resize

README

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Resize

Image resizing for the Go programming language with common interpolation methods.

Installation

$ go get github.com/nfnt/resize

It's that easy!

Usage

This package needs at least Go 1.1. Import package with

import "github.com/nfnt/resize"

The resize package provides 2 functions:

• resize.Resize creates a scaled image with new dimensions (width, height) using the interpolation function interp. If either width or height is set to 0, it will be set to an aspect ratio preserving value.
• resize.Thumbnail downscales an image preserving its aspect ratio to the maximum dimensions (maxWidth, maxHeight). It will return the original image if original sizes are smaller than the provided dimensions.

resize.Resize(width, height uint, img image.Image, interp resize.InterpolationFunction) image.Image
resize.Thumbnail(maxWidth, maxHeight uint, img image.Image, interp resize.InterpolationFunction) image.Image

The provided interpolation functions are (from fast to slow execution time)

• NearestNeighbor: Nearest-neighbor interpolation
• Bilinear: Bilinear interpolation
• Bicubic: Bicubic interpolation
• MitchellNetravali: Mitchell-Netravali interpolation
• Lanczos2: Lanczos resampling with a=2
• Lanczos3: Lanczos resampling with a=3

Which of these methods gives the best results depends on your use case.

Sample usage:

package main

import (
	"github.com/nfnt/resize"
	"image/jpeg"
	"log"
	"os"
)

func main() {
	// open "test.jpg"
	file, err := os.Open("test.jpg")
	if err != nil {
		log.Fatal(err)
	}

	// decode jpeg into image.Image
	img, err := jpeg.Decode(file)
	if err != nil {
		log.Fatal(err)
	}
	file.Close()

	// resize to width 1000 using Lanczos resampling
	// and preserve aspect ratio
	m := resize.Resize(1000, 0, img, resize.Lanczos3)

	out, err := os.Create("test_resized.jpg")
	if err != nil {
		log.Fatal(err)
	}
	defer out.Close()

	// write new image to file
	jpeg.Encode(out, m, nil)
}

Caveats

• Optimized access routines are used for image.RGBA, image.NRGBA, image.RGBA64, image.NRGBA64, image.YCbCr, image.Gray, and image.Gray16 types. All other image types are accessed in a generic way that will result in slow processing speed.
• JPEG images are stored in image.YCbCr. This image format stores data in a way that will decrease processing speed. A resize may be up to 2 times slower than with image.RGBA.

Downsizing Samples

Downsizing is not as simple as it might look like. Images have to be filtered before they are scaled down, otherwise aliasing might occur. Filtering is highly subjective: Applying too much will blur the whole image, too little will make aliasing become apparent. Resize tries to provide sane defaults that should suffice in most cases.

Artificial sample

Original image

Nearest-Neighbor	Bilinear
Bicubic	Mitchell-Netravali
Lanczos2	Lanczos3

Real-Life sample

Original image

Nearest-Neighbor	Bilinear
Bicubic	Mitchell-Netravali
Lanczos2	Lanczos3

License

Copyright (c) 2012 Jan Schlicht janschlicht@gmail.com Resize is released under a MIT style license.

Documentation

Overview

Package resize implements various image resizing methods.

The package works with the Image interface described in the image package. Various interpolation methods are provided and multiple processors may be utilized in the computations.

Example:

imgResized := resize.Resize(1000, 0, imgOld, resize.MitchellNetravali)

Index

• func Resize(width, height uint, img image.Image, interp InterpolationFunction) image.Image
• func ResizeFile(inputFile string, outputFile string, maxWidth, maxHeight int, ...) error
• func Thumbnail(maxWidth, maxHeight uint, img image.Image, interp InterpolationFunction) image.Image
• type InterpolationFunction

Functions

func Resize

func Resize(width, height uint, img image.Image, interp InterpolationFunction) image.Image

Resize scales an image to new width and height using the interpolation function interp. A new image with the given dimensions will be returned. If one of the parameters width or height is set to 0, its size will be calculated so that the aspect ratio is that of the originating image. The resizing algorithm uses channels for parallel computation. If the input image has width or height of 0, it is returned unchanged.

func ResizeFile

func ResizeFile(inputFile string, outputFile string, maxWidth, maxHeight int, interp InterpolationFunction, outputType string) error

ResizeFile - resizes the inputFile to outputFile with type outPutType and width and height if the maxWidth and/or maxHeight is less than the image's. Also, if outputType is different than the inputFile type the image will be converted to that output type (jpg, png).

func Thumbnail

func Thumbnail(maxWidth, maxHeight uint, img image.Image, interp InterpolationFunction) image.Image

Thumbnail will downscale provided image to max width and height preserving original aspect ratio and using the interpolation function interp. It will return original image, without processing it, if original sizes are already smaller than provided constraints.

Types

type InterpolationFunction

type InterpolationFunction int

An InterpolationFunction provides the parameters that describe an interpolation kernel. It returns the number of samples to take and the kernel function to use for sampling.

const (
	// Nearest-neighbor interpolation
	NearestNeighbor InterpolationFunction = iota
	// Bilinear interpolation
	Bilinear
	// Bicubic interpolation (with cubic hermite spline)
	Bicubic
	// Mitchell-Netravali interpolation
	MitchellNetravali
	// Lanczos interpolation (a=2)
	Lanczos2
	// Lanczos interpolation (a=3)
	Lanczos3
)

InterpolationFunction constants