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

Generation QR Code in Visual Studio .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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You can find plenty of calculators online to do the conversion for you.
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Arcs
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The canvas DOM method for drawing arcs is context.arc(). The method has several parameters that need to be understood in concert and individually:
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x,y: Circle s center radius: Radius of circle startAngle: Start point of arc expressed in radians endAngle: End point of arc expressed in radians anticlockwise: Boolean (true is counterclockwise and false is clockwise)
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I find it helpful to envision either a compass rose or a clock with the four cardinal directions and time/degrees north (12 o clock or 0 degrees), east (3 o clock or 90 degrees), south (6 o clock or 180 degrees), and west (9 o clock or 270 degrees). A full arc statement looks like the following:
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contextNow.arc(150,100,50,six,0,true);
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This arc has its center at x = 150 and y = 100, and it has a radius of 50. The start angle is set to 6, which is a variable that we ve created to represent the 6 o clock position of 180 degrees. The variable s value has been converted to radians. Both degrees and radians have the same value at the 12 o clock position (0), and it is used as the ending angle. Finally, the arc is set to true anticlockwise. This next program is one used to experiment with different arcs. Four variables 12, 3, 6, and 9 are set in radians corresponding to the positions on a clock. Certain statements are commented out but will be used later.
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<!DOCTYPE HTML> <html> <head> <script type= text/javascript > CanvasMaster=new Object(); CanvasMaster.showCanvas=function() { canvasNow = document.getElementById( beHappy ); contextNow = canvasNow.getContext( 2d ); contextNow.beginPath(); contextNow.moveTo(0,0); contextNow.lineTo(300,0); contextNow.lineTo(300,200); contextNow.lineTo(0,200); contextNow.closePath(); contextNow.stroke();
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CHAPTER 13: THUNDERING YOUR SITE WITH CANVAS
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// RADCON = (Math.PI/180) ; RADCON=0.01745329251994; twelve=0; three = RADCON * 90; six = RADCON * 180; nine = RADCON * 270; contextNow.beginPath(); contextNow.arc(125,100,50,six,twelve,true); //contextNow.closePath(); //contextNow.fill() contextNow.stroke(); } </script> <style type= text/css > body { font-family:Verdana, Geneva, sans-serif; color:#cc0000; } </style> <meta http-equiv= Content-Type content= text/html; charset=UTF-8 > <title>Smile</title> </head> <body onLoad= CanvasMaster.showCanvas() > <figure> <canvas id= beHappy width= 300 height= 200 > You don t see a smile because you don t have an HTML5 browser. No smile for you!</canvas> <figcaption> <p>Rectangle represents canvas boundaries</p> </figcaption> </figure> </body> </html>
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The RADCON variable is a constant ( 180), so all degrees were set to radians by multiplying their values by RADCON. As noted, the variable names represent the positions on a clock. In addition, a rectangle around the area where the arc is drawn represents the boundaries of the <canvas> tag s width and height. Figure 13-14 shows the result. The starting point of the arc is on the left, and it moved anticlockwise to the ending point on the right. Change the following line:
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contextNow.arc(125,100,50,six,twelve,true);
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contextNow.arc(125,100,50,six,twelve,false);
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PART IV: DYNAMIC HTML5 TAGS PLUS A LITTLE JAVASCRIPT AND PHP
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Figure 13-14: An arc in canvas.
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That changed the drawing from anticlockwise to clockwise, but it made a major difference, as you ll see when you test it. 278 Next, using the same program, change the line back to:
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contextNow.arc(125,100,50,six,twelve,true);
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Then remove the comment lines (//) from the following line:
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//contextNow.closePath();
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And test it again. The final change to the program will fill the arc. Uncomment the line from the following:
//contextNow.fill()
to this:
contextNow.fill()
And add comment lines so that the stroke statement reads:
//contextNow.stroke()
When the changes are made, your arc now looks like a kettle, as shown in Figure 13-15.
CHAPTER 13: THUNDERING YOUR SITE WITH CANVAS
Figure 13-15: An arc with a closed path and ll.
The only way to really learn to work with arcs is to practice with them. Use the script in this section to try different things.
Circles and gradients
Thus far, only a single type of fill has been used a solid one. In this section, you ll see how to make a circle using an arc and fill it with a gradient. First, making circles is easy using the context.arc() method. The radian parameters are 0 and Math.PI*2. And the anticlockwise parameter is false. (That s the trick.) For instance, this next example uses the following line to create a big circle that will be filled with a gradient, to make it look like a sunset:
contextNow.arc(200,200,150,0,Math.PI*2,false);
To create a gradient fill, both linear and radial, is fairly straightforward. The first step is using the canvas DOM context.createLinearGradient() method. The method expects four parameters: x0, y0, x1, y1. The gradient fill moves from x0 to x1 and from y0 to y1. A straight linear gradient from left to right would have a single value in x1, and the rest would be 0. A gradient from top to bottom would have value in either y0 or y1, with the rest set to 0. To set the gradient colors, use the gradient.addColorStop() method. It expects two parameters. The first is a zero-based number from 0 to 1 and the second is the color. Once that s completed, assign the context.fillStyle the gradient. The following snippet shows the steps in adding a gradient fill:
sunsetGradient=contextNow.createLinearGradient(0, 0, 0,379);