OpenGL ES 下采样 OES 纹理：代码实现和步骤

OES 纹理是 OpenGL ES 扩展中的一种纹理类型，可以通过下采样的方式得到新的 OES 纹理。下面是具体的代码实现：

创建 OES 纹理对象

GLuint textureId;
glGenTextures(1, &textureId);
glBindTexture(GL_TEXTURE_EXTERNAL_OES, textureId);

设置 OES 纹理参数

glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

绑定 OES 纹理到 FBO

GLuint fboId;
glGenFramebuffers(1, &fboId);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_EXTERNAL_OES, textureId, 0);

设置渲染目标和视口

glViewport(0, 0, width, height);
glBindFramebuffer(GL_FRAMEBUFFER, fboId);

渲染场景到 OES 纹理

// 绘制场景

生成新的 OES 纹理

GLuint newTextureId;
glGenTextures(1, &newTextureId);
glBindTexture(GL_TEXTURE_EXTERNAL_OES, newTextureId);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameterf(GL_TEXTURE_EXTERNAL_OES, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexImage2D(GL_TEXTURE_EXTERNAL_OES, 0, GL_RGBA, width / 2, height / 2, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

绑定新的 OES 纹理到 FBO

GLuint newFboId;
glGenFramebuffers(1, &newFboId);
glBindFramebuffer(GL_FRAMEBUFFER, newFboId);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_EXTERNAL_OES, newTextureId, 0);

设置新的渲染目标和视口

glViewport(0, 0, width / 2, height / 2);
glBindFramebuffer(GL_FRAMEBUFFER, newFboId);

渲染 OES 纹理到新的 OES 纹理

glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

解绑 FBO 和纹理

glBindFramebuffer(GL_FRAMEBUFFER, 0);
glBindTexture(GL_TEXTURE_EXTERNAL_OES, 0);

通过上述步骤可以得到经过下采样后的新的 OES 纹理。