mirror of https://github.com/SixLabors/ImageSharp
James Jackson-South
2 months ago
6 changed files with 393 additions and 12 deletions
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// Copyright (c) Six Labors.
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// Licensed under the Six Labors Split License.
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namespace SixLabors.ImageSharp.Formats.Heif.Hevc; |
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/// <summary>
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/// Reads fixed-width and Exp-Golomb HEVC syntax from a most-significant-bit-first byte span.
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/// </summary>
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internal ref struct HevcBitReader |
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{ |
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/// <summary>
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/// The complete raw byte sequence buffer.
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/// </summary>
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private readonly ReadOnlySpan<byte> data; |
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/// <summary>
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/// The zero-based position of the next bit to read.
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/// </summary>
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private int bitPosition; |
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/// <summary>
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/// Initializes a new instance of the <see cref="HevcBitReader"/> struct.
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/// </summary>
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/// <param name="data">The bounded HEVC syntax bytes.</param>
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public HevcBitReader(ReadOnlySpan<byte> data) |
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{ |
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this.data = data; |
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this.bitPosition = 0; |
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} |
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/// <summary>
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/// Gets the zero-based position of the next bit to read.
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/// </summary>
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public readonly int BitPosition => this.bitPosition; |
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/// <summary>
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/// Gets the number of unread bits in the bounded byte span.
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/// </summary>
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public readonly int BitsRemaining => (this.data.Length * 8) - this.bitPosition; |
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/// <summary>
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/// Gets a value indicating whether the next bit begins a byte.
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/// </summary>
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public readonly bool IsByteAligned => (this.bitPosition & 7) == 0; |
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/// <summary>
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/// Reads an unsigned fixed-width value in most-significant-bit-first order.
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/// </summary>
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/// <param name="bitCount">The number of bits to read.</param>
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/// <returns>The decoded unsigned value.</returns>
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/// <exception cref="InvalidImageContentException">
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/// The requested value extends beyond the bounded HEVC syntax.
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/// </exception>
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public uint ReadBits(int bitCount) |
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{ |
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DebugGuard.MustBeBetweenOrEqualTo(bitCount, 0, 32, nameof(bitCount)); |
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if (bitCount > this.BitsRemaining) |
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{ |
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throw new InvalidImageContentException("The HEVC bitstream is truncated."); |
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} |
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uint value = 0; |
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int remaining = bitCount; |
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while (remaining > 0) |
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{ |
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// HEVC fixed-width syntax is MSB-first. Reading only the available portion of each byte keeps the
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// same operation valid for both aligned parameter fields and fields that straddle byte boundaries.
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int byteOffset = this.bitPosition >> 3; |
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int bitOffset = this.bitPosition & 7; |
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int bitsFromByte = Math.Min(remaining, 8 - bitOffset); |
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int shift = 8 - bitOffset - bitsFromByte; |
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uint mask = (1U << bitsFromByte) - 1; |
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value = (value << bitsFromByte) | ((uint)(this.data[byteOffset] >> shift) & mask); |
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this.bitPosition += bitsFromByte; |
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remaining -= bitsFromByte; |
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} |
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return value; |
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} |
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/// <summary>
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/// Reads a one-bit HEVC flag.
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/// </summary>
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/// <returns><see langword="true"/> when the coded flag is one; otherwise, <see langword="false"/>.</returns>
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/// <exception cref="InvalidImageContentException">The flag extends beyond the bounded HEVC syntax.</exception>
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public bool ReadFlag() => this.ReadBits(1) != 0; |
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/// <summary>
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/// Reads an unsigned exponential-Golomb value.
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/// </summary>
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/// <returns>The decoded unsigned value.</returns>
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/// <exception cref="InvalidImageContentException">
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/// The code is truncated or exceeds the range of a 32-bit unsigned integer.
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/// </exception>
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public uint ReadUnsignedExpGolomb() |
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{ |
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int leadingZeroBits = 0; |
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while (!this.ReadFlag()) |
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{ |
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leadingZeroBits++; |
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if (leadingZeroBits > 32) |
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{ |
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throw new InvalidImageContentException("The HEVC unsigned Exp-Golomb value exceeds 32 bits."); |
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} |
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} |
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// In ue(v), the zero-prefix length selects an all-one basis and the equally wide suffix selects the
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// offset from that basis. Keeping those parts separate makes the 32-bit overflow boundary explicit.
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uint suffix = this.ReadBits(leadingZeroBits); |
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if (leadingZeroBits == 32) |
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{ |
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// Only an all-zero suffix fits after the 32-bit all-one basis.
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if (suffix != 0) |
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{ |
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throw new InvalidImageContentException("The HEVC unsigned Exp-Golomb value exceeds 32 bits."); |
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} |
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return uint.MaxValue; |
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} |
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return ((1U << leadingZeroBits) - 1) + suffix; |
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} |
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/// <summary>
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/// Reads a signed exponential-Golomb value.
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/// </summary>
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/// <returns>The decoded signed value.</returns>
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/// <exception cref="InvalidImageContentException">
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/// The code is truncated or exceeds the range of a 32-bit signed integer.
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/// </exception>
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public int ReadSignedExpGolomb() |
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{ |
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uint codeNumber = this.ReadUnsignedExpGolomb(); |
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// HEVC's se(v) mapping alternates positive and negative magnitudes: 0, 1, -1, 2, -2, and so on.
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if ((codeNumber & 1) == 0) |
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{ |
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return -(int)(codeNumber >> 1); |
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} |
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ulong magnitude = ((ulong)codeNumber + 1) >> 1; |
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if (magnitude > int.MaxValue) |
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{ |
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throw new InvalidImageContentException("The HEVC signed Exp-Golomb value exceeds 32 bits."); |
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} |
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return (int)magnitude; |
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} |
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/// <summary>
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/// Reads and validates the stop bit and zero alignment bits that terminate an HEVC raw byte sequence payload.
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/// </summary>
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/// <exception cref="InvalidImageContentException">
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/// The trailing-bit pattern is truncated, malformed, or followed by additional data.
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/// </exception>
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public void ReadRbspTrailingBits() |
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{ |
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// An RBSP ends with one stop bit followed only by zero bits up to the next byte boundary.
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if (!this.ReadFlag()) |
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{ |
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throw new InvalidImageContentException("The HEVC RBSP stop bit is not set."); |
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} |
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while (!this.IsByteAligned) |
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{ |
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if (this.ReadFlag()) |
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{ |
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throw new InvalidImageContentException("The HEVC RBSP has a nonzero alignment bit."); |
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} |
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} |
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// Each reader is bounded to one RBSP, so reaching alignment before the buffer end means the caller left
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// syntax unread or the NAL unit contains bytes beyond its normative terminator.
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if (this.BitsRemaining != 0) |
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{ |
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throw new InvalidImageContentException("The HEVC RBSP contains unexpected trailing data."); |
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} |
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} |
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} |
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@ -0,0 +1,93 @@ |
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// Copyright (c) Six Labors.
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// Licensed under the Six Labors Split License.
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namespace SixLabors.ImageSharp.Formats.Heif.Hevc; |
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/// <summary>
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/// Contains one decoded HEVC network abstraction layer unit.
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/// </summary>
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internal sealed class HevcNalUnit |
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{ |
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/// <summary>
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/// Initializes a new instance of the <see cref="HevcNalUnit"/> class.
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/// </summary>
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/// <param name="data">The complete NAL unit, including its two-byte header.</param>
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/// <exception cref="InvalidImageContentException">The NAL header or encoded payload is malformed.</exception>
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public HevcNalUnit(ReadOnlySpan<byte> data) |
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{ |
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this.Header = HevcNalUnitHeader.Parse(data); |
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// Container and configuration NAL units carry EBSP bytes. Decode them once at the boundary so every
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// parameter-set and slice parser observes the same validated RBSP representation.
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ReadOnlySpan<byte> encodedPayload = data[2..]; |
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byte[] rbspBuffer = new byte[encodedPayload.Length]; |
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int rbspLength = HevcRbspDecoder.Decode(encodedPayload, rbspBuffer); |
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this.Rbsp = rbspBuffer.AsMemory(0, rbspLength); |
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} |
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/// <summary>
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/// Gets the decoded two-byte NAL-unit header.
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/// </summary>
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public HevcNalUnitHeader Header { get; } |
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/// <summary>
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/// Gets the raw byte sequence payload after removal of emulation-prevention bytes.
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/// </summary>
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public ReadOnlyMemory<byte> Rbsp { get; } |
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} |
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/// <summary>
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/// Removes HEVC emulation-prevention bytes from an encoded raw byte sequence payload.
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/// </summary>
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internal static class HevcRbspDecoder |
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{ |
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/// <summary>
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/// Decodes an encoded byte sequence payload into a raw byte sequence payload.
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/// </summary>
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/// <param name="encodedPayload">The NAL payload following the two-byte header.</param>
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/// <param name="destination">A buffer at least as long as <paramref name="encodedPayload"/>.</param>
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/// <returns>The number of decoded bytes written to <paramref name="destination"/>.</returns>
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/// <exception cref="InvalidImageContentException">
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/// The payload contains a forbidden start-code-like byte sequence or an invalid emulation-prevention byte.
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/// </exception>
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public static int Decode(ReadOnlySpan<byte> encodedPayload, Span<byte> destination) |
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{ |
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DebugGuard.MustBeGreaterThanOrEqualTo(destination.Length, encodedPayload.Length, nameof(destination)); |
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int destinationOffset = 0; |
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int consecutiveZeroBytes = 0; |
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for (int sourceOffset = 0; sourceOffset < encodedPayload.Length; sourceOffset++) |
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{ |
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byte value = encodedPayload[sourceOffset]; |
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// HEVC section 7.3.1.1 forbids 00 00 00 through 00 00 02 in EBSP form. A 03 after two zeros is an
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// emulation-prevention byte only when another byte in the range 00 through 03 follows it.
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if (consecutiveZeroBytes == 2) |
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{ |
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if (value < 3) |
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{ |
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throw new InvalidImageContentException("The HEVC NAL unit contains a forbidden start-code-like byte sequence."); |
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} |
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if (value == 3) |
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{ |
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sourceOffset++; |
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if (sourceOffset == encodedPayload.Length || encodedPayload[sourceOffset] > 3) |
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{ |
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throw new InvalidImageContentException("The HEVC NAL unit contains an invalid emulation-prevention byte."); |
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} |
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// Removal depends on the preceding two decoded bytes, so this deliberately remains a single
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// scalar pass rather than introducing a second SIMD behavior model for a non-hot syntax path.
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value = encodedPayload[sourceOffset]; |
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consecutiveZeroBytes = 0; |
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} |
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} |
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destination[destinationOffset++] = value; |
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consecutiveZeroBytes = value == 0 ? consecutiveZeroBytes + 1 : 0; |
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} |
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return destinationOffset; |
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} |
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} |
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@ -0,0 +1,68 @@ |
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// Copyright (c) Six Labors.
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// Licensed under the Six Labors Split License.
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namespace SixLabors.ImageSharp.Formats.Heif.Hevc; |
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/// <summary>
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/// Contains the type, layer, and temporal identifier encoded by an HEVC NAL-unit header.
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/// </summary>
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internal readonly struct HevcNalUnitHeader |
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{ |
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/// <summary>
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/// Initializes a new instance of the <see cref="HevcNalUnitHeader"/> struct.
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/// </summary>
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/// <param name="nalUnitType">The six-bit NAL-unit type.</param>
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/// <param name="layerId">The six-bit layer identifier.</param>
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/// <param name="temporalId">The zero-based temporal identifier.</param>
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private HevcNalUnitHeader(byte nalUnitType, byte layerId, byte temporalId) |
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{ |
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this.NalUnitType = nalUnitType; |
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this.LayerId = layerId; |
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this.TemporalId = temporalId; |
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} |
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/// <summary>
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/// Gets the six-bit NAL-unit type.
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/// </summary>
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public byte NalUnitType { get; } |
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/// <summary>
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/// Gets the six-bit layer identifier.
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/// </summary>
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public byte LayerId { get; } |
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/// <summary>
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/// Gets the zero-based temporal identifier.
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/// </summary>
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public byte TemporalId { get; } |
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/// <summary>
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/// Reads and validates an HEVC NAL-unit header.
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/// </summary>
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/// <param name="data">The complete NAL unit beginning with its two-byte header.</param>
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/// <returns>The decoded header.</returns>
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/// <exception cref="InvalidImageContentException">
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/// The header is truncated, its forbidden bit is set, or its temporal identifier is reserved.
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/// </exception>
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public static HevcNalUnitHeader Parse(ReadOnlySpan<byte> data) |
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{ |
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if (data.Length < 2) |
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{ |
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throw new InvalidImageContentException("The HEVC NAL-unit header is truncated."); |
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} |
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// Use the same bounded MSB-first reader as the RBSP parsers so header truncation and field ordering have
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// one behavior model instead of a second set of shifts and masks.
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HevcBitReader reader = new(data[..2]); |
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bool forbiddenZeroBit = reader.ReadFlag(); |
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byte nalUnitType = (byte)reader.ReadBits(6); |
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byte layerId = (byte)reader.ReadBits(6); |
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byte temporalIdPlusOne = (byte)reader.ReadBits(3); |
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if (forbiddenZeroBit || temporalIdPlusOne == 0) |
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{ |
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throw new InvalidImageContentException("The HEVC NAL-unit header is invalid."); |
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} |
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return new HevcNalUnitHeader(nalUnitType, layerId, (byte)(temporalIdPlusOne - 1)); |
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} |
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} |
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