java - 为什么是 glTranslatef？

Question

我只是不明白为什么在这种情况下如果我摆脱这个笔划gl.glTranslatef(0.0f,(float)Math.sin(mTransY), -3.0f); square只是不画画？ 完整代码在这里(并忽略那些西里尔注释 - 我只是为自己写了每个函数的作用)

MainActivity 类

package opengl.tutorial.one;

import android.app.Activity;
import android.opengl.GLSurfaceView;
import android.os.Bundle;
import android.view.WindowManager;
public class MainActivity extends Activity
{
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
            getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                WindowManager.LayoutParams.FLAG_FULLSCREEN);
                GLSurfaceView view = new GLSurfaceView(this);
                view.setRenderer(new SquareRenderer(true));
                setContentView(view);
    }
}

方形类

package opengl.tutorial.one;

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.FloatBuffer;
import java.nio.IntBuffer;
import javax.microedition.khronos.opengles.GL10; //1
import javax.microedition.khronos.opengles.GL11;

public class Square {

    private FloatBuffer mFVertexBuffer;
    private ByteBuffer mColorBuffer;
    private ByteBuffer mIndexBuffer;

    public Square(){

        float vertices[] = {
            -1.0f, -1.0f,//0
            1.0f, -1.0f, //1
            -1.0f, 1.0f, //2
            1.0f, 1.0f  //3
        };//задали положение веришн квадрата по координатам

        byte maxColor=(byte)255;

        byte colors[] = //задали цвет веришнам
        {
            maxColor,maxColor, 0,maxColor,
            maxColor,maxColor, 0,maxColor,
            maxColor,maxColor, 0,maxColor,
            maxColor,maxColor, 0,maxColor,
        };

        byte indices[] = //порядок построения треугольников вершинами
        {
            0, 3, 1, //первый
            0, 2, 3  //второй
        };

        ByteBuffer vbb = ByteBuffer.allocateDirect(vertices.length * 4); //5
        vbb.order(ByteOrder.nativeOrder());

        mFVertexBuffer = vbb.asFloatBuffer();
        mFVertexBuffer.put(vertices);
        mFVertexBuffer.position(0);

        mColorBuffer = ByteBuffer.allocateDirect(colors.length);
        mColorBuffer.put(colors);
        mColorBuffer.position(0);

        mIndexBuffer = ByteBuffer.allocateDirect(indices.length);
        mIndexBuffer.put(indices);
        mIndexBuffer.position(0);
    }

    public void draw(GL10 gl) //6
    {
        gl.glFrontFace(GL11.GL_CW); //выводит полигон по заданной ориентации к экрану. CW - по часовой
        gl.glVertexPointer(2, GL11.GL_FLOAT, 0, mFVertexBuffer); //Установка массива для вершинных координат 
        gl.glColorPointer(4, GL11.GL_UNSIGNED_BYTE, 0, mColorBuffer); //9
        gl.glDrawElements(GL11.GL_TRIANGLES, 6,GL11.GL_UNSIGNED_BYTE, mIndexBuffer);
        gl.glFrontFace(GL11.GL_CCW); //выводит полигон по заданной ориентации к экрану. СCW - против часовой
    }

}

类 SquareRenderer

包 opengl.tutorial.one;

import javax.microedition.khronos.egl.EGL10; //1
import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;
import android.opengl.GLSurfaceView.Renderer;

import java.lang.Math;

public class SquareRenderer implements Renderer  {

    private boolean mTranslucentBackground;
    private Square mSquare;
    private float mTransY;
    private float mAngle;

    public SquareRenderer(boolean useTranslucentBackground)
    {
        mTranslucentBackground = useTranslucentBackground;//прозрачный фон
        mSquare = new Square(); //новый объект типа квадрат
    }

    public void onDrawFrame(GL10 gl) //4
    {
        gl.glClear(GL10.GL_COLOR_BUFFER_BIT | GL10.GL_DEPTH_BUFFER_BIT); //5
        gl.glMatrixMode(GL10.GL_MODELVIEW); //6
        gl.glLoadIdentity(); //7
        gl.glTranslatef(0.0f,(float)Math.sin(mTransY), -3.0f); //8
        gl.glEnableClientState(GL10.GL_VERTEX_ARRAY); //9
        gl.glEnableClientState(GL10.GL_COLOR_ARRAY);
        mSquare.draw(gl); //10
        mTransY += 0f;
    }

    public void onSurfaceChanged(GL10 gl, int width, int height) //11
    {
        gl.glViewport(0, 0, width, height); //12
        float ratio = (float) width / height;
        gl.glMatrixMode(GL10.GL_PROJECTION); //13
        gl.glLoadIdentity();
        gl.glFrustumf(-ratio, ratio, -1, 1, 1, 10); //14
    }

    public void onSurfaceCreated(GL10 gl, EGLConfig config) //15
    {
        gl.glDisable(GL10.GL_DITHER); //16
        gl.glHint(GL10.GL_PERSPECTIVE_CORRECTION_HINT, GL10.GL_FASTEST);
        if (mTranslucentBackground) //18
        {

        }
        else
        {
            gl.glClearColor(1,1,1,1);
        }
        gl.glEnable(GL10.GL_CULL_FACE); //19
        gl.glShadeModel(GL10.GL_SMOOTH); //20
        gl.glEnable(GL10.GL_DEPTH_TEST); //21
    }

}

Answer 1

整个正方形位于 z=0 平面。 glTranslatef 通过 z 轴定义视点 3.0。如果没有此调用，视点的坐标为 z=0，因此您正在查看正方形本身内部的正方形。

使用 glTranslatef 坐标来看看会发生什么。了解每个 OpenGL 函数含义的最佳方法是排除/包含它(好吧，您已经这样做了...)并设置不同的参数值。

glTranslatef 的 y 参数给出绕 y 轴的旋转效果。