How to custom a YUV texture downloader

[FinleyPan]

I use readPixels to read the rendered result back to host memory with PixelDataFormat::RGBA, everything goes well. My question is it possible to convert the resultant rendered texture from RGBA to NV12 in the material file(.mat)? I know how to do it with opengl fragment shader, but don't know whether it can be achieved with filament supported features or any customizable post-pocessing modules?