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:
<code class="hljs cpp">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, };</code>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，
对Android opengl ES世界坐标系和纹理坐标系的理解
。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
<code class="hljs cpp">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); }}</code>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，
电脑资料
《对Android opengl ES世界坐标系和纹理坐标系的理解》(https://www.unjs.com)。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：<code class="hljs cpp">static final float COORD[] = { -1.0f, -1.0f, //1 1.0f, -1.0f, //2 -1.0f, 1.0f, //3 1.0f, 1.0f, //4 };</code>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：
<code class="hljs cpp">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, };</code>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：
<code class="hljs cpp">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, };</code>不信的可以在这里都替换了试试：
<code class="hljs cs">float[] coord = COORD1; float[] texture_coord = TEXTURE_COORD1;</code>
为了加深理解，甚至可以玩点花样出来，比如这样
<code class="hljs cpp">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;</code>结果如下图：
4.Demo源码地址
https://github.com/yellowcath/GLCoordDemo.git