Hi Christian,
The ImageFormat.YUV_420_888 is a generic YCbCr forma, capable of describing any 4:2:0 chroma-subsampled planar or semiplanar buffer (but not fully interleaved), with 8 bits per color sample.
It only guarantees
1. that plane #0 is always Y, plane #1 is always U (Cb), and plane #2 is always V (Cr)
2. pixel stride is always 1 in yPlane.getPixelStride()
3. uPlane.getRowStride() == vPlane.getRowStride() and uPlane.getPixelStride() == vPlane.getPixelStride()
So YUV_420_888 could be nv21, nv12, I420, etc,.
To determine which pixel format is being used internally, you could refer to the code here
Unfortunately there is no public API to determine the format yet.
Best,
Kailiang