javascript - HTML5 canvas getImageData() can't work -

-

when use getimagedata() function data of image chrome, says uncaught indexsizeerror: failed execute 'getimagedata' on 'canvasrenderingcontext2d': source width 0. here code:

<!doctype html> <html> <head>     <title>canvastest</title> </head> <body> <img src="flower.jpg" hidden /> <canvas id="drawing" width="600" height="532">a drawing of something</canvas> <script type="text/javascript" >     var drawing = document.getelementbyid("drawing");     if (drawing.getcontext){         var context = drawing.getcontext("2d"),             image = document.images[0],             imagedata,data,             i,len,average,             red,blue,green,alpha;          image.onload = function(){             context.drawimage(image,0,0);         };          imagedata = context.getimagedata(0,0,image.width,image.height);         data = imagedata.data;          (i=0,len=data.length;i<len;i+=4){             red = data[i];             green = data[i+1];             blue = data[i+2];             alpha = data[i+3];              average = math.floor((red + green + blue) / 3);              data[i] = average;             data[i+1] = average;             data[i+2] = average;         }          imagedata.data = data;         context.putimagedata(imagedata,0,0);     } </script> </body> </html>

how happended? , how fix it?

you attaching onload callback draw image canvas. fires once image loaded in dom , has fetched server. makes think of happens before try pull image data canvas?

that's problem when mixing synchronous , asynchronous code, , there numerous techniques getting around it. i'd bet money if set break point in debugger @ line draws image , line tries read latter trigger before former.

you either need put processing in onload handler (not recommended because of coupling) or use promise make sure code reads data runs after code writes it.


Comments

Post a Comment

Popular posts from this blog

php - Invalid Cofiguration - yii\base\InvalidConfigException - Yii2 -

-
Image
i've installed yii2-user module/dektrium yii2-app-basic application command composer require "dektrium/yii2-user:0.9.*@dev" config/console.php return [ . . 'modules' => [ 'gii' => 'yii\gii\module', 'user' => [ 'class' => 'dektrium\user\module', ], ], . . ] config/web.php 'components' => [ . . /* 'user' => [ 'identityclass' => 'app\models\user', 'enableautologin' => true, ], */ 'modules' => [ 'user' => [ 'class' => 'dektrium\user\module', ], ], . . ] after that, run command $ php yii migrate/up --migrationpath=@vendor/dektrium/yii2-user/migrations updating database schema . but, when run http://localhost/mylawsuit/yii/web/index.php?r=u
Read more

How to show in django cms breadcrumbs full path? -

-
need advice - how show in breadcrumbs whole path article or page? i've got code: {% ance in ancestors %} <li> {% if not forloop.last %} <a href="{{ ance.get_absolute_url }}">{{ ance.get_menu_title }}</a> {% else %} <span class="active">{{ ance.get_menu_title }}</span> {% endif %} </li> {% endfor %} but shows "home" , current page. how modify show whole path in breadcrumbs? i've found in file menu_tags.py code calls ancestors. @ least there class showbreadcrumbs: class showbreadcrumb(inclusiontag): """ shows breadcrumb node has same url current request - start level: after level should breadcrumb start? 0=home - template: template used render breadcrumb """ name = 'show_breadcrumb' template = 'menu/dummy.html' options = options( argument('start_level', default=0, require
Read more

ruby on rails - npm error: tunneling socket could not be established, cause=connect ETIMEDOUT -

-
i have app uses rails framework , implements angularjs part of front end. i have pushed heroku , have heroku toolbelt installed, when try migrate db using "heroku run rake db:migrate" receive following error(s): installing core plugins heroku-cli-addons, heroku-apps, heroku-fork, heroku-git, heroku-local, heroku-run, heroku-status... error installing package. try running again gode_debug=info see more output. ! `run` not heroku command. ! perhaps meant `-h`, `2fa`, `auth`, `join`, `open`, `orgs`, `pg`, `ps` or `rake`. ! see `heroku help` list of available commands. i run command "gode_debug=info heroku run rake db:migrate" , receive error: npm err! darwin 14.5.0 npm err! argv "/users/christopher_pelnar/.heroku/node-v4.2.1-darwin-x64/bin/node" "/users/christopher_pelnar/.heroku/node-v4.2.1-darwin-x64/lib/node_modules/npm/cli.js" "install" "heroku-cli-addons" "heroku-apps" "heroku-fork
Read more