HTML5 Canvas Resize Downscale Image High Quality

Question

I use html5 canvas elements to resize images im my browser  It turns out that the quality is very low  I found this  Disable Interpolation when Scaling a  lt canvas gt  but it does not help to increase the quality   Below is my css and js code as well as the image scalled with Photoshop and scaled in the canvas API    What do I have to do to get optimal quality when scaling an image in the browser   Note  I want to scale down a large image to a small one  modify color in a canvas and send the result from the canvas to the server   CSS    canvas  img       image-rendering  optimizeQuality      image-rendering  -moz-crisp-edges      image-rendering  -webkit-optimize-contrast      image-rendering  optimize-contrast      -ms-interpolation-mode  nearest-neighbor      JS    var  img       lt img gt     var  originalCanvas       lt canvas gt      img load function          var originalContext    originalCanvas 0  getContext  2d          originalContext imageSmoothingEnabled   false     originalContext webkitImageSmoothingEnabled   false     originalContext mozImageSmoothingEnabled   false     originalContext drawImage this  0  0  379  500         The image resized with photoshop     The image resized on canvas     Edit    I tried to make downscaling in more than one steps as proposed in   Resizing an image in an HTML5 canvas and Html5 canvas drawImage  how to apply antialiasing  This is the function I have used    function resizeCanvasImage img  canvas  maxWidth  maxHeight        var imgWidth   img width           imgHeight   img height       var ratio   1  ratio1   1  ratio2   1      ratio1   maxWidth   imgWidth      ratio2   maxHeight   imgHeight          Use the smallest ratio that the image best fit into the maxWidth x maxHeight box      if  ratio1  lt  ratio2            ratio   ratio1            else           ratio   ratio2             var canvasContext   canvas getContext  2d        var canvasCopy   document createElement  canvas        var copyContext   canvasCopy getContext  2d        var canvasCopy2   document createElement  canvas        var copyContext2   canvasCopy2 getContext  2d        canvasCopy width   imgWidth      canvasCopy height   imgHeight        copyContext drawImage img  0  0           init     canvasCopy2 width   imgWidth      canvasCopy2 height   imgHeight              copyContext2 drawImage canvasCopy  0  0  canvasCopy width  canvasCopy height  0  0  canvasCopy2 width  canvasCopy2 height         var rounds   2      var roundRatio   ratio   rounds      for  var i   1  i  lt   rounds  i              console log  Step    i               tmp         canvasCopy width   imgWidth   roundRatio   i          canvasCopy height   imgHeight   roundRatio   i           copyContext drawImage canvasCopy2  0  0  canvasCopy2 width  canvasCopy2 height  0  0  canvasCopy width  canvasCopy height               copy back         canvasCopy2 width   imgWidth   roundRatio   i          canvasCopy2 height   imgHeight   roundRatio   i          copyContext2 drawImage canvasCopy  0  0  canvasCopy width  canvasCopy height  0  0  canvasCopy2 width  canvasCopy2 height             end for          copy back to canvas     canvas width   imgWidth   roundRatio   rounds      canvas height   imgHeight   roundRatio   rounds      canvasContext drawImage canvasCopy2  0  0  canvasCopy2 width  canvasCopy2 height  0  0  canvas width  canvas height         Here is the result if I use a 2 step down sizing      Here is the result if I use a 3 step down sizing      Here is the result if I use a 4 step down sizing      Here is the result if I use a 20 step down sizing      Note  It turns out that from 1 step to 2 steps there is a large improvement in image quality but the more steps you add to the process the more fuzzy the image becomes   Is there a way to solve the problem that the image gets more fuzzy the more steps you add     Edit 2013-10-04  I tried the algorithm of GameAlchemist  Here is the result compared to Photoshop   PhotoShop Image        GameAlchemist s Algorithm

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instead of  85  if we add 1 0  You will get exact answer   data canvas toDataURL  image jpeg   1 0     You can get clear and bright image  Please check

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Fast canvas resample with good quality  http   jsfiddle net 9g9Nv 442   Update  version 2 0  faster  web workers   transferable objects  - https   github com viliusle Hermite-resize         Hermite resize - fast image resize resample using Hermite filter  1 cpu version          param  HtmlElement  canvas     param  int  width     param  int  height     param  boolean  resize canvas if true  canvas will be resized  Optional      function resample single canvas  width  height  resize canvas        var width source   canvas width      var height source   canvas height      width   Math round width       height   Math round height        var ratio w   width source   width      var ratio h   height source   height      var ratio w half   Math ceil ratio w   2       var ratio h half   Math ceil ratio h   2        var ctx   canvas getContext  2d        var img   ctx getImageData 0  0  width source  height source       var img2   ctx createImageData width  height       var data   img data      var data2   img2 data       for  var j   0  j  lt  height  j              for  var i   0  i  lt  width  i                  var x2    i   j   width    4              var weight   0              var weights   0              var weights alpha   0              var gx r   0              var gx g   0              var gx b   0              var gx a   0              var center y    j   0 5    ratio h              var yy start   Math floor j   ratio h               var yy stop   Math ceil  j   1    ratio h               for  var yy   yy start  yy  lt  yy stop  yy                      var dy   Math abs center y -  yy   0 5     ratio h half                  var center x    i   0 5    ratio w                  var w0   dy   dy    pre-calc part of w                 var xx start   Math floor i   ratio w                   var xx stop   Math ceil  i   1    ratio w                   for  var xx   xx start  xx  lt  xx stop  xx                          var dx   Math abs center x -  xx   0 5     ratio w half                      var w   Math sqrt w0   dx   dx                       if  w  gt   1                              pixel too far                         continue                                              hermite filter                     weight   2   w   w   w - 3   w   w   1                      var pos x   4    xx   yy   width source                         alpha                     gx a    weight   data pos x   3                       weights alpha    weight                        colors                     if  data pos x   3   lt  255                          weight   weight   data pos x   3    250                      gx r    weight   data pos x                       gx g    weight   data pos x   1                       gx b    weight   data pos x   2                       weights    weight                                              data2 x2    gx r   weights              data2 x2   1    gx g   weights              data2 x2   2    gx b   weights              data2 x2   3    gx a   weights alpha                        clear and resize canvas     if  resize canvas     true            canvas width   width          canvas height   height        else           ctx clearRect 0  0  width source  height source                draw     ctx putImageData img2  0  0

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This is the improved Hermite resize filter that utilises 1 worker so that the window doesn t freeze   https   github com calvintwr blitz-hermite-resize  const blitz   Blitz create       Promise    blitz       source  DOM Image DOM Canvas jQuery DataURL File      width  400      height  600    then output   gt           handle output   catch error   gt           handle error        Await    let resized   await blizt            Old school callback    const blitz   Blitz create  callback   blitz        function output           run your callback

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Since your problem is to downscale your image  there is no point in talking about interpolation -which is about creating pixel-  The issue here is downsampling     To downsample an image  we need to turn each square of p   p pixels in the original image into a single pixel in the destination image     For performances reasons Browsers do a very simple downsampling   to build the smaller image  they will just pick ONE pixel in the source and use its value for the destination  which  forgets  some details and adds noise     Yet there s an exception to that   since the 2X image downsampling is very simple to compute  average 4 pixels to make one  and is used for retina HiDPI pixels  this case is handled properly -the Browser does make use of 4 pixels to make one-     BUT    if you use several time a 2X downsampling  you ll face the issue that the successive rounding errors will add too much noise  What s worse  you won t always resize by a power of two  and resizing to the nearest power   a last resizing is very noisy   What you seek is a pixel-perfect downsampling  that is   a re-sampling of the image that will take all input pixels into account -whatever the scale-  To do that we must compute  for each input pixel  its contribution to one  two  or four destination pixels depending wether the scaled projection of the input pixels is right inside a destination pixels  overlaps an X border  an Y border  or both    A scheme would be nice here  but i don t have one       Here s an example of canvas scale vs my pixel perfect scale on a 1 3 scale of a zombat   Notice that the picture might get scaled in your Browser  and is  jpegized by S O   Yet we see that there s much less noise especially in the grass behind the wombat  and the branches on its right  The noise in the fur makes it more contrasted  but it looks like he s got white hairs -unlike source picture-  Right image is less catchy but definitively nicer     Here s the code to do the pixel perfect downscaling    fiddle result    http   jsfiddle net gamealchemist r6aVp embedded result  fiddle itself   http   jsfiddle net gamealchemist r6aVp      scales the image by  float  scale  lt  1    returns a canvas containing the scaled image  function downScaleImage img  scale        var imgCV   document createElement  canvas        imgCV width   img width      imgCV height   img height      var imgCtx   imgCV getContext  2d        imgCtx drawImage img  0  0       return downScaleCanvas imgCV  scale         scales the canvas by  float  scale  lt  1    returns a new canvas containing the scaled image  function downScaleCanvas cv  scale        if    scale  lt  1       scale  gt  0   throw   scale must be a positive number  lt 1         var sqScale   scale   scale     square scale   area of source pixel within target     var sw   cv width     source image width     var sh   cv height     source image height     var tw   Math floor sw   scale      target image width     var th   Math floor sh   scale      target image height     var sx   0  sy   0  sIndex   0     source x y  index within source array     var tx   0  ty   0  yIndex   0  tIndex   0     target x y  x y index within target array     var tX   0  tY   0     rounded tx  ty     var w   0  nw   0  wx   0  nwx   0  wy   0  nwy   0     weight   next weight x   y        weight is weight of current source point within target         next weight is weight of current source point within next target s point      var crossX   false     does scaled px cross its current px right border       var crossY   false     does scaled px cross its current px bottom border       var sBuffer   cv getContext  2d        getImageData 0  0  sw  sh  data     source buffer 8 bit rgba     var tBuffer   new Float32Array 3   tw   th      target buffer Float32 rgb     var sR   0  sG   0   sB   0     source s current point r g b        untested       var sA   0     source alpha              for  sy   0  sy  lt  sh  sy              ty   sy   scale     y src position within target         tY   0   ty         rounded   target pixel s y         yIndex   3   tY   tw      line index within target array         crossY    tY     0   ty   scale             if  crossY       if pixel is crossing botton target pixel             wy    tY   1 - ty      weight of point within target pixel             nwy    ty   scale - tY - 1          within y 1 target pixel                   for  sx   0  sx  lt  sw  sx    sIndex    4                tx   sx   scale     x src position within target             tX   0     tx        rounded   target pixel s x             tIndex   yIndex   tX   3     target pixel index within target array             crossX    tX     0   tx   scale                if  crossX       if pixel is crossing target pixel s right                 wx    tX   1 - tx      weight of point within target pixel                 nwx    tx   scale - tX - 1          within x 1 target pixel                           sR   sBuffer sIndex            retrieving r g b for curr src px              sG   sBuffer sIndex   1               sB   sBuffer sIndex   2                      untested   handling alpha                   sA   sBuffer sIndex   3                  if   sA  continue                 if  sA    0xFF                       sR    sR   sA   gt  gt  8      or use  256 instead                       sG    sG   sA   gt  gt  8                     sB    sB   sA   gt  gt  8                                              if   crossX  amp  amp   crossY       pixel does not cross                    just add components weighted by squared scale                  tBuffer tIndex         sR   sqScale                  tBuffer tIndex   1     sG   sqScale                  tBuffer tIndex   2     sB   sqScale                else if  crossX  amp  amp   crossY       cross on X only                 w   wx   scale                     add weighted component for current px                 tBuffer tIndex         sR   w                  tBuffer tIndex   1     sG   w                  tBuffer tIndex   2     sB   w                     add weighted component for next  tX 1  px                                 nw   nwx   scale                 tBuffer tIndex   3     sR   nw                  tBuffer tIndex   4     sG   nw                  tBuffer tIndex   5     sB   nw                else if  crossY  amp  amp   crossX       cross on Y only                 w   wy   scale                     add weighted component for current px                 tBuffer tIndex         sR   w                  tBuffer tIndex   1     sG   w                  tBuffer tIndex   2     sB   w                     add weighted component for next  tY 1  px                                 nw   nwy   scale                 tBuffer tIndex   3   tw         sR   nw                  tBuffer tIndex   3   tw   1     sG   nw                  tBuffer tIndex   3   tw   2     sB   nw                else      crosses both x and y   four target points involved                    add weighted component for current px                 w   wx   wy                  tBuffer tIndex         sR   w                  tBuffer tIndex   1     sG   w                  tBuffer tIndex   2     sB   w                     for tX   1  tY px                 nw   nwx   wy                  tBuffer tIndex   3     sR   nw                  tBuffer tIndex   4     sG   nw                  tBuffer tIndex   5     sB   nw                     for tX   tY   1 px                 nw   wx   nwy                  tBuffer tIndex   3   tw         sR   nw                  tBuffer tIndex   3   tw   1     sG   nw                  tBuffer tIndex   3   tw   2     sB   nw                     for tX   1   tY  1 px                 nw   nwx   nwy                  tBuffer tIndex   3   tw   3     sR   nw                  tBuffer tIndex   3   tw   4     sG   nw                  tBuffer tIndex   3   tw   5     sB   nw                             end for sx           end for sy         create result canvas     var resCV   document createElement  canvas        resCV width   tw      resCV height   th      var resCtx   resCV getContext  2d        var imgRes   resCtx getImageData 0  0  tw  th       var tByteBuffer   imgRes data         convert float32 array into a UInt8Clamped Array     var pxIndex   0           for  sIndex   0  tIndex   0  pxIndex  lt  tw   th  sIndex    3  tIndex    4  pxIndex              tByteBuffer tIndex    Math ceil tBuffer sIndex            tByteBuffer tIndex   1    Math ceil tBuffer sIndex   1            tByteBuffer tIndex   2    Math ceil tBuffer sIndex   2            tByteBuffer tIndex   3    255               writing result to canvas      resCtx putImageData imgRes  0  0       return resCV      It is quite memory greedy  since a float buffer is required to store the intermediate values of the destination image  -  if we count the result canvas  we use 6 times the source image s memory in this algorithm   It is also quite expensive  since each source pixel is used whatever the destination size  and we have to pay for the getImageData   putImageDate  quite slow also  But there s no way to be faster than process each source value in this case  and situation is not that bad   For my 740   556 image of a wombat  processing takes between 30 and 40 ms

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Why use the canvas to resize images  Modern browsers all use bicubic interpolation     the same process used by Photoshop  if you re doing it right      and they do it faster than the canvas process  Just specify the image size you want  use only one dimension  height or width  to resize proportionally    This is supported by most browsers  including later versions of IE  Earlier versions may require browser-specific CSS   A simple function  using jQuery  to resize an image would be like this   function resizeImage img  percentage        var coeff   percentage 100          width     img  width            height     img  height         return   width   width coeff   height   height coeff                 Then just use the returned value to resize the image in one or both dimensions   Obviously there are different refinements you could make  but this gets the job done   Paste the following code into the console of this page and watch what happens to the gravatars   function resizeImage img  percentage        var coeff   percentage 100          width     img  width            height     img  height         return   width   width coeff   height   height coeff                    user-gravatar32 img   each function      var newDimensions   resizeImage  this  150     this style width   newDimensions width    px     this style height   newDimensions height    px

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Suggestion 1 - extend the process pipe-line You can use step-down as I describe in the links you refer to but you appear to use them in a wrong way  Step down is not needed to scale images to ratios above 1 2  typically  but not limited to   It is where you need to do a drastic down-scaling you need to split it up in two  and rarely  more  steps depending on content of the image  in particular where high-frequencies such as thin lines occur   Every time you down-sample an image you will loose details and information  You cannot expect the resulting image to be as clear as the original  If you are then scaling down the images in many steps you will loose a lot of information in total and the result will be poor as you already noticed  Try with just one extra step  or at tops two  Convolutions In case of Photoshop notice that it applies a convolution after the image has been re-sampled  such as sharpen  It s not just bi-cubic interpolation that takes place so in order to fully emulate Photoshop we need to also add the steps Photoshop is doing  with the default setup   For this example I will use my original answer that you refer to in your post  but I have added a sharpen convolution to it to improve quality as a post process  see demo at bottom   Here is code for adding sharpen filter  it s based on a generic convolution filter - I put the weight matrix for sharpen inside it as well as a mix factor to adjust the pronunciation of the effect   Usage  sharpen context  width  height  mixFactor    The mixFactor is a value between  0 0  1 0  and allow you do downplay the sharpen effect - rule-of-thumb  the less size the less of the effect is needed  Function  based on this snippet   function sharpen ctx  w  h  mix         var weights     0  -1  0   -1  5  -1   0  -1  0           katet   Math round Math sqrt weights length            half    katet   0 5   0          dstData   ctx createImageData w  h           dstBuff   dstData data          srcBuff   ctx getImageData 0  0  w  h  data          y   h               while y--             x   w           while x--                 var sy   y                  sx   x                  dstOff    y   w   x    4                  r   0  g   0  b   0  a   0               for  var cy   0  cy  lt  katet  cy                      for  var cx   0  cx  lt  katet  cx                           var scy   sy   cy - half                      var scx   sx   cx - half                       if  scy  gt   0  amp  amp  scy  lt  h  amp  amp  scx  gt   0  amp  amp  scx  lt  w                             var srcOff    scy   w   scx    4                          var wt   weights cy   katet   cx                            r    srcBuff srcOff    wt                          g    srcBuff srcOff   1    wt                          b    srcBuff srcOff   2    wt                          a    srcBuff srcOff   3    wt                                                                     dstBuff dstOff    r   mix   srcBuff dstOff     1 - mix               dstBuff dstOff   1    g   mix   srcBuff dstOff   1     1 - mix               dstBuff dstOff   2    b   mix   srcBuff dstOff   2     1 - mix              dstBuff dstOff   3    srcBuff dstOff   3                        ctx putImageData dstData  0  0      The result of using this combination will be  ONLINE DEMO HERE  Depending on how much of the sharpening you want to add to the blend you can get result from default  quot blurry quot  to very sharp   Suggestion 2 - low level algorithm implementation If you want to get the best result quality-wise you ll need to go low-level and consider to implement for example this brand new algorithm to do this  See Interpolation-Dependent Image Downsampling  2011  from IEEE  Here is a link to the paper in full  PDF   There are no implementations of this algorithm in JavaScript AFAIK of at this time so you re in for a hand-full if you want to throw yourself at this task  The essence is  excerpts from the paper   Abstract  An interpolation oriented adaptive down-sampling algorithm is proposed for low bit-rate image coding in this paper   Given an image  the proposed algorithm is able to obtain a low resolution image  from which a high quality image with  the same resolution as the input image can be interpolated  Different from the traditional down-sampling algorithms   which are independent from the interpolation process  the proposed down-sampling algorithm hinges the down-sampling  to the interpolation process  Consequently  the proposed down-sampling algorithm is able to maintain the original information of the input image to the largest extent  The down-sampled image is then fed into JPEG  A total variation  TV  based post processing is then applied to the decompressed low resolution image  Ultimately  the processed image is  interpolated to maintain the original resolution of the input image  Experimental results verify that utilizing the downsampled image by the proposed algorithm  an interpolated image with much higher quality can be achieved  Besides  the  proposed algorithm is able to achieve superior performance than JPEG for low bit rate image coding     see provided link for all details  formulas etc

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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 size 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

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I found a solution that doesn t need to access directly the pixel data and loop through it to perform the downsampling  Depending on the size of the image this can be very resource intensive  and it would be better to use the browser s internal algorithms   The drawImage   function is using a linear-interpolation  nearest-neighbor resampling method  That works well when you are not resizing down more than half the original size   If you loop to only resize max one half at a time  the results would be quite good  and much faster than accessing pixel data   This function downsample to half at a time until reaching the desired size     function resize image  src  dst  type  quality          var tmp   new Image             canvas  context  cW  cH        type   type     image jpeg        quality   quality    0 92        cW   src naturalWidth       cH   src naturalHeight        tmp src   src src       tmp onload   function              canvas   document createElement   canvas              cW    2          cH    2           if   cW  lt  src width   cW   src width          if   cH  lt  src height   cH   src height           canvas width   cW          canvas height   cH          context   canvas getContext   2d             context drawImage  tmp  0  0  cW  cH             dst src   canvas toDataURL  type  quality             if   cW  lt   src width    cH  lt   src height              return           tmp src   dst src                   The images sent as parameters can be in the DOM or be image objects   resize image       original    0        smaller    0

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If you wish to use canvas only  the best result will be with multiple downsteps  But that s not good enougth yet  For better quality you need pure js implementation  We just released pica - high speed downscaler with variable quality speed  In short  it resizes 1280 1024px in  0 1s  and 5000 3000px image in 1s  with highest quality  lanczos filter with 3 lobes   Pica has demo  where you can play with your images  quality levels  and even try it on mobile devices   Pica does not have unsharp mask yet  but that will be added very soon  That s much more easy than implement high speed convolution filter for resize

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Maybe man you can try this  which is I always use in my project In this way you can not only get high quality image  but any other element on your canvas           parame canvas   gt  canvas object     parame rate   gt  the pixel quality     function setCanvasSize canvas  rate        const scaleRate   rate      canvas width   window innerWidth   scaleRate      canvas height   window innerHeight   scaleRate      canvas style width   window innerWidth    px       canvas style height   window innerHeight    px       canvas getContext  2d   scale scaleRate  scaleRate

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Here is a reusable Angular service for high quality image   canvas resizing  https   gist github com fisch0920 37bac5e741eaec60e983  The service supports lanczos convolution and step-wise downscaling   The 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

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Not the right answer for people who really need to resize the image itself  but just to shrink the file size   I had a problem with  directly from the camera  pictures  that my customers often uploaded in  uncompressed  JPEG     Not so well known is  that the canvas supports  in most browsers 2017  to change the quality of JPEG  data canvas toDataURL  image jpeg    85     1  0  default 0 92   With this trick I could reduce 4k x 3k pics with  10Mb to 1 or 2Mb  sure it depends on your needs   look here

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context scale xScale  yScale    lt canvas id  c  gt  lt  canvas gt   lt hr  gt   lt img id  i    gt    lt script gt  var i   document getElementById  i     i onload   function        var width   this naturalWidth          height   this naturalHeight          canvas   document getElementById  c            ctx   canvas getContext  2d         canvas width   Math floor width   2       canvas height   Math floor height   2        ctx scale 0 5  0 5       ctx drawImage this  0  0       ctx rect 0 0 500 500       ctx stroke            restore original 1x1 scale     ctx scale 2  2       ctx rect 0 0 500 500       ctx stroke        i src    https   static md b70a511140758c63f07b618da5137b5d png    lt  script gt

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I really try to avoid running through image data  especially on larger images  Thus I came up with a rather simple way to decently reduce image size without any restrictions or limitations using a few extra steps  This routine goes down to the lowest possible half step before the desired target size  Then it scales it up to twice the target size and then half again  Sounds funny at first  but the results are astoundingly good and go there swiftly  function resizeCanvas canvas  newWidth  newHeight      let ctx   canvas getContext  2d      let buffer   document createElement  canvas      buffer width   ctx canvas width    buffer height   ctx canvas height    let ctxBuf   buffer getContext  2d          let scaleX   newWidth   ctx canvas width    let scaleY   newHeight   ctx canvas height     let scaler   Math min scaleX  scaleY       see if target scale is less than half      if  scaler  lt  0 5          while loop in case target scale is less than quarter        while  scaler  lt  0 5          ctxBuf canvas width   ctxBuf canvas width   0 5        ctxBuf canvas height   ctxBuf canvas height   0 5        ctxBuf scale 0 5  0 5         ctxBuf drawImage canvas  0  0         ctxBuf setTransform 1  0  0  1  0  0         ctx canvas width   ctxBuf canvas width        ctx canvas height   ctxBuf canvas height        ctx drawImage buffer  0  0          scaleX   newWidth   ctxBuf canvas width        scaleY   newHeight   ctxBuf canvas height        scaler   Math min scaleX  scaleY               only if the scaler is now larger than half  double target scale trick        if  scaler  gt  0 5          scaleX    2 0        scaleY    2 0        ctxBuf canvas width   ctxBuf canvas width   scaleX        ctxBuf canvas height   ctxBuf canvas height   scaleY        ctxBuf scale scaleX  scaleY         ctxBuf drawImage canvas  0  0         ctxBuf setTransform 1  0  0  1  0  0         scaleX   0 5        scaleY   0 5            else     ctxBuf drawImage canvas  0  0        wrapping things up      ctx canvas width   newWidth    ctx canvas height   newHeight    ctx scale scaleX  scaleY     ctx drawImage buffer  0  0     ctx setTransform 1  0  0  1  0  0

[javascript] HTML5 Canvas Resize (Downscale) Image High Quality?

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