Applying Projection and Camera Views 运用投影和相机视图

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

• Projection - This transformation adjusts the coordinates of drawn objects based on the width and height of the​​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 ​​onSurfaceChanged()​​​ method of your renderer. For more information about OpenGL ES projections and coordinate mapping, see​​Mapping Coordinates for Drawn Objects​​.
• Camera View - 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​​GLSurfaceView​​, or might change dynamically based on user actions or your application’s function.

This lesson describes how to create a projection and camera view and apply it to shapes drawn in your​​GLSurfaceView​​.

Define a Projection

The data for a projection transformation is calculated in the ​​onSurfaceChanged()​​​ method of your​​GLSurfaceView.Renderer​​​ class. The following example code takes the height and width of the​​GLSurfaceView​​​ and uses it to populate a projection transformation​​Matrix​​​ using the ​​Matrix.frustumM()​​ method:

@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(mProjMatrix, 0, -ratio, ratio, -1, 1, 3, 7);
}

This code populates a projection matrix, ​​mProjMatrix​​​ which you can then combine with a camera view transformation in the​​onDrawFrame()​​ method, which is shown in the next section.

Note: 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.

Define a Camera View

Complete the process of transforming your drawn objects by adding a camera view transformation as part of the drawing process. In the following example code, the camera view transformation is calculated using the​​Matrix.setLookAtM()​​ method and then combined with the previously calculated projection matrix. The combined transformation matrices are then passed to the drawn shape.

@Override
public void onDrawFrame(GL10 unused) {
    ...

    // Set the camera position (View matrix)
    Matrix.setLookAtM(mVMatrix, 0, 0, 0, -3, 0f, 0f, 0f, 0f, 1.0f, 0.0f);

    // Calculate the projection and view transformation
    Matrix.multiplyMM(mMVPMatrix, 0, mProjMatrix, 0, mVMatrix, 0);

    // Draw shape
    mTriangle.draw(mMVPMatrix);
}

Apply Projection and Camera Transformations

In order to use the combined projection and camera view transformation matrix shown in the previews sections, modify the​​draw()​​ method of your graphic objects to accept the combined transformation matrix and apply it to the shape:

public void draw(float[] mvpMatrix) { // pass in the calculated transformation matrix
    ...

    // get handle to shape's transformation matrix
    mMVPMatrixHandle = GLES20.glGetUniformLocation(mProgram, "uMVPMatrix");

    // Apply the projection and view transformation
    GLES20.glUniformMatrix4fv(mMVPMatrixHandle, 1, false, mvpMatrix, 0);

    // Draw the triangle
    GLES20.glDrawArrays(GLES20.GL_TRIANGLES, 0, vertexCount);
    ...
}

Once you have correctly calulated and applied the projection and camera view transformations, your graphic objects are drawn in correct proportions and should look like this:

Figure 1. Triangle drawn with a projection and camera view applied.

Now that you have an application that displays your shapes in correct proportions, it's time to add motion to your shapes.