graphics on the go

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Render 2D graphics in cross-platform mobile apps with HTML5, JavaScript, jQuery and PhoneGap. Learn how to draw in a way that will work across a variety of devices.

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  • 1. Graphics on the Go Gil Irizarry Conoa

2. About Me Launched VC News Daily app on iOS and Android. Over 1200 downloads so far. Owner and lead engineer at Conoa, a graphics and mobile software firm gil@conoa.com http://www.slideshare.net/conoagil/ 3. About Me All examples and sample code in this presentation can be found at: http://conoa.com/hidden/example.zip 4. Why? There are nearly 2 million mobile apps available today. (http://www.pureoxygenmobile.com/how-many-apps-in-each-app-store/ ) 1.5 years ago, there were 15K new mobile apps released each week. (http://www.nytimes.com/2011/12/12/technology/one-million-apps-and-counting.html ) For many, interacting with software means interacting with mobile devices (or at least devices that run mobile software). 5. What we will do We will learn how to build lightweight mobile apps quickly, using open source tools. The apps will be cross-platform. However, we must actually pick a platform on which to build and test the apps. For this presentation, we will work in Android since the tools are free and easily available. We will do this on Windows. 6. What we will do (Mostly) everything presented today in the Android environment will apply to iOS, or has an equivalent in that environment. So, lets get started 7. Assumptions Setting up a development environment is covered in the Making Mobile Apps Quickly workshop. For this workshop, we assume that a development environment is in place. 8. PhoneGap PhoneGap allows HTML pages to be built as mobile apps. HTML pages can include JavaScript and JavaScript libraries, such as jQuery. HTML pages can also include HTML5 elements. 9. Canvas HTML5 includes a new element. Canvas allows the rendering of 2D graphics via some low- level primitives. It does not include a scene graph API, so you must store rendering information yourself for redrawing a scene. 10. Canvas Canvas supports: Basic shapes Images Transparency Compositing Transforms Basic animation 11. Graphics Example 1 Simple Canvas Select the Graphics Example 1 project in the Package Explorer in Eclipse. Select Run from the top menu bar Once the emulator starts and is finished installing the app, you should see something like this: 12. Graphics Example 1 - Create a Canvas 13. Graphics Example 1 Simple Canvas This example creates a canvas, gets its drawing context, and draws into it. It follows a pattern that will soon be familiar: most of the action is happening in JavaScript. One problem with this example is that the canvas is a fixed size. Lets fix that with Example 2. 14. Graphics Example 2 - Better Canvas 15. Graphics Example 2 Better Canvas Example 2 queries the size of the window, then adds a canvas to fit the window. In this way, we can make a canvas that would fit any device. Notice that the position of the text is based on the size of the canvas, which is now based on the size of the device window. The text will be centered on any device the app is run, courtesy of a small amount of JavaScript. Lets see what else we can draw. 16. Graphics Example 3 - More Drawing 17. Graphics Example 3 More Drawing This example gives a sense of what can be drawn. Canvas uses a drawing system similar to Adobe PostScript or Mac OS QuickDraw. This is not surprising considering that Apple developed the initial implementation of canvas before it became a (nascent) standard. Obtain the graphics context of the canvas for issuing drawing commands 18. Canvas Drawing beginPath() fill() or stroke () paradigm moveTo() / lineTo() for paths arc() for full or partial circles bezierTo() for complex curves fillText() / strokeText() for text 19. Canvas Drawing drawImage() colors, either by name or hex values gradients, linear or radial, and patterns stroke styles, including stipple patterns and line end caps push and pop transformations and state 20. Canvas Coordinate System (0, 0) is in the upper left corner of the canvas. Increasing y is down from the screens perspective. Increasing x is to the right. (0, 0) (0, height) (width, 0) 21. Lets make a pattern Find an image from the internet and save it to GraphicsExample3/assets/www Change this code: context.fillStyle=gradient; context.beginPath(); context.arc(55,155,40,0,2*Math.PI); context.fill(); To this code: 22. Lets make a pattern var img = new Image(); img.src = 'clouds.jpg'; // use your image name here img.onload = function (e) { var pattern = context.createPattern(img, 'repeat'); context.fillStyle=pattern; context.beginPath(); context.arc(55,155,40,0,2*Math.PI); context.fill(); }; Shapes that you fill will become masks for the underlying pattern. 23. Graphics Example 4 Touch Everything so far has been static. JavaScript has an event system. We saw one in the previous example: onload Lets use other events to add some interactivity. 24. Graphics Example 4 - Touch 25. Touch Events HTML5 canvas adds new events for devices: ontouchstart ontouchmove ontouchend Note that events come back as arrays. Why? Because HTML5 supports multi-touch. We can have multiple simultaneous move events. 26. Touch Events Important to remember that event coordinates triggered by the canvas are relative to the full window. Events need to be converted to the canvas coordinate system in order to be relevant to the canvas. Look at function windowToCanvas (canvas, x, y) in Graphics Example 4 source code (found in assets/www folder). 27. Previewing Examples 1 through 3 work equally well as a PhoneGap- powered mobile app and as a web page. If you havent already done so, trying running assets/www/index.html in a browser. However, by incorporating touch events, we must run Example 4 and other examples incorporating touch in the mobile emulator. 28. Graphics Example 5 - Touch 29. Images In addition to stroked shapes, we can draw images. Images can be sourced locally or over the network. Note how images are loaded: image.src = filename; image.onload = function(e) { }; If you reference an image before it is loaded, you run the risk that you are trying to use an image before it is available. 30. Images Images can be scaled and transformed. Canvas supports compositing and alpha. Canvas also support shadows. The example uses an RGBA color value to render semi- transparent text and have it composited over the image. The text is given a drop shadow. 31. Images over the network To access images over the network (or any data at all), remember that network access must be enabled. On Android, network access is not the default. It must be enabled in the AndroidManifest.xml file: 32. Saving and restoring state save() pushes the current context onto a stack. In addition to the current transformation, attributes are also pushed. In the current example, because the text was given a shadow, the shadow applies to the paint object. If we want only the text to have a shadow, we could put the text rendering code inside a save() / restore() block. 33. Graphics Example 6 Paint and Menus Since we can render images into the canvas, we can use a set of images and canvases to create menus. We could also draw into the canvases to render the menu options dynamically. 34. Graphics Example 6 Paint and Menus 35. Graphics Example 6 Paint and Menus Example 6 allows the color and size of the paint brush. Separate canvases are used for the icon images, and touch event handlers are attached to the canvases. How else could we accomplish this? The image selector and scale checkbox are standard HTML controls that trigger JavaScript events. We could make

elements with elements containing the icons, and put event handlers on the images. 36. Animation Although weve added some interactivity, the examples so far have been a little static. Lets add some animation. 37. Graphics Example 7 Simple Animation 38. Animation self.setInterval () is used to have the page be called again. Note that the screen is not cleared and that we are adding to the canvas on each call. Instead, lets clear the canvas each frame. We can transform the previous example into a simple clock app. 39. Graphics Example 8 Simple Clock 40. Animation This example queries the time from the JavaScript Date() object and draws the hands accordingly. The canvas is cleared each frame and redrawn. If you havent already, try running the Graphics Example 8 in a browser (by running assets/www/index.html) There is something consistently missing in both the emulator and the browser. What is it? 41. Animation The answer is flicker. The same canvas is cleared and redrawn each frame, yet there is no flicker. This is because the canvas is inherently double-buffered. You could implement your own double buffering scheme because HTML5 allows drawing to off-screen drawables. You could draw off-screen, then swap the off-screen memory to the on-screen canvas. However, HTML5 canvas does this more efficiently. 42. Save and Restore Remember save() and restore()? Lets use them to change the clock face so the text is oriented around the face, much like text on a path in some applications. Do this by using translate and rotate to change the coordinate system of the fillText() call. If you cant figure out how to do this, look at the commented out code. 43. Graphics Example 8 Simple Clock 44. Lets Do Things the Right Way We have been cutting some corners, so lets do things the right way. PhoneGap should load the JavaScript when it is ready, otherwise you potentially have the problem similar to when you reference an image before it is loaded. Set a device ready event: document.addEventListener("deviceready", fnName, false); fnName is the name of the event handler 45. Lets Do Things the Right Way Setting the device ready event requires that the device get set in the config.xml file. This fil