195,7 +194,7 @@ async function scaledCopy(
   const made = await ensureGenerated(scaledPath, async () => {
     how ??= await describe(path);
     if (!how) throw new Error("Not an image that can be scaled");
-    await scaleImage(path, scaledPath, width, how.orientation, format);
+    await scaleImage(path, scaledPath, width, how, format);
   }, path);
   return made ? { path: scaledPath, mime: SCALED_MIMES[format] } : null;
 }

api/remote/image-proxy.ts

@@ -111,8 +111,7 @@ export async function fetchProxiedImage(
   const original = await fetchOriginal(url);
   if (!original) return null;
   const how = scalableImage(original.bytes);
-  const bytes = how &&
-    await scaleImageBytes(original.bytes, width, how.orientation);
+  const bytes = how && await scaleImageBytes(original.bytes, width, how);
   if (!bytes) return original;
   writeImageCache(key, bytes, "image/webp");
   return { bytes, contentType: "image/webp" };

api/test/lib/upload.test.ts

@@ -94,17 +94,27 @@ Deno.test("scalableImage: an animated WebP and what isn't an image are served as
   assertEquals(scalableImage(webp(0x02)), null);
   // A reader without WebP gets a PNG of what may be transparent, a JPEG of
   // the rest.
-  assertEquals(scalableImage(webp(0x10)), { orientation: 1, compat: "png" });
+  const still = { orientation: 1, animated: false };
+  assertEquals(scalableImage(webp(0x10)), { ...still, compat: "png" });
   assertEquals(
     scalableImage(new TextEncoder().encode("RIFF\0\0\0\0WEBPVP8 ")),
-    { orientation: 1, compat: "png" },
+    { ...still, compat: "png" },
   );
-  assertEquals(scalableImage(PNG), { orientation: 1, compat: "png" });
-  assertEquals(scalableImage(jpegWithOrientation(1, true))?.compat, "jpg");
-  assertEquals(scalableImage(new TextEncoder().encode("BM-a-bitmap")), {
-    orientation: 1,
+  assertEquals(scalableImage(PNG), { ...still, compat: "png" });
+  assertEquals(scalableImage(jpegWithOrientation(1, true)), {
+    ...still,
     compat: "jpg",
   });
+  assertEquals(scalableImage(new TextEncoder().encode("BM-a-bitmap")), {
+    ...still,
+    compat: "jpg",
+  });
+  // Only a GIF's frames are an animation to keep.
+  assertEquals(scalableImage(new TextEncoder().encode("GIF89a")), {
+    orientation: 1,
+    animated: true,
+    compat: "png",
+  });
   assertEquals(scalableImage(new TextEncoder().encode("<svg/>")), null);
   assertEquals(scalableImage(new Uint8Array()), null);
 });

api/test/routes/avatars.test.ts

@@ -38,6 +38,24 @@ test("avatars: upload, serve, and 404 when there is none", async () => {
   );
 });
 
+test("avatars: a still holding a second image is scaled to its first alone", async () => {
+  if (!await hasFfmpeg()) return;
+  const user = await createUser();
+  // As a phone's photo carries a gain map after it, which ffmpeg reads as a
+  // second frame.
+  const photo = await makeImage(600, 400, "red");
+  const extra = await makeImage(150, 100, "white");
+  const both = new Uint8Array(photo.length + extra.length);
+  both.set(photo);
+  both.set(extra, photo.length);
+  await put("/api/me/avatar", { auth: user.auth, form: formWithFile(both) });
+
+  const small = await get(`/api/users/${user.id}/avatar?w=128`);
+  assertEquals(small.headers.get("content-type"), "image/webp");
+  assertEquals(webpWidth(small.bytes), 128);
+  assertFalse(new TextDecoder().decode(small.bytes).includes("ANMF"));
+});
+
 test("avatars: `?w=` answers a scaled copy, made again from a new avatar", async () => {
   if (!await hasFfmpeg()) return;
   const user = await createUser();