AnnexBUtils.java
/*
* Copyright 2022 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.media3.muxer;
import com.google.common.collect.ImmutableList;
import java.nio.ByteBuffer;
/** NAL unit utilities for start codes and emulation prevention. */
/* package */ final class AnnexBUtils {
private AnnexBUtils() {}
/**
* Splits a {@link ByteBuffer} into individual NAL units (0x00000001 start code).
*
* <p>An empty list is returned if the input is not NAL units.
*
* <p>The position of the input buffer is unchanged after calling this method.
*/
public static ImmutableList<ByteBuffer> findNalUnits(ByteBuffer input) {
if (input.remaining() < 4 || input.getInt(0) != 1) {
return ImmutableList.of();
}
ImmutableList.Builder<ByteBuffer> nalUnits = new ImmutableList.Builder<>();
int lastStart = 4;
int zerosSeen = 0;
for (int i = 4; i < input.limit(); i++) {
if (input.get(i) == 1 && zerosSeen >= 3) {
// We're just looking at a start code.
nalUnits.add(getBytes(input, lastStart, i - 3 - lastStart));
lastStart = i + 1;
}
// Handle the end of the stream.
if (i == input.limit() - 1) {
nalUnits.add(getBytes(input, lastStart, input.limit() - lastStart));
}
if (input.get(i) == 0) {
zerosSeen++;
} else {
zerosSeen = 0;
}
}
input.rewind();
return nalUnits.build();
}
/** Removes Annex-B emulation prevention bytes from a buffer. */
public static ByteBuffer stripEmulationPrevention(ByteBuffer input) {
// For simplicity, we allocate the same number of bytes (although the eventual number might be
// smaller).
ByteBuffer output = ByteBuffer.allocate(input.limit());
int zerosSeen = 0;
for (int i = 0; i < input.limit(); i++) {
boolean lookingAtEmulationPreventionByte = input.get(i) == 0x03 && zerosSeen >= 2;
// Only copy bytes if they aren't emulation prevention bytes.
if (!lookingAtEmulationPreventionByte) {
output.put(input.get(i));
}
if (input.get(i) == 0) {
zerosSeen++;
} else {
zerosSeen = 0;
}
}
output.flip();
return output;
}
private static ByteBuffer getBytes(ByteBuffer buf, int offset, int length) {
ByteBuffer result = buf.duplicate();
result.position(offset);
result.limit(offset + length);
return result.slice();
}
}