1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
|
page.title=Applying Projection and Camera Views
parent.title=Displaying Graphics with OpenGL ES
parent.link=index.html
trainingnavtop=true
previous.title=Drawing Shapes
previous.link=draw.html
next.title=Applying Projection and Camera Views
next.link=projection.html
@jd:body
<div id="tb-wrapper">
<div id="tb">
<h2>This lesson teaches you to</h2>
<ol>
<li><a href="#projection">Define a Projection</a></li>
<li><a href="#camera-view">Define a Camera View</a></li>
<li><a href="#transform">Apply Projection and Camera Transformations</a></li>
</ol>
<h2>You should also read</h2>
<ul>
<li><a href="{@docRoot}guide/topics/graphics/opengl.html">OpenGL</a></li>
</ul>
<div class="download-box">
<a href="{@docRoot}shareables/training/OpenGLES.zip"
class="button">Download the sample</a>
<p class="filename">OpenGLES.zip</p>
</div>
</div>
</div>
<p>In the OpenGL ES environment, projection and camera views allow you to display drawn objects in a
way that more closely resembles how you see physical objects with your eyes. This simulation of
physical viewing is done with mathematical transformations of drawn object coordinates:</p>
<ul>
<li><em>Projection</em> - This transformation adjusts the coordinates of drawn objects based on
the width and height of the {@link android.opengl.GLSurfaceView} where they are displayed. Without
this calculation, objects drawn by OpenGL ES are skewed by the unequal proportions of the view
window. A projection transformation typically only has to be calculated when the proportions of the
OpenGL view are established or changed in the {@link
android.opengl.GLSurfaceView.Renderer#onSurfaceChanged
onSurfaceChanged()} method of your renderer. For more information about OpenGL ES projections and
coordinate mapping, see <a
href="{@docRoot}guide/topics/graphics/opengl.html#coordinate-mapping">Mapping Coordinates for Drawn
Objects</a>.</li>
<li><em>Camera View</em> - This transformation adjusts the coordinates of drawn objects based on a
virtual camera position. It’s important to note that OpenGL ES does not define an actual camera
object, but instead provides utility methods that simulate a camera by transforming the display of
drawn objects. A camera view transformation might be calculated only once when you establish your
{@link android.opengl.GLSurfaceView}, or might change dynamically based on user actions or your
application’s function.</li>
</ul>
<p>This lesson describes how to create a projection and camera view and apply it to shapes drawn in
your {@link android.opengl.GLSurfaceView}.</p>
<h2 id="projection">Define a Projection</h2>
<p>The data for a projection transformation is calculated in the {@link
android.opengl.GLSurfaceView.Renderer#onSurfaceChanged onSurfaceChanged()}
method of your {@link android.opengl.GLSurfaceView.Renderer} class. The following example code
takes the height and width of the {@link android.opengl.GLSurfaceView} and uses it to populate a
projection transformation {@link android.opengl.Matrix} using the {@link
android.opengl.Matrix#frustumM Matrix.frustumM()} method:</p>
<pre>
// mMVPMatrix is an abbreviation for "Model View Projection Matrix"
private final float[] mMVPMatrix = new float[16];
private final float[] mProjectionMatrix = new float[16];
private final float[] mViewMatrix = new float[16];
@Override
public void onSurfaceChanged(GL10 unused, int width, int height) {
GLES20.glViewport(0, 0, width, height);
float ratio = (float) width / height;
// this projection matrix is applied to object coordinates
// in the onDrawFrame() method
Matrix.frustumM(mProjectionMatrix, 0, -ratio, ratio, -1, 1, 3, 7);
}
</pre>
<p>This code populates a projection matrix, {@code mProjectionMatrix} which you can then combine
with a camera view transformation in the {@link android.opengl.GLSurfaceView.Renderer#onDrawFrame
onDrawFrame()} method, which is shown in the next section.</p>
<p class="note"><strong>Note:</strong> Just applying a projection transformation to your
drawing objects typically results in a very empty display. In general, you must also apply a camera
view transformation in order for anything to show up on screen.</p>
<h2 id="camera-view">Define a Camera View</h2>
<p>Complete the process of transforming your drawn objects by adding a camera view transformation as
part of the drawing process in your renderer. In the following example code, the camera view
transformation is calculated using the {@link android.opengl.Matrix#setLookAtM Matrix.setLookAtM()}
method and then combined with the previously calculated projection matrix. The combined
transformation matrices are then passed to the drawn shape.</p>
<pre>
@Override
public void onDrawFrame(GL10 unused) {
...
// Set the camera position (View matrix)
Matrix.setLookAtM(mViewMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);
// Calculate the projection and view transformation
Matrix.multiplyMM(mMVPMatrix, 0, mProjectionMatrix, 0, mViewMatrix, 0);
// Draw shape
mTriangle.draw(mMVPMatrix);
}
</pre>
<h2 id="#transform">Apply Projection and Camera Transformations</h2>
<p>In order to use the combined projection and camera view transformation matrix shown in the
previews sections, first add a matrix variable to the <em>vertex shader</em> previously defined
in the <code>Triangle</code> class:</p>
<pre>
public class Triangle {
private final String vertexShaderCode =
// This matrix member variable provides a hook to manipulate
// the coordinates of the objects that use this vertex shader
<strong>"uniform mat4 uMVPMatrix;" +</strong>
"attribute vec4 vPosition;" +
"void main() {" +
// the matrix must be included as a modifier of gl_Position
// Note that the uMVPMatrix factor *must be first* in order
// for the matrix multiplication product to be correct.
" gl_Position = <strong>uMVPMatrix</strong> * vPosition;" +
"}";
// Use to access and set the view transformation
private int mMVPMatrixHandle;
...
}
</pre>
<p>Next, modify the {@code draw()} method of your graphic objects to accept the combined
transformation matrix and apply it to the shape:</p>
<pre>
public void draw(float[] mvpMatrix) { // pass in the calculated transformation matrix
...
// get handle to shape's transformation matrix
<strong>mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");</strong>
// Pass the projection and view transformation to the shader
<strong>GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0);</strong>
// Draw the triangle
GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount);
// Disable vertex array
GLES20.glDisableVertexAttribArray(mPositionHandle);
}
</pre>
<p>Once you have correctly calculated and applied the projection and camera view transformations,
your graphic objects are drawn in correct proportions and should look like this:</p>
<img src="{@docRoot}images/opengl/ogl-triangle-projected.png">
<p class="img-caption">
<strong>Figure 1.</strong> Triangle drawn with a projection and camera view applied.</p>
<p>Now that you have an application that displays your shapes in correct proportions, it's time to
add motion to your shapes.</p>
|