You can decode a locally selected iPhone HEIC image in a browser without uploading it by trying the browser’s image decoder first and using a WebAssembly build of libheif as a fallback. For more control, an app can optionally try WebCodecs when the browser exposes an HEVC decoder. A support check is only a clue: the actual file still has to decode successfully.
What HEIC decoding involves
HEIF is a container format; HEIC commonly means a HEIF image whose picture data is encoded with HEVC. Reading one therefore involves interpreting the container and decoding its image data. A browser’s ability to display some HEIF or HEVC images does not guarantee it can handle every iPhone photo: support can vary with browser, operating system, installed codecs, hardware, and the file’s profile.
For a web app, the practical approach is a sequence of decode attempts: use a native browser image decoder if it accepts the file, optionally try WebCodecs where HEVC decoding is available, then fall back to libheif compiled to WebAssembly. The @su-engineering/heic project documents a native-to-WebCodecs-to-fallback strategy and lazy loading of libheif-js. The libheif project describes a HEIF/AVIF decoder and encoder and documents compiling it to JavaScript with Emscripten; its build documentation says, “libheif can also be compiled to JavaScript using emscripten.”
WebAssembly is not itself a HEIC codec. It is a way to run compiled code in the browser; the libheif build supplies the relevant container and image-decoding implementation. See MDN’s overview of loading and running WebAssembly code for the general browser model.
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Build a local-only decode flow
- Accept a local file. Read a user-selected
FileorBlobin the browser. The documented @su-engineering/heic API also accepts anArrayBufferorUint8Array. - Identify the input, then attempt a real decode. Do not treat a
.heicextension or a capability probe as proof that this particular image will work. Handle rejection for malformed, unsupported, or otherwise undecodable files. - Try the available decoding paths. Attempt native image decoding, optionally use WebCodecs if a suitable HEVC decoder is available, and load a libheif WebAssembly fallback only if earlier attempts fail. Lazy loading keeps the fallback optional until it is needed; actual asset splitting and download size depend on the bundler and libheif version.
- Produce the result locally. Render decoded pixels for preview or encode them as JPEG or PNG, then offer a browser-side download. Do not send the source or result to an application server if the goal is to avoid uploads.
- Show a useful failure state. If all paths fail, tell the user that this image could not be decoded and suggest selecting another file or using a different workflow. A friendly error is better than a blank preview or an indefinite loading state.
For an implementation rather than a low-level integration, the @su-engineering/heic project documents browser conversion, pixel output, JPEG/PNG options, and fallback behavior. Check the exact package version and build you deploy because repository documentation can change.
Choose the output with metadata and image content in mind
The documented browser library can convert the primary HEVC image to JPEG or PNG; it does not encode a new HEIC file. Its conversion output does not copy source EXIF metadata. That matters if the workflow depends on camera or location information, or if another system expects metadata to survive conversion. Preserve the original separately when users may need it.
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JPEG is the library’s documented default, and it permits a JPEG quality option. PNG is another available output. The browser’s encoders determine the resulting color and compression behavior, so do not promise identical output across browsers. A maximum output dimension can help control exported dimensions, but it is not a cap on the memory needed to decode the full-resolution source first.
The documented implementation targets the primary HEVC image. It does not decode AVIF, animation, or Live Photo video. It recognizes some auxiliary alpha, depth, and HDR gain-map items and may warn about them, but the project says warning coverage is not exhaustive. If those components are important to the application, test the precise files and workflow rather than assuming a JPEG/PNG conversion preserves them.
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Plan for compatibility, memory, and deployment
- Compatibility: Native decoding and WebCodecs HEVC availability vary across browser and device configurations. A WebAssembly fallback broadens the available path, but still handle file-specific decode failure.
- Memory and resilience: Full-resolution decoding and compositing can allocate substantial memory even when the exported image is resized. Apply file-size limits, limit concurrent decodes, and consider running work in a worker, especially for large or untrusted inputs. The libheif documentation also describes security limits intended to mitigate denial-of-service risks from very large or malicious images.
- Startup cost: Loading a fallback only when needed avoids making every user pay its startup cost. No independently measured bundle-size or speed comparison is established here; those depend on the build and deployment.
- Licensing: The libheif project identifies its licensing as LGPL, while optional bindings or packaged distributions can have separate terms. Check the exact package and build before redistribution.
What “no upload” does—and does not—promise
A client-side app can avoid sending a photo to a server if reading, decoding, compositing, and downloading all happen in the browser. That is a statement about the app’s implementation, not a guarantee about every online converter. To make the promise credible, keep both the source bytes and derived output on the client, and ensure the surrounding application does not transmit them through its own code or services. Do not infer that a third-party converter never transmits, stores, or logs images without independently verified evidence.
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