How To Test Canvas With Selenium

Selenium WebDriver is widely used for automating Web UI, with its capabilities extending to programming languages like Python and Java. However, when it comes to testing canvas elements in web applications, using Selenium can be complicated. This is because canvas is an HTML element that utilizes JavaScript to create graphics.

In this blog, we will explore how to test canvas with Selenium.

Understanding the Canvas Element

The canvas element is part of HTML5 and allows for dynamic, scriptable rendering of 2D shapes and bitmap images. It is a low level, procedural model that updates a bitmap and does not have a built-in scene graph.

Challenges in Testing Canvas with Selenium

Selenium WebDriver works by interacting with the HTML DOM to find elements and perform actions on them. The main challenge with the canvas element is that it’s just a blank space for drawing on the web page as far as Selenium is concerned. Selenium cannot directly interact with the drawn elements on the canvas. So, traditional methods of interacting and testing elements with Selenium will not work with the canvas element.

Approach to Test Canvas with Selenium

To test the canvas with Selenium, we need to use JavaScript Executor, which is a part of Selenium WebDriver. JavaScript Executor allows you to execute JavaScript directly on the browser. By using JavaScript Executor, we can interact with the canvas and test it.

Example: Drawing on the Canvas

Let’s consider an example where we need to draw something on the canvas. Here is how you can do it:

WebElement canvas = driver.findElement("canvasId"));
JavascriptExecutor js = (JavascriptExecutor) driver;
js.executeScript("var ctx = arguments[0].getContext('2d');"+
                 "ctx.moveTo(30, 50);"+
                 "ctx.lineTo(100, 100);"+
                 "ctx.stroke();", canvas);

In the above example, we first find the canvas element. Then we execute a JavaScript on it to draw a line from (30,50) to (100,100).

Verifying the Canvas:

To verify the changes on the canvas, we can take a screenshot of the canvas before and after the changes and compare them. Here is how you can take a screenshot with Selenium:

WebElement canvas = driver.findElement("canvasId"));
File screenshot = ((TakesScreenshot) driver).getScreenshotAs(OutputType.FILE);

In the above example, we take a screenshot of the entire browser window. To take a screenshot of the specific canvas element, we can use AShot, a WebDriver screenshot utility.


Testing a canvas with Selenium can be challenging due to the way Selenium interacts with the HTML DOM. However, by using JavaScript Executor, we can overcome these challenges. Remember that testing should also include verification, which can be done by taking screenshots before and after the test and comparing them.