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Canvas builds its drawings around paths, which are collections of lines, arcs, and invisible moves between them. You create a new path by describing any number of these lines, and then you finish up the path by deciding how it s going to look, writing out your whole stack of commands in the process. Nothing gets printed to the screen until you dump out everything with a completion command. This is all done with JavaScript commands that you include as part of a drawOnCanvas-like function, such as the one we included in listing 6.1. All Canvas drawing is done on a two-dimensional grid with an origin at the top left. This is depicted in figure 6.1. With these fundamentals of Canvas in hand, you can now begin drawing.
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Figure 6.1 Any Canvas object maintains its own two-dimensional grid.
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Basic path commands
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Table 6.1 lists the basic path commands. They re divided into three broad types: creation commands that get you going, draw commands that either draw or move while you re working on a path, and completion commands that are used when you re finishing a path.
Table 6.1 Method A variety of simple JavaScript commands help you create, draw, and finish basic Canvas paths Type Creation method Draw method Draw method Draw method Completion method x,y x,y Variables Starts a new path Moves the virtual pencil visibly Moves the virtual pencil invisibly Completes a path by drawing back to the first point Draws a path by filling in the space between visible lines Draws a path by just drawing the visible lines Summary
beginPath lineTo moveTo closePath fill stroke
Completion method
Listing 6.2 shows an example of how to use these commands to draw a simple banner. This is just the first step in putting together a Canvas application. Things will get more complex as we learn about additional methods.
Listing 6.2 Simple Canvas commands draw quick two-dimensional shapes
var context = canvas.getContext('2d'); context.beginPath(); context.moveTo(10,110); context.lineTo(10,10); context.lineTo(40,40); context.lineTo(70,10); context.lineTo(70,110); context.closePath(); context.stroke();
C D E F
You start by repeating the getContext line B from the setup example. The context is important because it s the object that gives you access to all of the drawing methods. For future examples, we ll always assume that we have defined a context by the name of context. After creating the context, you draw a path that defines an image, as shown in figure 6.2. Any path must start off with a beginPath line C. This clears off the drawing stack and resets your virtual pencil to the origin point of 0,0. As a result, most Canvas methods
Figure 6.2 This simple banner was drawn with eight path commands.
Drawing paths
will follow the beginPath with a moveTo D, to get the virtual pencil to where you want to start drawing without actually drawing anything in between. For this example, you next use a set of four lineTo methods E to draw an Mshape. Because these are lines, they ll display when you complete your path. The closePath F that ends the drawing is entirely optional. It s really just a shorthand way to draw a line between the final point that you explicitly designated and the point that you began drawing at. But none of this appears on the screen until you use a completion method G. You can use stroke, as in this example, to just draw the line, or alternatively you can use fill, to color everything in. Note that when you use a fill command, you don t need a closePath command; instead, Canvas will automatically close your shape for you.
Curve commands
Once you ve learned about lines, you ve just got one other fundamental drawing tool in Canvas: the curve. Three different curve commands are available to you: the arc (which is available through two different methods), the quadratic curve, and the Bezier curve. These are summarized in table 6.2.
Table 6.2 Canvas supplies four methods for drawing curved paths. Type Draw method Variables x, y, radius, startangle, endangle, anticlockwise x1,y1,x2,y2,radius cpx,cpy,x,y cpx1,cpy1,cpx2,cpy2,x,y Summary Draws a circle or an arc of a circle
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