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https://github.com/nrkn/object-fit-math

Just the math behind object-fit and object-position
https://github.com/nrkn/object-fit-math

contain cover fill fit object-fit object-position scale-down

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Just the math behind object-fit and object-position

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README 

        
# object-fit-math



Just the math behind object-fit and object-position



It has no dependencies and just exposes a few simple functions



Allows you to use the same math as the browser uses for `object-fit` and

`object-position`



Supports the following `object-fit` properties:



`contain`, `cover`, `fill`, `none`, `scale-down`



Supports `object-position` specified in `px` or `%` only



Use cases:



- using the same algorithm as the browser elsewhere, like on an HTML canvas

  or server side image rendering

- finding out the bounds for an object that's using `object-fit` and

  `object-position` - this is what I actually wrote this for, so I could

  translate the mouse/touch event locations to the actual pixel location on a

  fitted `img`/`canvas`/`video` element

- you could use it to make a polyfill I guess



## usage



`npm install object-fit-math`



### basic usage



```js

const { fit, position, fitAndPosition } = require( 'object-fit-math' )



const parentSize = { width: 320, height: 240 }

const childSize = { width: 90, height: 160 }



const fitted = fit( parentSize, childSize, 'contain' )



// { width: 135, height: 240 }

console.log( fitted )



const positioned = position( parentSize, fitted, '50%', '50%' )



// { x: 92.5, y: 0 }

console.log( positioned )



const rect = fitAndPosition( parentSize, childSize, 'contain', '50%', '50%' )



// { x: 92.5, y: 0, width: 135, height: 240 }

console.log( rect )

```



### translate a point on the fitted element to original pixel location



```js

const { transformFittedPoint } = require( 'object-fit-math' )



// get the object eg a canvas that uses object-fit from the DOM here



canvas.addEventListener( 'click', e => {

  const { offsetX: x, offsetY: y } = e



  const canvasSize = canvas.getBoundingClientRect()

  const { objectFit, objectPosition } = getComputedStyle( canvas )

  const [ left, top ] = objectPosition.split( ' ' )



  const childPoint = transformFittedPoint(

    { x, y }, canvasSize, canvas, objectFit, left, top

  )



  console.log( childPoint )

})

```



## types



See [/src/types.ts](/src/types.ts) for `Size`, `Point` etc



### exported functions



```ts

fit = ( parent: Size, child: Size, fitMode: FitMode = 'fill' ) => Size



position = ( parent: Size, child: Size, left = '50%', top = '50%' ) => Point



fitAndPosition = (

  parent: Size, child: Size,

  fitMode: FitMode = 'fill', left = '50%', top = '50%'

) => Rect



transformFittedPoint = (

  fittedPoint: Point, parent: Size, child: Size,

  fitMode: FitMode = 'fill', left = '50%', top = '50%'

) => Point



isFit = ( value: any ): value is FitMode => boolean

```



## examples



### transform event point to pixel



The [transform example](/examples/transform/) creates a small canvas then

fits it to its parent element - you can then click anywhere on the canvas to

get the actual pixel location. Useful for finding out the actual pixel that was

clicked on a `video`/`canvas`/`img` which uses `object-fit`



### compare



The [compare example](/examples/compare/) generates a long list of

combinations of parent and child sizes, fit modes, positions etc



Each comparison shows the result of the DOM's built in `object-fit` and

`object-position` on the left, and on the right it uses this module to get the

rectangle and uses absolute positioning and sizing to replicate the effect



## building



You will need `typescript` installed globally to build the module, and

`browserify` installed globally to build the examples



`npm install typescript -g`



`npm run build`



`npm install browserify -g`



`npm run build-examples`



## contributing



Please try to follow the existing style (sorry, no linting etc at the moment)

and keep your pull requests small and focussed



### testing your changes



The easiest way to tell if your changes are correct is to build the examples

and then scroll through the compare example - yes, this file is huge, but it

ensures that a good combination of inputs is represented. Once you verify

visually that the math matches the DOM behaviour, there is a `
` element at

the bottom that contains the fixture data used by the tests, copy and paste this

to [/src/test/fixtures.json](/src/test/fixtures.json) and run the tests.



## license



MIT License



Copyright (c) 2020 Nik Coughlin



Permission is hereby granted, free of charge, to any person obtaining a copy

of this software and associated documentation files (the "Software"), to deal

in the Software without restriction, including without limitation the rights

to use, copy, modify, merge, publish, distribute, sublicense, and/or sell

copies of the Software, and to permit persons to whom the Software is

furnished to do so, subject to the following conditions:



The above copyright notice and this permission notice shall be included in all

copies or substantial portions of the Software.



THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR

IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,

FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE

AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER

LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,

OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE

SOFTWARE.