1page.title=Defining Shapes 2parent.title=Displaying Graphics with OpenGL ES 3parent.link=index.html 4 5trainingnavtop=true 6previous.title=Building an OpenGL ES Environment 7previous.link=environment.html 8next.title=Drawing Shapes 9next.link=draw.html 10 11@jd:body 12 13<div id="tb-wrapper"> 14<div id="tb"> 15 16<h2>This lesson teaches you to</h2> 17<ol> 18 <li><a href="#triangle">Define a Triangle</a></li> 19 <li><a href="#square">Define a Square</a></li> 20</ol> 21 22<h2>You should also read</h2> 23<ul> 24 <li><a href="{@docRoot}guide/topics/graphics/opengl.html">OpenGL</a></li> 25</ul> 26 27<div class="download-box"> 28 <a href="{@docRoot}shareables/training/OpenGLES.zip" 29class="button">Download the sample</a> 30 <p class="filename">OpenGLES.zip</p> 31</div> 32 33</div> 34</div> 35 36<p>Being able to define shapes to be drawn in the context of an OpenGL ES view is the first step in 37creating your high-end graphics masterpiece. Drawing with OpenGL ES can be a little tricky without 38knowing a few basic things about how OpenGL ES expects you to define graphic objects.</p> 39 40<p>This lesson explains the OpenGL ES coordinate system relative to an Android device screen, the 41basics of defining a shape, shape faces, as well as defining a triangle and a square.</p> 42 43 44<h2 id="triangle">Define a Triangle</h2> 45 46<p>OpenGL ES allows you to define drawn objects using coordinates in three-dimensional space. So, 47before you can draw a triangle, you must define its coordinates. In OpenGL, the typical way to do 48this is to define a vertex array of floating point numbers for the coordinates. For maximum 49efficiency, you write these coordinates into a {@link java.nio.ByteBuffer}, that is passed into the 50OpenGL ES graphics pipeline for processing.</p> 51 52<pre> 53class Triangle { 54 55 private FloatBuffer vertexBuffer; 56 57 // number of coordinates per vertex in this array 58 static final int COORDS_PER_VERTEX = 3; 59 static float triangleCoords[] = { // in counterclockwise order: 60 0.0f, 0.622008459f, 0.0f, // top 61 -0.5f, -0.311004243f, 0.0f, // bottom left 62 0.5f, -0.311004243f, 0.0f // bottom right 63 }; 64 65 // Set color with red, green, blue and alpha (opacity) values 66 float color[] = { 0.63671875f, 0.76953125f, 0.22265625f, 1.0f }; 67 68 public Triangle() { 69 // initialize vertex byte buffer for shape coordinates 70 ByteBuffer bb = ByteBuffer.allocateDirect( 71 // (number of coordinate values * 4 bytes per float) 72 triangleCoords.length * 4); 73 // use the device hardware's native byte order 74 bb.order(ByteOrder.nativeOrder()); 75 76 // create a floating point buffer from the ByteBuffer 77 vertexBuffer = bb.asFloatBuffer(); 78 // add the coordinates to the FloatBuffer 79 vertexBuffer.put(triangleCoords); 80 // set the buffer to read the first coordinate 81 vertexBuffer.position(0); 82 } 83} 84</pre> 85 86<p>By default, OpenGL ES assumes a coordinate system where [0,0,0] (X,Y,Z) specifies the center of 87the {@link android.opengl.GLSurfaceView} frame, [1,1,0] is the top right corner of the frame and 88[-1,-1,0] is bottom left corner of the frame. For an illustration of this coordinate system, see the 89<a href="{@docRoot}guide/topics/graphics/opengl.html#coordinate-mapping">OpenGL ES</a> developer 90guide.</p> 91 92<p>Note that the coordinates of this shape are defined in a counterclockwise order. The drawing 93order is important because it defines which side is the front face of the shape, which you typically 94want to have drawn, and the back face, which you can choose to not draw using the OpenGL ES cull 95face feature. For more information about faces and culling, see the <a 96href="{@docRoot}guide/topics/graphics/opengl.html#faces-winding">OpenGL ES</a> developer guide.</p> 97 98 99<h2 id="square">Define a Square</h2> 100 101<p>Defining triangles is pretty easy in OpenGL, but what if you want to get a just a little more 102complex? Say, a square? There are a number of ways to do this, but a typical path to drawing such a 103shape in OpenGL ES is to use two triangles drawn together:</p> 104 105<img src="{@docRoot}images/opengl/ccw-square.png"> 106<p class="img-caption"> 107 <strong>Figure 1.</strong> Drawing a square using two triangles.</p> 108 109<p>Again, you should define the vertices in a counterclockwise order for both triangles that 110represent this shape, and put the values in a {@link java.nio.ByteBuffer}. In order to avoid 111defining the two coordinates shared by each triangle twice, use a drawing list to tell the 112OpenGL ES graphics pipeline how to draw these vertices. Here’s the code for this shape:</p> 113 114<pre> 115class Square { 116 117 private FloatBuffer vertexBuffer; 118 private ShortBuffer drawListBuffer; 119 120 // number of coordinates per vertex in this array 121 static final int COORDS_PER_VERTEX = 3; 122 static float squareCoords[] = { -0.5f, 0.5f, 0.0f, // top left 123 -0.5f, -0.5f, 0.0f, // bottom left 124 0.5f, -0.5f, 0.0f, // bottom right 125 0.5f, 0.5f, 0.0f }; // top right 126 127 private short drawOrder[] = { 0, 1, 2, 0, 2, 3 }; // order to draw vertices 128 129 public Square() { 130 // initialize vertex byte buffer for shape coordinates 131 ByteBuffer bb = ByteBuffer.allocateDirect( 132 // (# of coordinate values * 4 bytes per float) 133 squareCoords.length * 4); 134 bb.order(ByteOrder.nativeOrder()); 135 vertexBuffer = bb.asFloatBuffer(); 136 vertexBuffer.put(squareCoords); 137 vertexBuffer.position(0); 138 139 // initialize byte buffer for the draw list 140 ByteBuffer dlb = ByteBuffer.allocateDirect( 141 // (# of coordinate values * 2 bytes per short) 142 drawOrder.length * 2); 143 dlb.order(ByteOrder.nativeOrder()); 144 drawListBuffer = dlb.asShortBuffer(); 145 drawListBuffer.put(drawOrder); 146 drawListBuffer.position(0); 147 } 148} 149</pre> 150 151<p>This example gives you a peek at what it takes to create more complex shapes with OpenGL. In 152general, you use collections of triangles to draw objects. In the next lesson, you learn how to draw 153these shapes on screen.</p> 154
