Image binary interpretation: unknown image format

Question

Let\'s say I\'ve got an image of some format (its binary representation, say, cv::Mat from OpenCV or YuvImage from Android, not compressed), and interpreted its data as YUV NV21

Answer 1

Well, at the end, we've found some inconsistencies in image handling.

YuvImage of Android expects you to feed it with NV21, so I had a working conversion method from I420 to NV21. When I changed a video source (same vendor, different model), I expected it to come with the same format, but, in fact, it was NV12 instead.

Reinterpret-casting NV12 as I420 and then converting it to NV21 is what happened here.

Answer 2

The YUV image output uses an Android library which results in different YUV formats depending on the device used.

In my case, depending on which of the two devices I used, I needed to switch the bytes of the two color channels (NV12 to NV21).

What really helped me understanding which format I have was using the following YUV player: https://github.com/Tee0125/yuvplayer

In the player you can actively change the way the data is interpreted as, just test different settings until you find the right one.

Although I don't know if you need it, I'll provide the method I used to switch the channels.

/**
 * Changes a NV12 yuv Frame to the NV21 format. Basically the two color channels are switched.
 *
 * @param yuvFrameNV12 The YUV Frame in the NV12 format.
 * @param width The width of the frame.
 * @param height The height of the frame.
 * @return The YUV Frame in the NV21 format.
 */
private static byte[] convertNV12toNV21(byte[] yuvFrameNV12, int width, int height) {
    final int length = yuvFrameNV12.length;
    for (int i1 = 0; i1 < length; i1 += 2) {
        if (i1 >= width * height) { //start after the luminance bytes
            byte tmp = yuvFrameNV12[i1];
            yuvFrameNV12[i1] = yuvFrameNV12[i1+1];
            yuvFrameNV12[i1+1] = tmp;
        }
    }
    return yuvFrameNV12;
}