Files, codecs and assets

Examples use named imports after one await initOpenCV() call. See the initialization guide.

Each OpenCV instance owns an in-memory filesystem, FS. Native paths refer to this filesystem. They are not host paths, URLs or browser file handles.

import { dnn_readNetFromONNX, FS } from '@banou/opencv-wasm'
const response = await fetch('/assets/model.onnx')
if (!response.ok) throw new Error(`Model request failed: ${response.status}`)
FS.writeFile('/model.onnx', new Uint8Array(await response.arrayBuffer()))
using model = dnn_readNetFromONNX('/model.onnx')
FS.unlink('/model.onnx')

Image formats

Native image codecs include PNG, JPEG, WebP, TIFF, JPEG 2000 and OpenEXR, plus OpenCV’s built-in formats. AVIF is not included. Use decodeImage and encodeImage when the application already owns the bytes.

Decoded colour images use BGR or BGRA. The helpers can preserve 16-bit pixels; a round trip through an 8-bit canvas cannot. Be deliberate about precision when porting scientific images or reconstruction pipelines.

Video

Native VideoCapture and VideoWriter support MJPEG AVI files in FS using CAP_OPENCV_MJPEG, and image sequences through CAP_IMAGES. FFmpeg, GStreamer and native camera devices are not linked.

In a browser, obtain camera frames with browser media APIs. Decode other video containers with a separate media pipeline, then give OpenCV the decoded frames. In Node, applications such as Cadence can use FFmpeg externally for video packaging.

Fonts, OCR and data files

Feature	Application asset	Important limit
FreeType text	TTF or OTF file	Some compressed and bitmap font dependencies are disabled
Tesseract OCR	Matching .traineddata language files	Language data is not bundled
DNN	Model files and any required auxiliary data	Operator support varies by model
HDF5	Dataset file in FS	This build omits gzip/SZIP filters and parallel HDF5

Close HDF5 files before reading their final bytes. Some native APIs borrow an asset buffer; follow the parameter’s lifetime note instead of disposing it immediately after loading.