pixel-scale

📐 Get the pixel scale of an image, or scale it up or down without quality loss. Useful for pixel art!

• 👀 Zero loss of quality
• 🤖 Auto-detects current scale by default
• ⏫ Can scale by multiplier or to a specific scale
• 0️⃣ Zero dependencies
• 🖼️ Works directly on ImageData

Table of contents

• Demo
• Installation
• Usage
  • scalePixels(imageData, to, options)
  • getPixelScale(imageData, options)
  • multiplyPixelScale(imageData, by, options)
  • dividePixelScale(imageData, by, options)
• About
  • ImageData
  • Pixel scale
  • How does it work?

Demo

https://pixel-scale.netlify.app

Installation

npm	yarn
npm install pixel-scale	yarn add pixel-scale

Usage

scalePixels(imageData, to, options)

Up- or downscales an image to the specified scale, without losing quality or changing any colors.

Parameters:

• imageData (ImageData instance) - The ImageData instance to scale.

• to (number) - The desired scale of the new image.

• options (object, optional)

  • from (number) - The current scale of the image. If no from value is provided, the scale is calculated with getPixelScale. Only provide a from value if you are sure of the current pixel scale and want to save time.
  • maxColorDiff (number, default: 0) - Passed along to getPixelScale if no from value is set, otherwise does nothing.

Return value:

A new, scaled ImageData-instance.

Examples:

import { scalePixels } from 'pixel-scale';

// detect an image's current scale, and rescale it to 1
const scale1ImageData = scalePixels(imageData, 1);

// increase an image's scale from 5 to 10
const scale10ImageData = scalePixels(imageData, 10, { from: 5 });

// detect an image's current scale, allowing a color diff of 10,
// and rescale it to 2
const scale2ImageData = scalePixels(imageData, 2, { maxColorDiff: 10 });

getPixelScale(imageData, options)

Get the current pixel scale of an image.

Parameters:

• imageData (ImageData instance) - The ImageData instance to scale.

• options (object, optional)

  • maxColorDiff (number, default: 0) - See scalePixels's maxColorDiff option.

Return value:

A number indicating the image's pixel scale.

Examples:

import { getPixelScale } from 'pixel-scale';

// get an image's pixel scale
const imageScale = getPixelScale(imageData);

// get an image's pixel scale, allowing a maximum difference of 10 when comparing
// color channels of individual pixels
const imageScale = getPixelScale(imageData, { maxColorDiff: 10 });

multiplyPixelScale(imageData, by, options)

Similar to scalePixels, but upscales the image by the specified multiplier instead of to a specific scale. Detects the current scale of the image if no options.from value is provided.

Parameters:

• imageData (ImageData instance) - The ImageData instance to upscale.

• by (number) - The amount to multiply the image's current scale by.

• options (object, optional) - See scalePixels's options.

Return value:

A new, scaled ImageData-instance.

Examples:

import { multiplyPixelScale } from 'pixel-scale';

// detect an image's current scale, and double it's size
const doubledImageData = multiplyPixelScale(imageData, 2);

// take an image of scale 5, and multiply it by 10
const tenfoldImageData = multiplyPixelScale(imageData, 10, { from: 5 });

dividePixelScale(imageData, by, options)

Similar to scalePixels, but downscales the image by the specified amount of times instead of to a specific scale. Detects the current scale of the image if no options.from value is provided.

Parameters:

• imageData (ImageData instance) - The ImageData instance to downscale.

• by (number) - The amount to divide the image's current scale by.

• options (object, optional) - See scalePixels's options.

Return value:

A new, scaled ImageData-instance.

Examples:

import { dividePixelScale } from 'pixel-scale';

// detect an image's current scale, and make it half as big
const doubledImageData = dividePixelScale(imageData, 2);

// take an image of scale 8 and divide it by 4
const tenfoldImageData = dividePixelScale(imageData, 4, { from: 8 });

About

ImageData

All functions operate on ImageData, which can be retrieved from a canvas in the browser or e.g. node-canvas or sharp on Node.

Pixel scale

The pixel scale referred to in this readme is the amount of times a pixel of e.g. a pixel art image has been multiplied to increase the image size. For example, this image has a pixel scale of 1, while this image has a pixel scale of 10.

How does it work?

To get the pixel scale of an image, pixel-scale first figures out the common divisors of the image's height and width. This is done using Euclid's algorithm for finding the greatest common divisor, and then counting down. A more performant method (e.g. prime factorization/Pollard rho) is not deemed necessary here, as image dimensions never go up to those sizes that big.

It then tests each common divisor, starting with the largest one, by chunking up the image equally sized pieces and verifying that they each chunk is in a solid color (scaled pixels). If a chunk contains a different color outside of the max allowed span (maxColorDiff), then it will go to the next divisor. If every chunk is a solid color, then the divisor is most likely the pixel scale of the image.