Android OpenGL 坐标系 <2>

原文地址：http://blog.csdn.net/qq_31726827/article/details/51265186

计算机中任何绘图都离不开坐标系只是在每种绘图中坐标系形式有些差异,比如平时做Android的View,在自定义View中绘制直线,那么这个时候默认的是采用的是屏幕坐标系,即坐标原点在屏幕左上角,X轴向右,Y向下:

而在opengl中采用的是3维坐标:

坐标原点默认在屏幕的中间,即(width/2,height/2)位置上,z轴是从屏幕"内"指向屏幕外,而且还要注意原点和x,y轴平面是在屏幕的"表面",有引号,这个"表面"刚好是人看不到的面,所以如果你要画一条线,线端点(1,0,0),(0,1,0),直接画到屏幕上,将看不到显示,怎么办?很简单,这个坐标系是三维空间的,那么就将坐标系沿Z轴的负轴方向移动一点,将X,Y平面稍微向屏幕内部移动一点,就能够看到直线了,想象一下在三维空间中,将坐标系往屏幕里面推一下,那么x,y轴形成的平面就在屏幕里面了,在它上面的直线就可以看见了.后面有例子.

纹理坐标系:

一般坐标系都自由操作移动(或旋转).

基本了解了坐标系,后面就可以开始绘图了

根据上面的可以通过画直线画出OPENGL坐标系,这样就可以被看见了:

<1> : android studio新建PumpKinBasicGL10工程:程序如下:

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1. package org.durian.pumpkinbasicgl10;  
2.   
3. import android.app.Activity;  
4. import android.opengl.GLSurfaceView;  
5. import android.os.Bundle;  
6.   
7. import org.durian.pumpkinbasicgl10.draw2d.PumpKinRenderer;  
8.   
9.   
10. public class MainActivity extends Activity {  
11.   
12.     private GLSurfaceView mSurfaceView;  
13.     @Override  
14.     protected void onCreate(Bundle savedInstanceState) {  
15.         super.onCreate(savedInstanceState);  
16.   
17.         mSurfaceView=new GLSurfaceView(this);  
18.         mSurfaceView.setRenderer(new PumpKinRenderer()/*new PumpkinDotRenderer()*//*new PumpKinLineRenderer(this)*//*new PumpKinTriangleRenderer()*//*new PumpKinPyramidRenderer()*/);  
19.         setContentView(mSurfaceView/*R.layout.activity_main*/);  
20.     }  
21.   
22.     @Override  
23.     protected void onResume() {  
24.         super.onResume();  
25.         mSurfaceView.onResume();  
26.     }  
27.   
28.     @Override  
29.     protected void onPause() {  
30.         super.onPause();  
31.         mSurfaceView.onPause();  
32.     }  
33. }  

渲染类:

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1. </pre><pre class="html" name="code">package org.durian.pumpkinbasicgl10.draw2d;  
2.   
3. import android.opengl.GLSurfaceView;  
4. import android.opengl.GLU;  
5.   
6. import javax.microedition.khronos.egl.EGLConfig;  
7. import javax.microedition.khronos.opengles.GL10;  
8.   
9. /**  
10.  * Created by Administrator on 2016/4/28.  
11.  */  
12. public class PumpKinRenderer implements GLSurfaceView.Renderer {  
13.   
14.     private PumpKin pumpKin;  
15.   
16.     public PumpKinRenderer(){  
17.         pumpKin=new PumpKin();  
18.     }  
19.     @Override  
20.     public void onSurfaceCreated(GL10 gl, EGLConfig config) {  
21.   
22.         gl.glClearColor(0.0f,0.0f,0.0f,1.0f);  
23.         gl.glClearDepthf(1.0f);  
24.         gl.glEnable(GL10.GL_DEPTH_TEST);  
25.         gl.glDepthFunc(GL10.GL_LEQUAL);  
26.         gl.glHint(GL10.GL_PERSPECTIVE_CORRECTION_HINT,GL10.GL_NICEST);  
27.         gl.glShadeModel(GL10.GL_SMOOTH);  
28.         gl.glDisable(GL10.GL_DITHER);  
29.   
30.     }  
31.   
32.     @Override  
33.     public void onSurfaceChanged(GL10 gl, int width, int height) {  
34.   
35.         if(height==0){  
36.             height=1;  
37.         }  
38.         float aspect=(float)width/height;  
39.   
40.         gl.glViewport(0,0,width,height);  
41.   
42.         gl.glMatrixMode(GL10.GL_PROJECTION);  
43.         gl.glLoadIdentity();  
44.         GLU.gluPerspective(gl, 45, aspect, 0.1f, 100.0f);  
45.   
46.         gl.glMatrixMode(GL10.GL_MODELVIEW);  
47.         gl.glLoadIdentity();  
48.   
49.     }  
50.   
51.     @Override  
52.     public void onDrawFrame(GL10 gl) {  
53.   
54.         gl.glClear(GL10.GL_COLOR_BUFFER_BIT|GL10.GL_DEPTH_BUFFER_BIT);  
55.   
56.         gl.glLoadIdentity();  
57.         gl.glTranslatef(0.0f,0.0f,-2f);  
58.         //gl.glRotatef(45,1.0f,1.0f,1.0f);  
59.         pumpKin.draw(gl);  
60.   
61.     }  
62. }  

这个渲染类里面稍微注意将坐标系"下沉"-2f单位.这样方便看到图形.可以试一下不执行:

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1. gl.glTranslatef(0.0f,0.0f,-2f);  

就能够体会了.

绘图类:

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1. package org.durian.pumpkinbasicgl10.draw2d;  
2.   
3. import java.nio.ByteBuffer;  
4. import java.nio.ByteOrder;  
5. import java.nio.FloatBuffer;  
6.   
7. import javax.microedition.khronos.opengles.GL10;  
8.   
9. /**  
10.  * Created by Administrator on 2016/4/28.  
11.  */  
12. public class PumpKin {  
13.   
14.     private FloatBuffer vertexsBuffer;  
15.     private FloatBuffer colorsBuffer;  
16.   
17.     private ByteBuffer indicesBuffer;  
18.   
19.     private float vertexs[]={  
20.         0.0f,0.0f,0.0f,  
21.             1.0f,0.0f,0.0f,  
22.             0.0f,0.0f,0.0f,  
23.             0.0f,1.0f,0.0f,  
24.             0.0f,0.0f,0.0f,  
25.             0.0f,0.0f,1.0f  
26.     };  
27.   
28.     private float colors[]={  
29.             1.0f,0.0f,0.0f,1.0f,  
30.             1.0f,0.0f,0.0f,1.0f,  
31.             0.0f,1.0f,0.0f,1.0f,  
32.             0.0f,1.0f,0.0f,1.0f,  
33.             0.0f,0.0f,1.0f,1.0f,  
34.             0.0f,0.0f,1.0f,1.0f  
35.     };  
36.   
37.     private byte indices[]={0,1,2};  
38.   
39.     public PumpKin(){  
40.   
41.         ByteBuffer vbb=ByteBuffer.allocateDirect(vertexs.length*4);  
42.         vbb.order(ByteOrder.nativeOrder());  
43.         vertexsBuffer=vbb.asFloatBuffer();  
44.         vertexsBuffer.put(vertexs);  
45.         vertexsBuffer.position(0);  
46.   
47.         ByteBuffer cbb=ByteBuffer.allocateDirect(colors.length*4);  
48.         cbb.order(ByteOrder.nativeOrder());  
49.         colorsBuffer=cbb.asFloatBuffer();  
50.         colorsBuffer.put(colors);  
51.         colorsBuffer.position(0);  
52.   
53.     }  
54.   
55.     public void draw(GL10 gl){  
56.   
57.         gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);  
58.         gl.glEnableClientState(GL10.GL_COLOR_ARRAY);  
59.   
60.         gl.glVertexPointer(3,GL10.GL_FLOAT,0,vertexsBuffer);  
61.         gl.glColorPointer(4,GL10.GL_FLOAT,0,colorsBuffer);  
62.   
63.         gl.glDrawArrays(GL10.GL_LINES,0,vertexs.length/3);  
64.   
65.         gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);  
66.   
67.     }  
68.   
69. }  

给线添加颜色:

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1. private float colors[]={  
2.             1.0f,0.0f,0.0f,1.0f,  
3.             1.0f,0.0f,0.0f,1.0f,  
4.             0.0f,1.0f,0.0f,1.0f,  
5.             0.0f,1.0f,0.0f,1.0f,  
6.             0.0f,0.0f,1.0f,1.0f,  
7.             0.0f,0.0f,1.0f,1.0f  
8.     };  

colors[0]和colors[1]表示从原点到坐标(1,0,0)的颜色,如果[0]和[1]颜色不同,将会产生颜色渐变.

如果加入上面数组改为:

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1. private float colors[]={  
2.             1.0f,0.0f,0.0f,1.0f,  
3.             1.0f,0.0f,0.0f,1.0f,  
4.             0.0f,1.0f,0.0f,1.0f,  
5.             0.0f,1.0f,0.0f,1.0f  
6.     };  

试一下,画Z会是什么颜色?

将会是(0.0f,1.0f,0.0f,1.0f),即绿色.

运行结果:

发现只有两个坐标,Z轴没有发现,其实Z轴目前正垂直于屏幕,指向屏幕外,所以看不见,可以修改程序,让坐标旋转一下:

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1. @Override  
2.     public void onDrawFrame(GL10 gl) {  
3.   
4.         gl.glClear(GL10.GL_COLOR_BUFFER_BIT|GL10.GL_DEPTH_BUFFER_BIT);  
5.   
6.         gl.glLoadIdentity();  
7.         gl.glTranslatef(0.0f,0.0f,-2f);  
8.         gl.glRotatef(45,1.0f,1.0f,1.0f);  
9.         pumpKin.draw(gl);  
10.   
11.     }  

沿着矢量(1.0f,1.0f,1.0f)做右手定则式旋转,即手握住这个矢量方向,顺手指的方向旋转45度.也可以说做了矩阵变换后得到的.

途中虚线是截图的问题,实际屏幕是连续的.

红线 : X轴方向;

绿线: Y轴方向;

蓝线: Z轴方向;