Resize image with javascript canvas smoothly

Question

I m trying to resize some images with canvas but I m clueless on how to smoothen them  On photoshop  browsers etc   there are a few algorithms they use  e g  bicubic  bilinear  but I don t know if these are built into canvas or not   Here s my fiddle  http   jsfiddle net EWupT   var canvas   document createElement  canvas    var ctx   canvas getContext  2d    canvas width 300 canvas height 234 ctx drawImage img  0  0  300  234   document body appendChild canvas     The first one is a normal resized image tag  and the second one is canvas  Notice how the canvas one is not as smooth  How can I achieve  smoothness

User · Accepted Answer

You can use down-stepping to achieve better results. Most browsers seem to use linear interpolation rather than bi-cubic when resizing images.

(Update There has been added a quality property to the specs, imageSmoothingQuality which is currently available in Chrome only.)

Unless one chooses no smoothing or nearest neighbor the browser will always interpolate the image after down-scaling it as this function as a low-pass filter to avoid aliasing.

Bi-linear uses 2x2 pixels to do the interpolation while bi-cubic uses 4x4 so by doing it in steps you can get close to bi-cubic result while using bi-linear interpolation as seen in the resulting images.

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var canvas = document.getElementById("canvas");_x000D_
var ctx = canvas.getContext("2d");_x000D_
var img = new Image();_x000D_
_x000D_
img.onload = function () {_x000D_
_x000D_
    // set size proportional to image_x000D_
    canvas.height = canvas.width * (img.height / img.width);_x000D_
_x000D_
    // step 1 - resize to 50%_x000D_
    var oc = document.createElement('canvas'),_x000D_
        octx = oc.getContext('2d');_x000D_
_x000D_
    oc.width = img.width * 0.5;_x000D_
    oc.height = img.height * 0.5;_x000D_
    octx.drawImage(img, 0, 0, oc.width, oc.height);_x000D_
_x000D_
    // step 2_x000D_
    octx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5);_x000D_
_x000D_
    // step 3, resize to final size_x000D_
    ctx.drawImage(oc, 0, 0, oc.width * 0.5, oc.height * 0.5,_x000D_
    0, 0, canvas.width, canvas.height);_x000D_
}_x000D_
img.src = "//i.imgur.com/SHo6Fub.jpg";

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<img src="//i.imgur.com/SHo6Fub.jpg" width="300" height="234">_x000D_
<canvas id="canvas" width=300></canvas>

_x000D_

Depending on how drastic your resize is you can might skip step 2 if the difference is less.

In the demo you can see the new result is now much similar to the image element.

User · Answer

While some of those code-snippets are short and working  they aren t trivial to follow and understand   As i am not a fan of  copy-paste  from stack-overflow  i would like developers to understand the code they are push into they software  hope you ll find the below useful   DEMO  Resizing images with JS and HTML Canvas Demo fiddler   You may find 3 different methods to do this resize  that will help you understand how the code is working and why   https   jsfiddle net 1b68eLdr 93089   Full code of both demo  and TypeScript method that you may want to use in your code  can be found in the GitHub project   https   github com eyalc4 ts-image-resizer  This is the final code   export class ImageTools   base64ResizedImage  string   null   constructor        ResizeImage base64image  string  width  number   1080  height  number   1080        let img   new Image        img src   base64image       img onload        gt                Check if the image require resize at all         if img height  lt   height  amp  amp  img width  lt   width                this base64ResizedImage   base64image                  TODO  Call method to do something with the resize image                   else                  Make sure the width and height preserve the original aspect ratio and adjust if needed             if img height  gt  img width                    width   Math floor height    img width   img height                              else                   height   Math floor width    img height   img width                               let resizingCanvas  HTMLCanvasElement   document createElement  canvas                let resizingCanvasContext   resizingCanvas getContext  2d                    Start with original image size             resizingCanvas width   img width              resizingCanvas height   img height                   Draw the original image on the  temp  resizing canvas             resizingCanvasContext drawImage img  0  0  resizingCanvas width  resizingCanvas height                let curImageDimensions                     width  Math floor img width                   height  Math floor img height                              let halfImageDimensions                     width  null                  height  null                                Quickly reduce the dize by 50  each time in few iterations until the size is less then                2x time the target size - the motivation for it  is to reduce the aliasing that would have been                created with direct reduction of very big image to small image             while  curImageDimensions width   0 5  gt  width                       Reduce the resizing canvas by half and refresh the image                 halfImageDimensions width   Math floor curImageDimensions width   0 5                   halfImageDimensions height   Math floor curImageDimensions height   0 5                    resizingCanvasContext drawImage resizingCanvas  0  0  curImageDimensions width  curImageDimensions height                      0  0  halfImageDimensions width  halfImageDimensions height                    curImageDimensions width   halfImageDimensions width                  curImageDimensions height   halfImageDimensions height                                Now do final resize for the resizingCanvas to meet the dimension requirments                directly to the output canvas  that will output the final image             let outputCanvas  HTMLCanvasElement   document createElement  canvas                let outputCanvasContext   outputCanvas getContext  2d                 outputCanvas width   width              outputCanvas height   height               outputCanvasContext drawImage resizingCanvas  0  0  curImageDimensions width  curImageDimensions height                  0  0  width  height                   output the canvas pixels as an image  params  format  quality             this base64ResizedImage   outputCanvas toDataURL  image jpeg   0 85                   TODO  Call method to do something with the resize image

User · Answer

I wrote small js-utility to crop and resize image on front-end  Here is link on GitHub project  Also you can get blob from final image to send it   import imageSqResizer from    image-square-resizer js   let resizer   new imageSqResizer       image-input       300       dataUrl    gt           document getElementById  image-output   src   dataUrl       Get blob let formData   new FormData    formData append  files 0    resizer blob      get dataUrl document getElementById  image-output   src   resizer dataUrl

User · Answer

Based on K3N answer  I rewrite code generally for anyone wants  var oc   document createElement  canvas    octx   oc getContext  2d        oc width   img width      oc height   img height      octx drawImage img  0  0       while  oc width   0 5  gt  width           oc width    0 5         oc height    0 5         octx drawImage oc  0  0  oc width  oc height             oc width   width      oc height   oc width   img height   img width      octx drawImage img  0  0  oc width  oc height     UPDATE JSFIDDLE DEMO  Here is my ONLINE DEMO

User · Answer

I created a reusable Angular service to handle high quality resizing of images   canvases for anyone who s interested  https   gist github com transitive-bullshit 37bac5e741eaec60e983  The service includes two solutions because they both have their own pros   cons  The lanczos convolution approach is higher quality at the cost of being slower  whereas the step-wise downscaling approach produces reasonably antialiased results and is significantly faster   Example usage   angular module  demo   controller  ExampleCtrl   function  imageService         EXAMPLE USAGE      NOTE  it s bad practice to access the DOM inside a controller        but this is just to show the example usage        resize by lanczos-sinc filter   imageService resize     myimg   0   256  256       then function  resizedImage             do something with resized image              resize by stepping down image size in increments of 2x   imageService resizeStep     myimg   0   256  256       then function  resizedImage             do something with resized image

User · Answer

Since Trung Le Nguyen Nhat s fiddle isn t correct at all   it just uses the original image in the last step  I wrote my own general fiddle with performance comparison   FIDDLE  Basically it s   img onload   function        var canvas   document createElement  canvas           ctx   canvas getContext  2d           oc   document createElement  canvas           octx   oc getContext  2d        canvas width   width     destination canvas size    canvas height   canvas width   img height   img width      var cur          width  Math floor img width   0 5        height  Math floor img height   0 5           oc width   cur width     oc height   cur height      octx drawImage img  0  0  cur width  cur height       while  cur width   0 5  gt  width         cur            width  Math floor cur width   0 5          height  Math floor cur height   0 5               octx drawImage oc  0  0  cur width   2  cur height   2  0  0  cur width  cur height            ctx drawImage oc  0  0  cur width  cur height  0  0  canvas width  canvas height

User · Answer

I don t understand why nobody is suggesting createImageBitmap    createImageBitmap      document getElementById  image           resizeWidth  300  resizeHeight  234  resizeQuality   high       then imageBitmap   gt       document getElementById  canvas   getContext  2d   drawImage imageBitmap  0  0       works beautifully  assuming you set ids for image and canvas

User · Answer

I created a library that allows you to downstep any percentage while keeping all the color data   https   github com danschumann limby-resize blob master lib canvas resize js  That file you can include in the browser   The results will look like photoshop or image magick  preserving all the color data  averaging pixels  rather than taking nearby ones and dropping others   It doesn t use a formula to guess the averages  it takes the exact average

[javascript] Resize image with javascript canvas (smoothly)

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