PART IV: DYNAMIC HTML5 TAGS PLUS A LITTLE JAVASCRIPT AND PHP in VS .NET

Drawer QR Code in VS .NET PART IV: DYNAMIC HTML5 TAGS PLUS A LITTLE JAVASCRIPT AND PHP
PART IV: DYNAMIC HTML5 TAGS PLUS A LITTLE JAVASCRIPT AND PHP
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Figure 13-3: Providing alternative materials for non-HTML5 browsers.
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A SIMPLE CANVAS IMPLEMENTATION
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When you re working with Adobe Dreamweaver to create an HTML5 page, you can view the page in the Design mode to preview what will show up on the screen. However, with material inside a <canvas> container, all you see is the outline. That outline provides an excellent visual picture of how canvas allocates a certain part of the page for rendering images even thought it appears as a blank rectangle. Basically, you re starting off with an empty canvas defined by the width and height attributes of the <canvas> tag. If you think about the first step in creating a canvas on your Web page in terms of stretching a canvas on a frame, it helps you visualize the process.
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Understanding the grid
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To work successfully with canvas, you have to understand the grid and the Cartesian coordinates. Basically, the upper-left corner is the 0,0 position on your page. As you move to
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CHAPTER 13: THUNDERING YOUR SITE WITH CANVAS
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the right, the first value increases. If you move 15 pixels to the right, the value becomes 15,0 this is the x-axis. As you move down, the second value (y-axis) increases. If you moved down 20, the position would be expressed as 15,20. Suppose, that you wanted to use that position as your starting point and create a 100-pixel square. It helps to visualize the position and size relative to the Web page with the grids, but you get a clearer idea of the image you re creating without the grid marks. Using both will help.
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Setting up for canvas drawings
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Now we re set to fill the blank box. To do so requires JavaScript. The only thing you do with the <canvas> tag is describe the area where you can place your graphics in a rendering context and a reference ID. So, starting small, this first little drawing will begin with the following tag:
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<canvas id= redHot width= 100 height= 100 >
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This should be pretty familiar. The width and height were simplified to equal 100 pixels, and the new name of the canvas object is redHot. I ve already covered the closing </canvas> tag and message in the container. And the rest of the work is all JavaScript programming working with the DOM. As noted earlier, I m going to try to simplify things by using a little OOP in the JavaScript to reflect the programming structure of the DOM. So, the first task is to create an object and a method for it.
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CanvasMaster=new Object(); CanvasMaster.showCanvas=function()...
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As you saw in 12, all that does is set up an object and a method for the object a function that will call the JavaScript operations when we need it. Next, the program needs a way to access the canvas DOM node. That s the part of the DOM that has canvas and canvas-related methods and properties. The first step is to create an object that holds the DOM node. Instead of thinking of assigning a node to a variable, think of it as creating an instance of an object that has the properties and methods of the canvas object.
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canvasNow = document.getElementById( redHot );
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That line creates an object that contains the canvas object named redHot. Once we have an instance of a canvas object, the program needs a rendering context. About the only context available is one called 2d, suggesting a two-dimensional drawing context. The canvas object (canvasNow) has a method called getContext() to do what it says: get the rendering context.
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contextNow = canvasNow.getContext( 2d );
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PART IV: DYNAMIC HTML5 TAGS PLUS A LITTLE JAVASCRIPT AND PHP
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The instance of the rendering context is named contextNow. It has the methods and properties of the 2d rendering context.
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Making the drawing
Before going on to the actual drawing, you may be wondering about the canvasNow and contextNow objects. Aren t those really variables After all, variables can be assigned objects. Well, that s one way to think about them, but the variables are assigned objects with their own methods and properties. So, aren t they actually instances of objects When a variable is assigned a real number, it is, for all intents and purposes, a number. You can do math operations just as you can with a literal number. Instead of quibbling about whether the program structures are really variables or objects, just treat them as objects (just as variables with text or numbers can be treated as strings or numbers). First, assign the drawing a color. You can use any of the techniques available to create a color as described in 4. This example uses the hexadecimal format:
contextNow.fillStyle = #cc0000 ;
The fillStyle property is only for the fill color and not the stroke (outline) of the object. Next, the fill color needs a shape to fill. To fill a rectangle, use the following: 258
contextNow.fillRect(5,20,100,100);