flutter_bicubic_resize

flutter_bicubic_resize runs the entire image pipeline - decode, crop, resize and encode - in native C, so a 4K photo becomes a 224×224 tensor in roughly 15-30 ms and the output is byte-identical on Android, iOS and macOS. It uses stb_image_resize2 with the Catmull-Rom spline, the same interpolation as OpenCV cv2.INTER_CUBIC and PIL Image.BICUBIC, which is why it is a good fit for ML preprocessing that must match a Python training pipeline.

• 3-4x faster than other Flutter image libraries (pure native C pipeline)
• 100% native C performance (stb_image + stb_image_resize + stb_image_write)
• Identical results on iOS, Android and macOS
• Bicubic interpolation (Catmull-Rom, same as OpenCV INTER_CUBIC and PIL BICUBIC)
• RGB and RGBA support, JPEG and PNG with alpha preservation
• EXIF orientation support - rotates JPEG photos correctly
• Flexible crop system - anchor position, aspect ratio modes, custom ratios
• Edge handling modes - clamp, wrap, reflect, zero
• ML preprocessing - tensor normalization, HWC/CHW layouts, RGB/BGR ordering
• Async wrappers - every method has an *Async() variant using Isolate.run()
• File I/O and JPEG↔PNG format conversion without resizing
• Zero external Dart dependencies (only ffi)

The whole pipeline runs in native C - there is no Dart image-processing overhead. Operations are synchronous but extremely fast, and there is no memory copying between Dart and native code thanks to direct FFI.

• Resize a 4K JPEG to 224×224 in about 15-30 ms.
• Crop, resize and compress in a single pass.
• No memory copying between Dart and native (direct FFI).

A single call walks the image through five native stages. No Dart image libraries are involved at any point.

1. 1

   Decode

   stb_image decodes JPEG/PNG to raw pixels.

2. 2

   EXIF orientation

   For JPEG: parses EXIF metadata and applies the correct rotation/flip (optional).

3. 3

   Crop

   Extracts a region based on anchor position and aspect ratio mode.

4. 4

   Resize

   stb_image_resize2 applies bicubic interpolation with the selected edge mode.

5. 5

   Encode

   stb_image_write encodes back to JPEG/PNG.

Add the dependency to your pubspec.yaml:

dependencies:
  flutter_bicubic_resize: ^1.7.0

Or add it from the command line:

flutter pub add flutter_bicubic_resize

Every operation is a static method on BicubicResizer. Pass the encoded bytes and the target size - the return value is the resized, re-encoded bytes.

import 'package:flutter_bicubic_resize/flutter_bicubic_resize.dart';

final resized = BicubicResizer.resizeJpeg(
  jpegBytes: originalBytes,
  outputWidth: 224,
  outputHeight: 224,
  quality: 95, // optional, default 95
);

final resized = BicubicResizer.resizePng(
  pngBytes: originalBytes,
  outputWidth: 224,
  outputHeight: 224,
  compressionLevel: 6, // optional, 0-9 (default: 6)
);

These constraints come from pubspec.yaml at v1.7.0 and are authoritative - the README lists an older, looser baseline.

• Dart SDK >=3.5.0 <4.0.0, Flutter >=3.24.0.
• Android SDK 21+, iOS 13.0+, macOS 10.15+.
• FFI plugin platforms: Android, iOS and macOS (macOS added in 1.7.0).
• Only one external dependency: ffi: ^2.1.0.