将纹理映射到正方形上：为什么图像会旋转？

Question

我成功地将图像映射到 OpenGL ES 中的正方形上...但它旋转了 90 度。我对 OpenGL ES 还很陌生，想知道是否有人可以指出它到底旋转的原因。谢谢！

package se.jayway.opengl.tutorial;

import java.io.IOException;
import java.io.InputStream;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.FloatBuffer;
import java.nio.ShortBuffer;

import javax.microedition.khronos.opengles.GL10;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.opengl.GLUtils;

public class Square {
// Our vertices.
private float vertices[] = {
        -1.0f, -1.0f, 0.0f, //Vertex 0
        1.0f, -1.0f, 0.0f,  //v1
        -1.0f, 1.0f, 0.0f,  //v2
        1.0f, 1.0f, 0.0f,   //v3
    };

//Our texture.
private float texture[] = {
        //Mapping coordinates for the vertices
        0.0f, 0.0f,
        0.0f, 1.0f,
        1.0f, 0.0f,
        1.0f, 1.0f, 
};

// The order we like to connect them.
private byte indices[] = {
        //Faces definition
        0,1,3, 0,3,2,           //Face front

                            };

// Our vertex buffer.
private FloatBuffer vertexBuffer;

// Our index buffer.
private ByteBuffer indexBuffer;

//texture buffer.
private FloatBuffer textureBuffer;

//Our texture pointer.
private int[] textures = new int[1];

public Square() {
    ByteBuffer byteBuf = ByteBuffer.allocateDirect(vertices.length * 4);
    byteBuf.order(ByteOrder.nativeOrder());
    vertexBuffer = byteBuf.asFloatBuffer();
    vertexBuffer.put(vertices);
    vertexBuffer.position(0);

    //
    byteBuf = ByteBuffer.allocateDirect(texture.length * 4);
    byteBuf.order(ByteOrder.nativeOrder());
    textureBuffer = byteBuf.asFloatBuffer();
    textureBuffer.put(texture);
    textureBuffer.position(0);

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

/**
 * This function draws our square on screen.
 * @param gl
 */
public void draw(GL10 gl) {
    //Bind our only previously generated texture in this case
    gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);

    //Point to our buffers
    gl.glEnableClientState(GL10.GL_VERTEX_ARRAY);
    gl.glEnableClientState(GL10.GL_TEXTURE_COORD_ARRAY);

    //Set the face rotation
    gl.glFrontFace(GL10.GL_CCW);

    //Enable the vertex and texture state
    gl.glVertexPointer(3, GL10.GL_FLOAT, 0, vertexBuffer);
    gl.glTexCoordPointer(2, GL10.GL_FLOAT, 0, textureBuffer);

    //Draw the vertices as triangles, based on the Index Buffer information

    gl.glDrawElements(GL10.GL_TRIANGLES, indices.length, GL10.GL_UNSIGNED_BYTE, indexBuffer);

    // Disable the vertices buffer.
    gl.glDisableClientState(GL10.GL_VERTEX_ARRAY);
    //Disable the texture buffer.
    gl.glDisableClientState(GL10.GL_TEXTURE_COORD_ARRAY);
    // Disable face culling.
    gl.glDisable(GL10.GL_CULL_FACE);
}
/**
 * Load the textures
 * 
 * @param gl - The GL Context
 * @param context - The Activity context
 */
public void loadGLTexture(GL10 gl, Context context) {
    //Get the texture from the Android resource directory
    InputStream is = context.getResources().openRawResource(R.drawable.android);
    Bitmap bitmap = null;
    try {
        //BitmapFactory is an Android graphics utility for images
        bitmap = BitmapFactory.decodeStream(is);

    } finally {
        //Always clear and close
        try {
            is.close();
            is = null;
        } catch (IOException e) {
        }
    }

    //Generate one texture pointer...
    gl.glGenTextures(1, textures, 0);
    //...and bind it to our array
    gl.glBindTexture(GL10.GL_TEXTURE_2D, textures[0]);

    //Create Nearest Filtered Texture
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MIN_FILTER, GL10.GL_NEAREST);
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_MAG_FILTER, GL10.GL_LINEAR);

    //Different possible texture parameters, e.g. GL10.GL_CLAMP_TO_EDGE
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_WRAP_S, GL10.GL_REPEAT);
    gl.glTexParameterf(GL10.GL_TEXTURE_2D, GL10.GL_TEXTURE_WRAP_T, GL10.GL_REPEAT);

    //Use the Android GLUtils to specify a two-dimensional texture image from our bitmap
    GLUtils.texImage2D(GL10.GL_TEXTURE_2D, 0, bitmap, 0);

    //Clean up
    bitmap.recycle();
}
}

Answer 1

vertices[] 对应于屏幕上的以下坐标：左下、右下、左上、右上（按顺序）。

texture[] 对应于以下纹理空间坐标：左下、左上、右下、右上。

你那里有一个不匹配的地方；更改任一数组中的顺序以匹配另一个数组。

Answer 2

纹理坐标已关闭，正确的坐标是：

    private float texture[] = {
        //Mapping coordinates for the vertices
        0.0f, 1.0f,
        1.0f, 1.0f,
        0.0f, 0.0f,
        1.0f, 0.0f, 
   };