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fix: use image's MIME type instead of default image/png#27

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issue-23
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fix: use image's MIME type instead of default image/png#27
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issue-23

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@flang flang commented Aug 19, 2025

Close #23

Summary by CodeRabbit

  • New Features

    • Cropped images now preserve the source image format when detectable (JPEG/PNG/etc.).
    • Cropping flow is asynchronous and waits for format detection before exporting.
  • Bug Fixes

    • Per-source MIME-type detection with caching for faster, consistent exports.
    • Improved error handling and logging during image load and crop completion.
    • Reliable fallback to PNG if format detection fails.

Review Change Stack

@flang flang requested review from javier-godoy and paodb August 19, 2025 13:49
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coderabbitai Bot commented Aug 19, 2025

No actionable comments were generated in the recent review. 🎉

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Reviewing files that changed from the base of the PR and between a226022 and 5c3f93a.

📒 Files selected for processing (1)
  • src/main/resources/META-INF/resources/frontend/src/image-crop.tsx

Walkthrough

Adds per-source MIME-type detection and caching, makes _updateCroppedImage async, and uses the detected MIME type (falling back to image/png) when encoding the cropped canvas; callers now catch and log Promise rejections.

Changes

Image crop async + MIME detection

Layer / File(s) Summary
MIME detection & cache
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx
Adds private `#mimeTypeCache: Map<string,string
Async crop update wiring
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx
Changes public _updateCroppedImage(...)public async _updateCroppedImage(...); onImageLoad and onComplete invoke it and attach .catch(console.error) to surface Promise rejections.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Assessment against linked issues

Objective Addressed Explanation
Preserve original image format when cropping (JPEGs not converted to PNGs) (#23) Cropped output uses detected MIME type (falls back to image/png only if detection fails).

Possibly related PRs

  • FlowingCode/ImageCrop#20 — Modifies the same client-side _updateCroppedImage pipeline; closely related to MIME/encoding and update trigger behavior.

Suggested reviewers

  • paodb
  • mlopezFC
🚥 Pre-merge checks | ✅ 5
✅ Passed checks (5 passed)
Check name Status Explanation
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly and directly describes the main change: using the image's MIME type instead of defaulting to image/png, which matches the core objective of fixing the JPEG-to-PNG conversion bug.
Linked Issues check ✅ Passed The code changes directly address issue #23 by detecting and preserving the original image MIME type instead of forcing PNG output, fully meeting the expected behavior requirement.
Out of Scope Changes check ✅ Passed All changes are scoped to implementing MIME type detection and preservation for cropped images, with no unrelated modifications present in the changeset.
Docstring Coverage ✅ Passed No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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Actionable comments posted: 1

🧹 Nitpick comments (1)
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (1)

210-264: Avoid stale-crop races introduced by async MIME detection

Now that _updateCroppedImage awaits network, a slower earlier call could dispatch a cropped image after a newer call, overwriting the latest result. Guard dispatches with a request sequence.

Apply this diff inside the method:

-	public async _updateCroppedImage(crop: PixelCrop|PercentCrop) {
+	public async _updateCroppedImage(crop: PixelCrop|PercentCrop) {
+			// Guard against out-of-order async completions
+			const seq = ++this.#updateSeq;
 			const image = this.querySelector("img");
 			if (crop && image) {
 				crop = convertToPixelCrop(crop, image.width, image.height);
@@
-					// dispatch the event containing cropped image
-					this.fireCroppedImageEvent(croppedImageDataUri);
+					// Only dispatch if this is still the latest update
+					if (seq === this.#updateSeq) {
+						this.fireCroppedImageEvent(croppedImageDataUri);
+					}
 				}
 			}
 	}

Add the backing field elsewhere in the class:

#updateSeq = 0;
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📒 Files selected for processing (1)
  • src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (4 hunks)
🔇 Additional comments (4)
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (4)

73-73: Good: prevent unhandled Promise rejections in onImageLoad

Chaining .catch(console.error) avoids noisy console errors and aligns with the new async flow.


126-126: Good: non-blocking error handling on crop completion

Consistent with the async update; avoids unhandled rejections during user interactions.


255-259: Preserving source MIME in toDataURL meets the PR objective

Using the detected type (with 'image/png' fallback) ensures JPEGs remain JPEGs when possible. With the normalization change above, this will also handle parameterized headers and common aliases reliably.


210-210: Verified async signature usage: no action required

  • The only invocations of _updateCroppedImage are in src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (lines 73 and 126).
  • Both calls already chain .catch(console.error) for error handling.
  • No other callers assume synchronous behavior.

Comment thread src/main/resources/META-INF/resources/frontend/src/image-crop.tsx
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Actionable comments posted: 0

♻️ Duplicate comments (1)
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (1)

166-225: Normalize MIME (strip parameters, handle data: with comma) to prevent silent PNG fallback

Canvas toDataURL only recognizes bare types (e.g., "image/jpeg"). If you pass something like "image/jpeg; charset=binary" or parse a data URL that uses a comma delimiter (common in SVG), the browser will silently fall back to PNG, undermining the PR goal in those cases. Minimal normalization keeps logic simple while avoiding regressions.

Apply this minimal diff inside the method:

@@
-    // Case 1: data URL (e.g., data:image/png;base64,...)
+    // Case 1: data URL (e.g., data:image/png;base64,... or data:image/svg+xml,<...>)
     if (img.src.startsWith("data:")) {
-      const semiIndex = img.src.indexOf(";");
-      if (semiIndex > 5) {
-        const mimeType = img.src.substring(5, semiIndex);
-        this.#mimeTypeCache?.set(cacheKey, mimeType);
-        return mimeType;
-      }
-      this.#mimeTypeCache?.set(cacheKey, null);
-      return null;
+      const m = /^data:([^;,]+)[;,]/i.exec(img.src);
+      const mt = m ? m[1].trim().toLowerCase() : null;
+      const norm = mt === "image/jpg" ? "image/jpeg" : mt;
+      this.#mimeTypeCache?.set(cacheKey, norm);
+      return norm;
     }
@@
-    const headRes = await fetch(img.src, { method: "HEAD" });
-    let mimeType = headRes.headers.get("Content-Type");
-    if (mimeType) {
-      this.#mimeTypeCache?.set(cacheKey, mimeType);
-      return mimeType;
-    }
+    const headRes = await fetch(img.src, { method: "HEAD" });
+    let mimeType = headRes.headers.get("Content-Type");
+    if (mimeType) {
+      const base = mimeType.split(";")[0].trim().toLowerCase();
+      const norm = base === "image/jpg" ? "image/jpeg" : base;
+      this.#mimeTypeCache?.set(cacheKey, norm);
+      return norm;
+    }
@@
-    const blobRes = await fetch(img.src);
+    const blobRes = await fetch(img.src, { cache: "force-cache" });
     const blob = await blobRes.blob();
-    mimeType = blob.type || null;
-    this.#mimeTypeCache?.set(cacheKey, mimeType);
-    return mimeType;
+    const base = (blob.type || "").split(";")[0].trim().toLowerCase() || null;
+    const norm = base === "image/jpg" ? "image/jpeg" : base;
+    this.#mimeTypeCache?.set(cacheKey, norm);
+    return norm;

Notes:

  • Keeps logic simple (no broad alias tables), per your preference, while fixing common pitfalls that cause PNG fallback.
  • Optional: If you want stricter behavior, return null for unsupported types so the caller explicitly defaults to image/png instead of passing through unknown types like application/octet-stream.
🧹 Nitpick comments (2)
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (2)

227-281: Avoid stale events when multiple async updates overlap (latest-wins guard)

With the async MIME lookup, multiple _updateCroppedImage calls can race; an earlier request might resolve after a later one and dispatch an outdated crop. Add a simple sequence guard so only the latest invocation fires the event.

Apply this diff within the method:

 public async _updateCroppedImage(crop: PixelCrop|PercentCrop) {
-      const image = this.querySelector("img");
+      // Increment sequence to identify the latest in-flight update
+      const seq = ++this.#updateSeq;
+      const image = this.querySelector("img");
@@
-          // dispatch the event containing cropped image
-          this.fireCroppedImageEvent(croppedImageDataUri);
+          // dispatch only if this is still the latest update
+          if (seq === this.#updateSeq) {
+            this.fireCroppedImageEvent(croppedImageDataUri);
+          }

And add this field to the class (outside the selected range):

// Monotonic sequence to keep only the latest async crop result
#updateSeq = 0;

This keeps behavior deterministic when users drag the crop rapidly.


272-276: Nit: Pass quality only for lossy formats

Quality is only meaningful for image/jpeg (and image/webp). For other types it’s ignored; passing it is harmless but slightly noisy.

Optional tweak:

-      let croppedImageDataUri = canvas.toDataURL(imgMimeType, 1.0);
+      const needsQuality = imgMimeType === "image/jpeg" || imgMimeType === "image/webp";
+      const croppedImageDataUri = needsQuality
+        ? canvas.toDataURL(imgMimeType, 1.0)
+        : canvas.toDataURL(imgMimeType);
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📥 Commits

Reviewing files that changed from the base of the PR and between 615e37a and ed0e45b.

📒 Files selected for processing (1)
  • src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (5 hunks)
🧰 Additional context used
🧠 Learnings (2)
📓 Common learnings
Learnt from: flang
PR: FlowingCode/ImageCrop#27
File: src/main/resources/META-INF/resources/frontend/src/image-crop.tsx:0-0
Timestamp: 2025-08-19T15:50:59.505Z
Learning: In the ImageCrop add-on project, flang prefers to keep MIME type handling simple without normalization of Content-Type headers or alias mapping, focusing on the core caching performance improvement.
📚 Learning: 2025-08-19T15:50:59.505Z
Learnt from: flang
PR: FlowingCode/ImageCrop#27
File: src/main/resources/META-INF/resources/frontend/src/image-crop.tsx:0-0
Timestamp: 2025-08-19T15:50:59.505Z
Learning: In the ImageCrop add-on project, flang prefers to keep MIME type handling simple without normalization of Content-Type headers or alias mapping, focusing on the core caching performance improvement.

Applied to files:

  • src/main/resources/META-INF/resources/frontend/src/image-crop.tsx
🔇 Additional comments (2)
src/main/resources/META-INF/resources/frontend/src/image-crop.tsx (2)

28-30: Solid perf win with per-src MIME cache

Good addition. This avoids repeated HEAD/GET on the same image and keeps the hot path fast once resolved.


76-76: Call sites correctly handle async; verify downstream expectations of event timing

Switching to an async update path means the "cropped-image" event will fire later than before (network-dependent for non-data URLs). Please verify that any server listeners or UI flows relying on immediate availability still behave as expected.

Suggested validation:

  • Manually crop rapidly several times; ensure the final event corresponds to the last crop.
  • Confirm server-side code does not assume synchronous dispatch within onComplete/onLoad.

Also applies to: 128-130

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@paodb paodb marked this pull request as ready for review May 18, 2026 13:58
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paodb commented May 18, 2026

@coderabbitai full review

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paodb commented Jun 5, 2026

Thanks for tackling this! The root cause is correctly identified — the hard-coded canvas.toDataURL("image/png", 1.0) — and the happy path works for same-origin JPEG/PNG/WebP. Before we merge though, I'd like to reconsider the approach, because detecting the MIME type with extra network requests at crop time is heavier than the problem needs and leaves a few edge cases that re-introduce silent conversion.

Concerns with the current approach

  1. canvas.toDataURL() only supports png/jpeg/webp. Any other detected type (gif, bmp, svg+xml, avif, ...) is silently ignored and the browser falls back to PNG — i.e. the exact silent-conversion symptom from JPEGs are silently converted to PNGs #23, just for a different set of inputs. The detected type needs to be validated against a supported allow-list.

  2. Content-Type is used unnormalized. Headers can carry parameters (image/jpeg; charset=binary), and toDataURL("image/jpeg; charset=binary", ...) is not a recognized type → silent PNG fallback again. Needs lowercasing + stripping of parameters.

  3. Circular crop + JPEG source → black corners (regression). A circular crop leaves transparent corners; JPEG has no alpha, so encoding as image/jpeg fills them with black. The previous always-PNG behavior preserved transparency. We should force a transparency-capable format (PNG/WebP) whenever circularCrop is on, regardless of source.

  4. Async ordering. _updateCroppedImage is now async with an await between draw and dispatch; rapid onComplete calls during dragging can dispatch out of order and leave the server with a stale crop. A monotonic sequence guard around the await would close the window.

  5. Network detection is fragile for our typical inputs. The image is already downloaded for the <img>; the extra HEAD (and full-blob GET fallback) re-hits the URL. For cross-origin images the fetch throws on CORS (and the canvas is already tainted anyway), and for Vaadin StreamResource the HEAD may return no Content-Type or re-stream the whole resource.

Suggested alternative — server-controlled output format

Rather than detecting at runtime, I'd lean toward making the output format an explicit, server-side property. It's deterministic, needs no extra network calls, and resolves 1–3 for free (the caller picks the format, and we override to a transparent format when circular). It also lets the caller control JPEG quality, since toDataURL("image/jpeg", 1.0) is still lossy re-encoding.

Java (ImageCrop):

/** Sets the MIME type used to encode the cropped output (default {@code image/png}). */
public void setOutputMimeType(String mimeType) {
  setState("outputMimeType", mimeType);
}

/** Sets the encoding quality (0..1) for lossy formats such as image/jpeg. */
public void setOutputQuality(double quality) {
  setState("outputQuality", quality);
}

TSX:

const [outputMimeType] = hooks.useState<string>("outputMimeType", "image/png");
const [outputQuality]  = hooks.useState<number>("outputQuality", 1.0);

const SUPPORTED = new Set(["image/png", "image/jpeg", "image/webp"]);

function resolveOutputType(requested: string, circular: boolean): string {
  const type = (requested || "image/png").toLowerCase().split(";")[0].trim();
  // JPEG has no alpha: a circular crop would get black corners.
  if (circular && type === "image/jpeg") return "image/png";
  return SUPPORTED.has(type) ? type : "image/png";
}

// in _updateCroppedImage (no longer needs to be async):
const type = resolveOutputType(outputMimeType, this.circularCrop);
const croppedImageDataUri = canvas.toDataURL(type, outputQuality);

Default stays image/png so this is backward compatible, and callers who want to preserve a JPEG just call setOutputMimeType("image/jpeg").

If we do want auto-detection instead of an explicit property, a zero-network heuristic (parse + normalize the extension from img.src, validate against the allow-list) covers the same common cases without the CORS/StreamResource pitfalls — but the explicit property is my preference for predictability.

Either way, let's add a couple of unit tests for the format paths. Happy to pair on whichever direction we pick.

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JPEGs are silently converted to PNGs

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