使用glReadPixels检索位图时会失去抗锯齿功能

我使用OpenGL编写程序。它实现了一个简单的功能：绘制一个茶壶。为了使它在屏幕上看起来不错，我启用了多重取样抗锯齿功能。它确实如此。请看下面的位图：使用glReadPixels检索位图时会失去抗锯齿功能

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但是当我将它保存为bmp图片，它看起来不错。我使用FBO和PBO来做到这一点。现在，我在后部分我的代码在这里：

glGenFramebuffers(1,&m_frameBuffer); 
glBindFramebuffer(GL_FRAMEBUFFER,m_frameBuffer); 

glGenRenderbuffers(1,&m_renderBufferColor); 
glBindRenderbuffer(GL_RENDERBUFFER,m_renderBufferColor); 
glRenderbufferStorage(GL_RENDERBUFFER,GL_RGB, 
    m_subImageWidth,m_subImageHeight); 
glFramebufferRenderbuffer(GL_FRAMEBUFFER,GL_COLOR_ATTACHMENT0, 
    GL_RENDERBUFFER,m_renderBufferColor); 

glGenRenderbuffers(1,&m_renderBufferDepth); 
glBindRenderbuffer(GL_RENDERBUFFER,m_renderBufferDepth); 
glRenderbufferStorage(GL_RENDERBUFFER,GL_DEPTH_COMPONENT, 
    m_subImageWidth,m_subImageHeight); 
glFramebufferRenderbuffer(GL_FRAMEBUFFER,GL_DEPTH_ATTACHMENT, 
    GL_RENDERBUFFER,m_renderBufferDepth); 

glBindFramebuffer(GL_FRAMEBUFFER,0); 

glBindFramebuffer(GL_FRAMEBUFFER,m_frameBuffer); 



glGenBuffers(1,m_subImageBuffer); 
glBindBuffer(GL_PIXEL_PACK_BUFFER,m_subImageBuffer); 
     glBufferData(GL_PIXEL_PACK_BUFFER,m_bufferSize, 
      NULL,GL_STREAM_READ); 

glBindFramebuffer(GL_FRAMEBUFFER,m_frameBuffer); 

glBindBuffer(GL_PIXEL_PACK_BUFFER,m_subImageBuffer); 
glPixelStorei(GL_PACK_ALIGNMENT,1); 
//注意：以BGR的顺序读取 
glReadPixels(0,0,m_subImageWidth,m_subImageHeight, 
     GL_BGR,GL_UNSIGNED_BYTE,bufferOffset(0)); 
GLUtils::checkForOpenGLError(__FILE__,__LINE__); 
m_subPixels[i] = static_cast<GLubyte*>(glMapBuffer(GL_PIXEL_PACK_BUFFER,GL_READ_ONLY)); 
gltGenBMP(subImageFile,GLT_BGR,m_subImageWidth,m_subImageHeight,m_subPixels[i]); 
glBindBuffer(GL_PIXEL_PACK_BUFFER,0); 
glUnmapBuffer(GL_PIXEL_PACK_BUFFER); 
glBindBuffer(GL_PIXEL_PACK_BUFFER,0);

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