FBO：在绘制渲染纹理时渲染到纹理，错误的纹理映射
我在Mac OS X应用程序上使用OpengGL在NSOpenGLView上绘制纹理。
该应用程序是一个电影播放器。 它将电影帧解码为CVOpenGLTextureRef （OpenGL纹理），并使用GL_QUAD将它们直接绘制到视图中。 一切工作正常。
以下是代码的相关部分。
// "image" is the CVOpenGLTextureRef containing the movie frame as texture
GLenum textureTarget = CVOpenGLTextureGetTarget(image);
GLuint textureName = CVOpenGLTextureGetName(image);
glEnable(textureTarget);
glBindTexture(textureTarget, textureName);
glBegin(GL_QUADS);
// Draw the quads
//Note: textureTagret is NOT GL_TEXTURE_2D, therefore texture coordinates
//are NOT scaled to [0, 1]
glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex2f (0.0f, windowRect.size.height);
glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (windowRect.size.width, windowRect.size.height);
glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (windowRect.size.width,0.0f);
glEnd();
glDisable(textureTarget);
glFlush();
它工作得很好，我可以调整窗口大小，纹理被正确地映射到较小的窗口。
在此处查看从全屏到500x280像素的不同窗口大小： 
我现在想要使用FBO渲染纹理，并开始制作一个非常简单的实现，其中包括将电影帧渲染为离屏FBO（纹理），然后绑定该纹理以在屏幕上绘制。
代码如下：
// "image" is the CVOpenGLTextureRef containing the movie frame as texture
GLenum textureTarget = CVOpenGLTextureGetTarget(image);
GLuint textureName = CVOpenGLTextureGetName(image);
////////////////////////////////////////////////////////////////////
// the creation on the FBO is done only once on program start:
GLuint fboId;
GLuint textureId;
float targetWidth = 2048;
float targetHeight = 2048;
// create a texture object
glGenTextures(1, &textureId);
glBindTexture(textureTarget, textureId);
glTexParameterf(textureTarget, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexParameterf(textureTarget, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameterf(textureTarget, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameterf(textureTarget, GL_TEXTURE_WRAP_T, GL_CLAMP);
glTexParameteri(textureTarget, GL_GENERATE_MIPMAP, GL_TRUE); // automatic mipmap
glTexImage2D(textureTarget, 0, GL_RGBA8, targetWidth, targetHeight, 0,
GL_RGBA, GL_UNSIGNED_BYTE, NULL);
glBindTexture(textureTarget, 0);
// create a framebuffer object
glGenFramebuffersEXT(1, &fboId);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
// attach the texture to FBO color attachment point
glFramebufferTexture2DEXT(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT,
textureTarget, textureId, 0);
// check FBO status
GLenum status = glCheckFramebufferStatusEXT(GL_FRAMEBUFFER_EXT);
if(status != GL_FRAMEBUFFER_COMPLETE_EXT)
return FALSE;
// switch back to window-system-provided framebuffer
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
///////////////////////////////////////////////////////////////////////////////
// Render to texture
glEnable(textureTarget);
glBindTexture(textureTarget, textureName);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_QUADS);
// Draw the quads
glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex2f (0.0f,imageRect.size.height);
glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (imageRect.size.width, imageRect.size.height);
glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (imageRect.size.width,0.0f);
glEnd();
glFlush();
// Bind newly rendered texture
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, 0);
glClear(GL_COLOR_BUFFER_BIT);
glBindTexture(textureTarget, textureId);
glGenerateMipmapEXT(textureTarget);
// draw on-screen
glBegin(GL_QUADS);
// Draw the quads
//Note: textureTagret is NOT GL_TEXTURE_2D, therefore texture coordinates
//are NOT scaled to [0, 1]
glTexCoord2f(0.0f, imageRect.size.height);
glVertex2f (0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex2f (0.0f, windowRect.size.height);
glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (windowRect.size.width, windowRect.size.height);
glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (windowRect.size.width,0.0f);
glEnd();
glDisable(textureTarget);
glFlush();
代码无法正常工作，因为图像/纹理不仅颠倒，而且纹理映射也是错误的。 只有当窗口全屏时它才能正常工作，否则它的行为真的很奇怪。
见下图： 
正如您所看到的，纹理在窗口内部正确缩放，但会按照全屏窗口大小和实际窗口大小之间的差异成比例地“裁剪”。
我试过一切都没有成功。
由于这是我第一次使用OpenGL，我错过了什么？ 我变得疯了..
// Render to texture
glEnable(textureTarget);
glBindTexture(textureTarget, textureName);
glBindFramebufferEXT(GL_FRAMEBUFFER_EXT, fboId);
glClear(GL_COLOR_BUFFER_BIT);
我缺少在这里设置视窗和framebuffer对象的投影。
glBegin(GL_QUADS);
// Draw the quads
glTexCoord2f(0.0f, imageRect.size.height);
g lVertex2f (0.0f, 0.0f);
glTexCoord2f(0.0f, 0.0f);
glVertex2f (0.0f,imageRect.size.height);
glTexCoord2f(imageRect.size.width, 0.0f);
glVertex2f (imageRect.size.width, imageRect.size.height);
glTexCoord2f(imageRect.size.width, imageRect.size.height);
glVertex2f (imageRect.size.width,0.0f);
glEnd();
glFlush();
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