对Android opengl ES世界坐标系和纹理坐标系的理解

初学opengl ES,每一个教你在屏幕上贴图的opengl版hello world都有这么两数组:

    static final float COORD[] = {
            -1.0f, -1.0f,
            1.0f, -1.0f,
            -1.0f, 1.0f,
            1.0f, 1.0f,
    };

    static final float TEXTURE_COORD[] = {
            0.0f, 1.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            1.0f, 0.0f,
    };

但是几乎都不解释，所以我学的时候都不明白这些点为什么要这么写，前后顺序有没有什么规律。于是各种查资料试验，终于搞懂了。

1.坐标系

PS：本人学opengl es主要是为了2D贴图，所以不涉及Z轴

如图，图一是opengl的世界坐标系，这个基本没啥问题，主要是很多教程说纹理坐标是左下原点。实践得出在Android上应该是最右边的图那样，以左上为原点。
个人猜测纹理吧其实就是一组颜色点组成的数组，Android由于UI坐标是以左上为原点，所以把数组里颜色点的存储顺序改了一下，于是坐标系就不一样了。

2.示例代码

public class Filter {

    protected static final String VERTEX_SHADER = "" +
            "attribute vec4 position;\n" +
            "attribute vec4 inputTextureCoordinate;\n" +
            " \n" +
            "varying vec2 textureCoordinate;\n" +
            " \n" +
            "void main()\n" +
            "{\n" +
            "    gl_Position = position;\n" +
            "    textureCoordinate = inputTextureCoordinate.xy;\n" +
            "}";
    protected static final String FRAGMENT_SHADER = "" +
            "varying highp vec2 textureCoordinate;\n" +
            " \n" +
            "uniform sampler2D inputImageTexture;\n" +
            " \n" +
            "void main()\n" +
            "{\n" +
            "     gl_FragColor = texture2D(inputImageTexture, textureCoordinate);\n" +
            "}";

    static final float COORD1[] = {
            -1.0f, -1.0f,
            1.0f, -1.0f,
            -1.0f, 1.0f,
            1.0f, 1.0f,
    };

    static final float TEXTURE_COORD1[] = {
            0.0f, 1.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            1.0f, 0.0f,
    };

    static final float COORD2[] = {
            -1.0f, 1.0f,
            -1.0f, -1.0f,
            1.0f, 1.0f,
            1.0f, -1.0f,
    };

    static final float TEXTURE_COORD2[] = {
            0.0f, 0.0f,
            0.0f, 1.0f,
            1.0f, 0.0f,
            1.0f, 1.0f,
    };

    static final float COORD3[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD3[] = {
            1.0f, 1.0f,
            1.0f, 0.0f,
            0.0f, 1.0f,
            0.0f, 0.0f,
    };


    static final float COORD4[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD4[] = {
            1.0f, 1.0f,
            1.0f, 0.0f,
            0.0f, 1.0f,
            0.0f, 0.0f,
    };

    static final float COORD_REVERSE[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD_REVERSE[] = {
            1.0f, 0.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            0.0f, 1.0f,
    };

    static final float COORD_FLIP[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD_FLIP[] = {
            0.0f, 1.0f,
            0.0f, 0.0f,
            1.0f, 1.0f,
            1.0f, 0.0f,
    };

    private String mVertexShader;
    private String mFragmentShader;

    private FloatBuffer mCubeBuffer;
    private FloatBuffer mTextureCubeBuffer;

    protected int mProgId;
    protected int mAttribPosition;
    protected int mAttribTexCoord;
    protected int mUniformTexture;


    public Filter() {
        this(VERTEX_SHADER, FRAGMENT_SHADER);
    }

    public Filter(String vertexShader, String fragmentShader) {
        mVertexShader = vertexShader;
        mFragmentShader = fragmentShader;
    }

    public void init() {
        loadVertex();
        initShader();
        GLES20.glBlendFunc(GLES20.GL_ONE, GLES20.GL_ONE_MINUS_SRC_ALPHA);
    }

    public void loadVertex() {
        float[] coord = COORD1;
        float[] texture_coord = TEXTURE_COORD1;

        mCubeBuffer = ByteBuffer.allocateDirect(coord.length * 4)
                .order(ByteOrder.nativeOrder())
                .asFloatBuffer();
        mCubeBuffer.put(coord).position(0);

        mTextureCubeBuffer = ByteBuffer.allocateDirect(texture_coord.length * 4)
                .order(ByteOrder.nativeOrder())
                .asFloatBuffer();
        mTextureCubeBuffer.put(texture_coord).position(0);
    }

    public void initShader() {
        mProgId = GLHelper.loadProgram(mVertexShader, mFragmentShader);
        mAttribPosition = GLES20.glGetAttribLocation(mProgId, "position");
        mUniformTexture = GLES20.glGetUniformLocation(mProgId, "inputImageTexture");
        mAttribTexCoord = GLES20.glGetAttribLocation(mProgId,
                "inputTextureCoordinate");
    }

    public void drawFrame(int glTextureId) {
        if (!GLES20.glIsProgram(mProgId)) {
            initShader();
        }
        GLES20.glUseProgram(mProgId);

        mCubeBuffer.position(0);
        GLES20.glVertexAttribPointer(mAttribPosition, 2, GLES20.GL_FLOAT, false, 0, mCubeBuffer);
        GLES20.glEnableVertexAttribArray(mAttribPosition);

        mTextureCubeBuffer.position(0);
        GLES20.glVertexAttribPointer(mAttribTexCoord, 2, GLES20.GL_FLOAT, false, 0,
                mTextureCubeBuffer);
        GLES20.glEnableVertexAttribArray(mAttribTexCoord);

        if (glTextureId != GLHelper.NO_TEXTURE) {
            GLES20.glActiveTexture(GLES20.GL_TEXTURE0);
            GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, glTextureId);
            GLES20.glUniform1i(mUniformTexture, 0);
        }
        GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4);

        GLES20.glDisableVertexAttribArray(mAttribPosition);
        GLES20.glDisableVertexAttribArray(mAttribTexCoord);
        GLES20.glBindTexture(GLES20.GL_TEXTURE_2D, 0);

        GLES20.glDisable(GLES20.GL_BLEND);

    }
}

其中
GLES20.glDrawArrays(GLES20.GL_TRIANGLE_STRIP, 0, 4);
由于openglES本身就是opengl的缩略版，所以能直接画的形状就只有三角形，别的复杂的都要由三角形来组成。GLES20.GL_TRIANGLE_STRIP指的就是一种三角形的绘制模式，对应这个顶点数组：

  static final float COORD[] = {
            -1.0f, -1.0f,  //1
            1.0f, -1.0f,   //2
            -1.0f, 1.0f,   //3
            1.0f, 1.0f,    //4
    };

实际绘制的就是顶点1,2,3组成的三角形和顶点2,3,4组成的三角形合并成的一个矩形，如果有更多点，依次类推（比如有5个点，就是1,2,3 2,3,4 3,4,5三个三角形组成的图案）。如下图：

3.纹理顶点顺序

纹理的点和世界坐标的点之间是对应的：

    static final float COORD1[] = {
            -1.0f, -1.0f,
            1.0f, -1.0f,
            -1.0f, 1.0f,
            1.0f, 1.0f,
    };

    static final float TEXTURE_COORD1[] = {
            0.0f, 1.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            1.0f, 0.0f,
    };

显示结果如图：

如图中箭头，opengl会把纹理中颜色顶点绘到对应的世界坐标顶点上，中间的点则按一定的规律取个平均值什么的，所以可见实际显示的图被上下拉伸了，因为原图是1:1，而在该程序里
GLES20.glViewport(0, 0, width, height);
赋予的显示区域是高大于宽的（这里涉及到opengl世界坐标和屏幕坐标的映射，和本文主旨关系不大就不多说了）。

其实也就是只要世界坐标和纹理坐标数组里的点能够对的上，顺序不是问题
代码里的四组坐标的显示效果都是一样的：

 static final float COORD1[] = {
            -1.0f, -1.0f,
            1.0f, -1.0f,
            -1.0f, 1.0f,
            1.0f, 1.0f,
    };

    static final float TEXTURE_COORD1[] = {
            0.0f, 1.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            1.0f, 0.0f,
    };

    static final float COORD2[] = {
            -1.0f, 1.0f,
            -1.0f, -1.0f,
            1.0f, 1.0f,
            1.0f, -1.0f,
    };

    static final float TEXTURE_COORD2[] = {
            0.0f, 0.0f,
            0.0f, 1.0f,
            1.0f, 0.0f,
            1.0f, 1.0f,
    };

    static final float COORD3[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD3[] = {
            1.0f, 1.0f,
            1.0f, 0.0f,
            0.0f, 1.0f,
            0.0f, 0.0f,
    };


    static final float COORD4[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD4[] = {
            1.0f, 1.0f,
            1.0f, 0.0f,
            0.0f, 1.0f,
            0.0f, 0.0f,
    };

不信的可以在这里都替换了试试：

      float[] coord = COORD1;
      float[] texture_coord = TEXTURE_COORD1;

为了加深理解，甚至可以玩点花样出来，比如这样

    static final float COORD_REVERSE[] = {
            1.0f, -1.0f,
            1.0f, 1.0f,
            -1.0f, -1.0f,
            -1.0f, 1.0f,
    };

    static final float TEXTURE_COORD_REVERSE[] = {
            1.0f, 0.0f,
            1.0f, 1.0f,
            0.0f, 0.0f,
            0.0f, 1.0f,
    };
  。。。。。。。。。。。。。。。。
      float[] coord = COORD_REVERSE;
      float[] texture_coord = TEXTURE_COORD_REVERSE;

结果如下图：

4.Demo源码地址

https://github.com/yellowcath/GLCoordDemo.git