image - Canvas - get pixel colour - what am I missing? -


using code base http://www.script-tutorials.com/demos/158/index.html

i code following:

    $canvas = $('<canvas/>');     ctx = $canvas[0].getcontext('2d');     var image = new image(); // create image object in memory     image.onload = function () {       // render image on canvas       $canvas.width(this.width).height(this.height); // resize canvas       ctx.drawimage(image, 0, 0, this.width, this.height);        var nav =  $("#navigation"); // navigation container       // find pixel in middle navigation       var pos = {         left: nav.position().left+(nav.width()/2),          top: nav.position().top+(nav.height()/2)        }       var pixel = ctx.getimagedata(pos.left,pos.top, 1, 1).data;       canvas=null; // no longer need canvas        // invert pixel colour, ignoring alpha channel       var invertedpixelcolor = "rgba("+(255-pixel[0])+", "+                                        (255-pixel[1])+", "+                                        (255-pixel[2])+", "+                                        1+")";        nav.css("color",invertedpixelcolor); // set nav text inverted color     }     image.src = imageurl; // load image, triggering calc

running example

i have following issues example:

  1. why image not render correctly canvas? (scroll down in demo see canvas rendered)
  2. why 0s image data?

i have 

window.console &&     console.log("before: rgba("+pixel[0]+", "+pixel[1]+", "+pixel[2]+", "+pixel[3]+")"); window.console &&     console.log("after: rgba("+(255-pixel[0])+", "+(255-pixel[1])+", "+(255-pixel[2])+", "+1+")");

and returns 

before: rgba(0, 0, 0, 0)  after: rgba(255, 255, 255, 1)

i hope missing simple

update - seems $("<canvas/>)[0].getcontext('2d'); not 100% happy context...

on 2 questions.

(1). use use 3 arguments when calling ctx.drawimage

ctx.drawimage(image,0,0);

(2). jquery $canvas object you're creating doesn't appear html canvas element.

so you're jquery element creation needs fixin' or use old fashioned document.createelement.

canvas=document.createelement("canvas");

those couple of tweaks you're go.

<!doctype html> <html> <head> <title>testing setting text colour depending on background</title> <script src="http://code.jquery.com/jquery-latest.min.js"></script> <script> var ctx;    $(function(){ // on page load, should on image transition     $(".nav").on("click", function(e) {         e.preventdefault();         var imageurl = $(this).data("img");         $("#content").css("background","url("+imageurl+")");         // creating canvas object in memory.          // since not append anywhere not rendered         canvas = document.createelement("canvas");         ctx = canvas.getcontext('2d');         var image = new image(); // create image object in memory         image.onload = function () {            // render image on canvas           canvas.width=this.width;           canvas.height=this.height; // resize canvas           ctx.drawimage(image, 0, 0);            var nav =  $("#navigation"); // navigation container           // find pixel in middle navigation           var pos = {left: nav.position().left+(nav.width()/2), top: nav.position().top+(nav.height()/2) }           var pixel = ctx.getimagedata(pos.left,pos.top, 1, 1).data;           canvas=null; // no longer need canvas             var invertedpixelcolor = "rgba("+(255-pixel[0])+", "+(255-pixel[1])+", "+(255-pixel[2])+", "+1+")"; // invert it, ignoring alpha channel           nav.css("color",invertedpixelcolor); // set nav text inverted color           // here save colour , reuse            // if user navigates same image         }         image.src = imageurl; // load image, triggering calc     }); }); </script> <style> #navigation { text-align:center } #content { width:640px; height:480px; background: url(city-q-c-640-480-6.jpg) } </style> </head> <body> <div id="content">     <div id="navigation">this text should have different color depending on background<br/>         <a href="#" class="nav" data-img="city-q-c-640-480-5.jpg">city 1</a> | <a href="#" class="nav" data-img="city-q-c-640-480-6.jpg">city 2</a> | <a href="#" class="nav" data-img="city-q-c-640-480-2.jpg">city 3</a>     </div> </div> <div id="test"></div> </body> </html>

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