Graphics -- To be discussed

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Lesley Dickerson
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1 Graphics -- To be discussed 1 Canvas Class Graphics Class Painting Color Models Animation's Worst Enemy: Flicker Working with Java Images Image Loading Chain of Events ImageObserver Interface MediaTracker Class Algorithmic Image Generation Producer Consumer Design Image Filtering Canvas Class - General GUI component for drawing images and text on the screen. - It does not support any drawing methods - Provides access to a Graphics object through its paint() method - The paint() method is invoked upon the creation and update of canvas so that Graphics object associated with the Canvas object can be updated. - paint() Should not be invoked - repaint() Indirect access of paint() method - Custom drawing and event handling 2 Graphics Class - Low level drawing methods are implemented - Draw objects themselves or can be combined to display more elaborate screen objects - A large number of geometrical objects to be drawn and filled lines, arcs, ovals, rectangles and polygons. 3 Painting Painting: How an applet actually draws something on the screen. public void paint(graphics g){ } The object Graphics is created automatically by the browser.

2 import java.awt.graphics; The Graphics Class Gives one the ability to draw lines, shapes, characters! Coordinate system (0,0) +x +y Drawing and Filling Built-in graphic primitives fo r drawing lines, rectangles, ovals, arcs. Drawing Lines and Rectangles Basic rendering of lines and rectangles public void drawline(int x1, int y1, int x2, int y2); public void fillrect(int x, int y, int width, int height); public void drawrect(int x, int y, int width, int height); public void clearrect(int x, int y, int width, int height); public void drawroundrect(int x, int y, int width, int height, int arcwidth, int archeight); public void fillroundrect(int x, int y, int width, int height, int arcwidth, int archeight); public void draw3drect(int x, int y, int width, int height, boolean raised); public void fill3drect(int x,int y, int width, int height, boolean raised); Drawing Polygons

3 Outlined or filled polygons public void drawpolygon(int xpoints[], int ypoints[], int npoints); public void drawpolygon(polygon p); public void fillpolygon(int xpoints[], int ypoints[], int npoints); public void drawpolyline(int xpoints[], int ypoints[], int npoints); Definition of polygon class Variable Index npoints The total number of points. xpoints The array of x coordinates. ypoints The array of y coordinates. Constructor Index Polygon () Creates an empty polygon. Polygon(int[], int[], int) Constructs and initializes a Polygon from the specified parameters. Method Index addpoint (int, int)appends a point to a polygon. getboundingbox() Determines the area spanned by this Polygon. inside(int, int) Determines whether the point (x,y) is inside the Polygon. Drawing Ovals Outlined and filled ovals public void drawoval(int x, int y, int width, int height); public void filloval(int x, int y, int width, int height); Animation Animation - Moving pictures ~ A sequence of still images played fast enough at a rate 24 frames/second, 50 Hz, 100 Hz Redrawing an entire screen - Time constraints! Do not draw the entire screen, but rather only the changing parts.

4 paint() - In a loop Determine the randomly determined start and end points. Determine the randomly determined color components. Draw 4 Color Models The hardware encoding of color - The number of bits allocated per primary color. Uniform distribution - 8 bits per primary Nonuniform distribution - 4 bits for R, 12 bits for G The class ColorModel defines the model. DirectColorModel: Pixels hold the value they represent. IndexColorModel: Pixels hold an index value in a CLUT. 5 Animation's Worst Enemy: Flicker Graphics class rendering methods to construct an image from scratch createimage() DEMO: Ball public Image createimage(int width, int height) Creates an off-screen drawable Image to be used for double buffering. Creates a blank image. To start drawing, you must get the Graphics handle associated with this image. getgraphics() - Returns an object of type Graphics. Smooth Animation Using Double Buffering

5 To use a second off-screen image buffer to do all the new graphics. Copy the finished product (a single animation frame) to the screen. Minimize the computation and update the screen as fast as possible. 6 Working with Java Images Animation based on images Several packages contain image handling stuff: java.awt.image - abstract class java.awt.image package contains image processing stuff. java.applet - getimage() methods The class Component defines two createimage() methods. The class Graphics contains several drawimage () methods. The class Image - Abstract Cannot be the source for Image instances itself. Other classes must create instances of Image! Factory methods - Method that construct and return objects java.applet - getimage() method Create new Image object. Loading an external image file, turning it into a runtime usable Image object ready to be rendered.

6 drawimage - To render the image! The image is not loaded until some part of the code actually tries to draw it or the loading is explicitly forced! (ie. Component.prepareImage()) public interface ImageObserver An asynchronous update interface for receiving notifications about Image information as the Image is constructed. Internet's reality into mind Seperation of loading and using images 6.1 Image Loading Chain of Events Method drawimage() kicks off the asynchronous loading of the image it wants to render. Another invisible Java thread actually does all the work of transferring the image over the network. The logic of the applets does not need to be concerned with the multithreaded, asynchronous loading of image data. Each time a new chunk of new image data becomes available, it tells the ImageObserver for the image that some extra data is available for displaying. In detail, the repaint method which then invokes the update which then calls the paint method which then displays the new image data available. The AWT defines an incremental draw rate property that determines how long the system can buffer image data before physically updating it on the screen. 6.2 ImageObserver Interface To keep users informed regarding the loading of an image ImageObserver Interface Enables the monitoring of the loading process so that users can be informed and the image can be used asap once it is loaded. Loading an image asynchronously how to know when the image is ready o An image is ready getimage() method returns, long before anything is known about the image

7 imageupdate(image img, int infoflags, int x, int y, int width, int height) Note java.awt.component implements ImageObserver all the subclasses do as well! g.drawimage(imge, 0,0, this) -- this refers to the ImageObserver instance. imageupdate() Called by the imageobserver whenever necessary. You do not call it explicitly! o If the image is complete returns false o If the image is not complete and needs to be updated returns true ImageObserver.ALLBITS = 32 Various constants are combined to form the infoflags argument which indicates whether all information is available or not! Flag Meaning if the flag is set ImageObserver.WIDTH The width of the image is available in this flag until then value is -1 ImageObserver.HEIGHT The height of the image is available in this flag until then value is -1 ImageObserver.PROPERTIES Properties are available and accessed via getproperty() method of the image object ImageObserver.SOMEBITS Some portion of the data is available ImageObserver.FRAMEBITS Another frame of the multi-frame image is now available ImageObserver.ALLBITS Image is now complete ImageObserver.ERROR Error encountered while loading ImgeObserver.ABORT Loading aborted before the image was complete 6.3 MediaTracker Class Multiple images or non-image media types like audio java.awt.mediatracker class track the loading status of different images, monitor audio clips or other multimedia content. To use it o Simply create an instance of the MediaTracker

8 o Use MediaTracker s addimage() method to place each image under its control o When an image is added, a numeric ID is given o Before using the image, use checked() to see if it is ready The thread is spawned just to make sure that the applet is repainted as soon as the image is ready o waitforid() blocks the thread until the image becomes ready! The IDs control the priority order in which the images are fetched as well as identifying unique subsets of the images that can be waited on independently. 7 Algorithmic Image Generation Image Processing Applications Graphics Applications The need to compute individual pixel values. MemoryImageSource This class is an implementation of the ImageProducer interface which uses an array to produce pixel values for an Image. Here is an example which calculates a 100x100 image representing a fade from black to blue along the X axis and a fade from black to red along the Y axis: int w = 100; int h = 100; int pix[] = new int[w * h]; int index = 0; for (int y = 0; y < h; y++) { int red = (y * 255) / (h - 1); for (int x = 0; x < w; x++) { int blue = (x * 255) / (w - 1); pix[index++] = (255 << 24) (red << 16) blue;

9 } } Image img = createimage(new MemoryImageSource(w, h, pix, 0, w)); DEMO: MandelWindow 8 Producer Consumer Design java.awt.image relies on Producer-Consumer design pattern. Decoupling ImageProducer - ImageConsumer ImageProducer - Calls the methods of ImageConsumer to transfer image data and other information. setpixels method Image data is 1-D array, an offset into it, the size of scanline ImageConsumer - Creates the producer-consumer connection by announcing itself as receiving data from an ImageProducer via calling addconsumer() method. For example, MemoryImageSource - ImageProducer createimage - Uses a hidden ImageConsumer 9 Image Filtering Class FilteredImageSource An image + A Filter Image A Combined Image FilteredImage: ImageFilter {CropImageFilter,RGBImageFilter,ReplicateScaleFilter,AverageScaleFilter} Or create your own filter!

Rendering with Java. Jim Graham Staff Engineer JavaSoft

Rendering with Java. Jim Graham Staff Engineer JavaSoft Rendering with Java Jim Graham Staff Engineer JavaSoft Overview AWT paint/update callback model Graphics rendering Image rendering and manipulation Basic image fetching and drawing Off-screen images for