Explain how this encoder deals with PPS and SPS?

Question

I found this code online can someone please explain the PPS and SPS part?

Everything after the else of if (sps != null && pps != null) I understand that since we check if (spsPpsBuffer.getInt() == 0x00000001) because a NALU starts with 0x00000001 but after that I really don't understand the following:

• Why is the ppsIndex is set to 0 at first then it's set to spsPpsBuffer.position()?

• Why is the SPS buffer size is ppsIndex - 8?

• Why is the size of the PPS buffer is outData.length - ppsIndex?

This is code:

@Override
public void offerEncoder(byte[] input) {
    try {
        ByteBuffer[] inputBuffers = mediaCodec.getInputBuffers();
        ByteBuffer[] outputBuffers = mediaCodec.getOutputBuffers();
        int inputBufferIndex = mediaCodec.dequeueInputBuffer(-1);
        if (inputBufferIndex >= 0) {
            ByteBuffer inputBuffer = inputBuffers[inputBufferIndex];
            inputBuffer.clear();
            inputBuffer.put(input);
            mediaCodec.queueInputBuffer(inputBufferIndex, 0, input.length, 0, 0);
        }
        MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();
        int outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
        while (outputBufferIndex >= 0) {
            ByteBuffer outputBuffer = outputBuffers[outputBufferIndex];
            byte[] outData = new byte[bufferInfo.size];
            outputBuffer.get(outData);
            if (sps != null && pps != null) {
                ByteBuffer frameBuffer = ByteBuffer.wrap(outData);
                frameBuffer.putInt(bufferInfo.size - 4);
                frameListener.frameReceived(outData, 0, outData.length);
            } else {
                ByteBuffer spsPpsBuffer = ByteBuffer.wrap(outData);
                if (spsPpsBuffer.getInt() == 0x00000001) {
                    System.out.println("parsing sps/pps");
                } else {
                    System.out.println("something is amiss?");
                }
                int ppsIndex = 0;
                while(!(spsPpsBuffer.get() == 0x00 && spsPpsBuffer.get() == 0x00 && spsPpsBuffer.get() == 0x00 && spsPpsBuffer.get() == 0x01)) {
                }
                ppsIndex = spsPpsBuffer.position();
                sps = new byte[ppsIndex - 8];
                System.arraycopy(outData, 4, sps, 0, sps.length);
                pps = new byte[outData.length - ppsIndex];
                System.arraycopy(outData, ppsIndex, pps, 0, pps.length);
                if (null != parameterSetsListener) {
                    parameterSetsListener.avcParametersSetsEstablished(sps, pps);
                }
            }
            mediaCodec.releaseOutputBuffer(outputBufferIndex, false);
            outputBufferIndex = mediaCodec.dequeueOutputBuffer(bufferInfo, 0);
        }
    } catch (Throwable t) {
        t.printStackTrace();
    }
}

Thank you very much.

Answer 1

You can get a general idea about PPS/SPS from an earlier answer: H264 with multiple PPS and SPS

The above code is highly specialized and will work only with a small subset of H.264 streams. The code assumes a fixed length SPS (8 bytes) and makes a few more invalid assumptions. Unless the code is for one specific encoder - I'd probably not use it.

This seems to be a decent H.264 parser: https://github.com/aizvorski/h264bitstream