flutter_bicubic_resize 1.2.1

Fast, consistent bicubic image resizing across iOS and Android using native C code with Catmull-Rom interpolation.

flutter_bicubic_resize

Fast, consistent bicubic image resizing for Flutter.

Features

• 100% Native C performance (stb_image + stb_image_resize + stb_image_write)
• Identical results on iOS and Android
• Bicubic interpolation (Catmull-Rom, same as OpenCV)
• Full native pipeline: decode -> resize -> encode (no Dart image libraries)
• RGB and RGBA support
• JPEG and PNG support with alpha channel preservation
• EXIF orientation support - automatically rotates JPEG images correctly
• 1:1 aspect ratio crop - square center crop without stretching
• Zero external Dart dependencies (only ffi)

Installation

Add to your pubspec.yaml:

dependencies:
  flutter_bicubic_resize: ^1.2.1

Or run:

flutter pub add flutter_bicubic_resize

Usage

Resize JPEG

import 'package:flutter_bicubic_resize/flutter_bicubic_resize.dart';

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

Resize PNG

final resized = BicubicResizer.resizePng(
  pngBytes: originalBytes,
  outputWidth: 224,
  outputHeight: 224,
);

Resize raw RGB/RGBA bytes

// RGB (3 bytes per pixel)
final resizedRgb = BicubicResizer.resizeRgb(
  input: rgbBytes,
  inputWidth: 1920,
  inputHeight: 1080,
  outputWidth: 224,
  outputHeight: 224,
);

// RGBA (4 bytes per pixel)
final resizedRgba = BicubicResizer.resizeRgba(
  input: rgbaBytes,
  inputWidth: 1920,
  inputHeight: 1080,
  outputWidth: 224,
  outputHeight: 224,
);

Custom filter selection

// Use different bicubic filter
final resized = BicubicResizer.resizeJpeg(
  jpegBytes: originalBytes,
  outputWidth: 224,
  outputHeight: 224,
  filter: BicubicFilter.mitchell, // or .cubicBSpline
);

Available filters:

• BicubicFilter.catmullRom - Default. Same as OpenCV/PIL. Best for ML.
• BicubicFilter.cubicBSpline - Smoother, more blurry.
• BicubicFilter.mitchell - Balanced between sharp and smooth.

Center crop (1:1 aspect ratio)

The crop parameter extracts a square region from the center of the image, ensuring no stretching or distortion.

// Crop square from center, then resize to 224x224
final cropped = BicubicResizer.resizeJpeg(
  jpegBytes: originalBytes,
  outputWidth: 224,
  outputHeight: 224,
  crop: 1.0, // Full square (min dimension)
);

How it works:

For an image 1920x1080:

• crop: 1.0 → crops 1080x1080 square from center
• crop: 0.8 → crops 864x864 square from center (80% of 1080)
• crop: 0.5 → crops 540x540 square from center (50% of 1080)

The crop always uses the minimum dimension to ensure a perfect square without stretching.

Why?

Default platform APIs use different algorithms:

• Android: Typically Bilinear
• iOS: Depends on context (Lanczos, Bilinear, etc.)

This package uses the same C code on both platforms, ensuring identical output for the same input.

Architecture

The entire image processing pipeline runs in native C code:

1. Decode - stb_image decodes JPEG/PNG to raw pixels
2. EXIF orientation - For JPEG: parses EXIF metadata and applies correct rotation/flip
3. Center crop - Extracts square region from center (1:1 aspect ratio)
4. Resize - stb_image_resize2 applies bicubic interpolation
5. Encode - stb_image_write encodes back to JPEG/PNG

This means:

• No Dart image libraries needed
• Minimal memory overhead
• Maximum performance
• Consistent results across platforms
• Photos from mobile cameras display correctly (no rotation issues)

Algorithm

Uses stb_image_resize2 with STBIR_FILTER_CATMULLROM (Catmull-Rom spline).

This is the same algorithm used by:

• OpenCV cv2.INTER_CUBIC
• PIL/Pillow Image.BICUBIC

Perfect for ML preprocessing (OpenCLIP, ResNet, etc.) where consistent results with training pipeline matter.

Performance

The entire pipeline is native C, making it significantly faster than pure Dart solutions. Operations are synchronous but very fast due to native performance.

Requirements

• Flutter 3.0+
• Android SDK 21+
• iOS 11.0+

Sponsor

CODIGEE.COM

Documentation

• API Reference - Complete API documentation
• Example App - Working demo application

License

MIT License - see LICENSE file.