Rotate an ASP.NET Image Control using HTML 5 Canvas
Posted by: Suprotim Agarwal
in Category ASP.NET
In this article, we will see how to use the HTML 5 canvas element to rotate an Image served by the ASP.NET Image Control.
Step 1: Open VS 2010 and create an ASP.NET Website. Set it’s target schema for validation as HTML 5 by going to Tools > Options > Text Editor > HTML > Validation
Step 2: Declare the following markup
As you can see, we have declared an HTML 5 canvas element of dimensions 400x400 and an ASP.NET Image control with display set to none. I will explain shortly, why the display is set to none.
Let us understand this code line by line
The window.onload is to wait for the page to load before trying to access the canvas tag by its ID. To draw on the canvas, we need a reference to it. We have used getElementById to get this reference from the DOM. We then set a variable context to reference the 2D context of the canvas, which is used for all drawing purposes. In other words, we are asking the Canvas to give us a context to draw on.
Note: If you are wondering if there is 3D context too, then the answer is not yet! Take a look at WebGL and SceneJS.
The final step is to rotate the image
We are using a translate transform to move the center point of the coordinate system, which is in the top-left corner of the canvas (0,0). We have to move the center position because we have to rotate the canvas context by 45 degrees, around this current position. After moving the center point, it is now (200,200).
Note: the canvas.width and height is 400 and 400/2 = 200.
After moving your reference point to (200,200), we are now centering our image over that point, by calling the drawImage() method. We are drawing the image from above and to the left, of your current reference point.
context.drawImage(img, x, y, width, height)
This function makes a 200 × 200 pixel box for the image, with the top-left corner at point (-100,-100).
The last step is to set the image source to the desired image. The reason we set the display property of the ASP.NET image control to none is to avoid displaying the image twice; once where the image control is declared outside the canvas and second, where we are drawing the image inside the canvas.
Detecting Browser Support and Graceful Degradation
The latest versions of IE (9 onwards), Firefox, Safari, Chrome and Opera all provide reasonably good support for the Canvas element. However for users who are using older browser versions, we can easily detect browser support or gracefully display a message, if the user’s browser does not support Canvas. For eg: to check if your browser supports canvas and the getContext method exists, change the line of code to:
The entire source code of this article can be downloaded over here
This article has been editorially reviewed by Suprotim Agarwal.
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