由glReadPixels快照。然后翻转像素内容。但图像质量却有所下降

All. Now I am using unity3D to develop the game. And I want to save the content of each frame by AVFoundation as mp4 file. But I met some problem while I process the snapshot. After I use glReadPixels to obtain the data saved in render buffer, vertex shader and fragment shader is used to help me turn update side down the pixels content. But, after flipping each frame, I found that the quality of each frame has been decreased a lot. So, anyone has met this kind of case before. Here is the code related. The snapshot part,All. Now I am using unity3D to develop the game