在 OpenGL ES 环境中,通过投影和相机视图,显示的绘制对象更接近于眼睛看到的实物。这种对实物查看的模拟是通过对绘制对象坐标进行数学转换完成的:
- 投影 - 这种转换可根据显示绘制对象的
GLSurfaceView
的宽度和高度调整绘制对象的坐标。如果不进行这种计算,OpenGL ES 绘制的对象会被不等比例的视图窗口所扭曲。通常只有在渲染程序的onSurfaceChanged()
方法中确定或更改 OpenGL 视图的比例时,才需要计算投影转换。如需详细了解 OpenGL ES 投影和坐标映射,请参阅映射绘制对象的坐标。 - 相机视图 - 这种转换可根据虚拟相机的位置调整绘制对象的坐标。请务必注意,OpenGL ES 不会定义实际的相机对象,而是通过转换绘制对象的显示方式提供模拟相机的实用程序方法。相机视图转换可能仅在您确定
GLSurfaceView
时计算一次,也可能会根据用户操作或应用的功能动态变化。
本课程介绍了如何创建投影和相机视图,并将其应用于 GLSurfaceView
中绘制的形状。
定义投影
用于投影转换的数据使用 GLSurfaceView.Renderer
的 onSurfaceChanged()
方法计算。以下示例代码采用 GLSurfaceView
的高度和宽度,并使用它通过 Matrix.frustumM()
方法填充投影转换 Matrix
:
Kotlin
// vPMatrix is an abbreviation for "Model View Projection Matrix" private val vPMatrix = FloatArray(16) private val projectionMatrix = FloatArray(16) private val viewMatrix = FloatArray(16) override fun onSurfaceChanged(unused: GL10, width: Int, height: Int) { GLES20.glViewport(0, 0, width, height) val ratio: Float = width.toFloat() / height.toFloat() // this projection matrix is applied to object coordinates // in the onDrawFrame() method Matrix.frustumM(projectionMatrix, 0, -ratio, ratio, -1f, 1f, 3f, 7f) }
Java
// vPMatrix is an abbreviation for "Model View Projection Matrix" private final float[] vPMatrix = new float[16]; private final float[] projectionMatrix = new float[16]; private final float[] viewMatrix = 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(projectionMatrix, 0, -ratio, ratio, -1, 1, 3, 7); }
该代码填充了一个投影矩阵 mProjectionMatrix
,您之后可以将其与 onDrawFrame()
方法中的相机视图转换合并,这在下一部分中进行介绍。
注意:仅将投影转换应用于绘制的对象通常会导致显示画面过于空旷。一般而言,要在屏幕上显示任何内容,您还必须应用相机视图转换。
定义相机视图
通过在渲染程序中添加相机视图转换作为绘制流程的一部分,完成绘制对象的转换流程。在以下示例代码中,相机视图转换使用 Matrix.setLookAtM()
方法进行计算,然后与之前计算的投影矩阵合并。之后,系统会将合并后的转换矩阵传递到绘制的形状。
Kotlin
override fun onDrawFrame(unused: GL10) { ... // Set the camera position (View matrix) Matrix.setLookAtM(viewMatrix, 0, 0f, 0f, -3f, 0f, 0f, 0f, 0f, 1.0f, 0.0f) // Calculate the projection and view transformation Matrix.multiplyMM(vPMatrix, 0, projectionMatrix, 0, viewMatrix, 0) // Draw shape triangle.draw(vPMatrix)
Java
@Override public void onDrawFrame(GL10 unused) { ... // Set the camera position (View matrix) Matrix.setLookAtM(viewMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f); // Calculate the projection and view transformation Matrix.multiplyMM(vPMatrix, 0, projectionMatrix, 0, viewMatrix, 0); // Draw shape triangle.draw(vPMatrix); }
应用投影和相机转换
为了使用预览部分中显示的合并后的投影和相机视图转换矩阵,请先将矩阵变体添加到之前在 Triangle
类中定义的顶点着色程序。
Kotlin
class Triangle { private val vertexShaderCode = // This matrix member variable provides a hook to manipulate // the coordinates of the objects that use this vertex shader "uniform mat4 uMVPMatrix;" + "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 = uMVPMatrix * vPosition;" + "}" // Use to access and set the view transformation private var vPMatrixHandle: Int = 0 ... }
Java
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 "uniform mat4 uMVPMatrix;" + "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 = uMVPMatrix * vPosition;" + "}"; // Use to access and set the view transformation private int vPMatrixHandle; ... }
接下来,修改图形对象的 draw()
方法以接受合并后的转换矩阵,并将其应用于形状:
Kotlin
fun draw(mvpMatrix: FloatArray) { // pass in the calculated transformation matrix // get handle to shape's transformation matrix vPMatrixHandle = GLES20.glGetUniformLocation(program, "uMVPMatrix") // Pass the projection and view transformation to the shader GLES20.glUniformMatrix4fv(vPMatrixHandle, 1, false, mvpMatrix, 0) // Draw the triangle GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount) // Disable vertex array GLES20.glDisableVertexAttribArray(positionHandle) }
Java
public void draw(float[] mvpMatrix) { // pass in the calculated transformation matrix ... // get handle to shape's transformation matrix vPMatrixHandle = GLES20.glGetUniformLocation(program, "uMVPMatrix"); // Pass the projection and view transformation to the shader GLES20.glUniformMatrix4fv(vPMatrixHandle, 1, false, mvpMatrix, 0); // Draw the triangle GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount); // Disable vertex array GLES20.glDisableVertexAttribArray(positionHandle); }
正确计算并应用投影和相机视图转换之后,系统会以正确的比例绘制您的图形对象,这些对象应如下所示:

图 1. 应用了投影和相机视图后绘制的三角形。
现在,您的应用已经能够按照正确的比例显示形状,接下来该为形状添加动画了。