How to check if a specific pixel of an image is transparent?

by

Building on Jeff’s answer, your first step would be to create a canvas representation of your PNG. The following creates an off-screen canvas that is the same width and height as your image and has the image drawn on it.

var img = document.getElementById('my-image');
var canvas = document.createElement('canvas');
canvas.width = img.width;
canvas.height = img.height;
canvas.getContext('2d').drawImage(img, 0, 0, img.width, img.height);

After that, when a user clicks, use event.offsetX and event.offsetY to get the position. This can then be used to acquire the pixel:

var pixelData = canvas.getContext('2d').getImageData(event.offsetX, event.offsetY, 1, 1).data;

Because you are only grabbing one pixel, pixelData is a four entry array containing the pixel’s R, G, B, and A values. For alpha, anything less than 255 represents some level of transparency with 0 being fully transparent.

Here is a jsFiddle example: http://jsfiddle.net/thirtydot/9SEMf/869/ I used jQuery for convenience in all of this, but it is by no means required.

Note: getImageData falls under the browser’s same-origin policy to prevent data leaks, meaning this technique will fail if you dirty the canvas with an image from another domain or (I believe, but some browsers may have solved this) SVG from any domain. This protects against cases where a site serves up a custom image asset for a logged in user and an attacker wants to read the image to get information. You can solve the problem by either serving the image from the same server or implementing Cross-origin resource sharing.

More Related Contents:

  1. What is wrong with this basic Javascript code? [closed]
  2. How can you programmatically tell an HTML SELECT to drop down (for example, due to mouseover)?
  3. How do you read CSS rule values with JavaScript?
  4. jQuery Mobile: Markup Enhancement of dynamically added content
  5. Convert json data to a html table [closed]
  6. HTML5 Local storage vs. Session storage
  7. Fastest method to escape HTML tags as HTML entities?
  8. Placeholder in IE9
  9. how to save canvas as png image?
  10. Changing nav-bar color after scrolling?
  11. Onclick javascript to make browser go back to previous page?
  12. How to maintain a WebSockets connection between pages?
  13. Template does not update when using ui-router and ion-tabs
  14. HTML input type=”number” still returning a string when accessed from javascript
  15. Checkbox Check Event Listener
  16. Is there any “on DOM change” event? [duplicate]
  17. event.offsetX in Firefox
  18. How to select all text in contenteditable div?
  19. shorthand http:// as // for script and link tags? anyone see / use this before?
  20. Javascript – Dynamically assign onclick event in the loop
  21. javascript code not work in HEAD tag
  22. ‘img-src’ was not explicitly set, so ‘default-src’ is used as a fallback
  23. Finding number of lines in an html textarea
  24. How do I determine if a checkbox is checked?
  25. Impossible to hide navigation bars in Safari iOS 7 for iPhone/iPod touch
  26. Trigger a button click inside an iframe [duplicate]
  27. How to determine if user selected a file for file upload?
  28. How can I get DPI from image in JS?
  29. How to use LUT with JavaScript?
  30. Maximum Call Stack Size Exceeded During a setTimeout Call

Leave a Comment