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LibAudio: Use channel coupling for stereo FLAC encoding

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The estimation for this is fast but not very accurate, meaning we save
around 5-10% storage space. (We also don’t try other channel coupling
methods, but I am sceptical of how much benefit that actually provides.)
This commit is contained in:
kleines Filmröllchen 2023-08-13 14:14:23 +02:00 • committed by Jelle Raaijmakers
parent 3d0da734ee
commit 1a1cba1c3f
2 changed files with 71 additions and 21 deletions
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83
Userland/Libraries/LibAudio/FlacWriter.cpp
View file

@ -6,9 +6,11 @@
#include "FlacWriter.h"
#include <AK/BitStream.h>
#include <AK/DisjointChunks.h>
#include <AK/Endian.h>
#include <AK/IntegralMath.h>
#include <AK/MemoryStream.h>
#include <AK/Statistics.h>
#include <LibCrypto/Checksum/ChecksummingStream.h>
namespace Audio {
@ -315,13 +317,51 @@ ErrorOr<void> FlacWriter::write_frame()
TRY(subframe_samples[i].try_append(static_cast<i64>(sample.right * sample_rescale)));
}
auto channel_type = static_cast<FlacFrameChannelType>(m_num_channels - 1);
if (channel_type == FlacFrameChannelType::Stereo) {
auto const& left_channel = subframe_samples[0];
auto const& right_channel = subframe_samples[1];
Vector<i64, block_size> mid_channel;
Vector<i64, block_size> side_channel;
TRY(mid_channel.try_ensure_capacity(left_channel.size()));
TRY(side_channel.try_ensure_capacity(left_channel.size()));
for (auto i = 0u; i < left_channel.size(); ++i) {
auto mid = (left_channel[i] + right_channel[i]) / 2;
auto side = left_channel[i] - right_channel[i];
mid_channel.unchecked_append(mid);
side_channel.unchecked_append(side);
}
AK::Statistics<i64, AK::DisjointSpans<i64>> normal_costs {
AK::DisjointSpans<i64> { { subframe_samples[0], subframe_samples[1] } }
};
AK::Statistics<i64, AK::DisjointSpans<i64>> correlated_costs {
AK::DisjointSpans<i64> { { mid_channel, side_channel } }
};
if (correlated_costs.standard_deviation() < normal_costs.standard_deviation()) {
dbgln_if(FLAC_ENCODER_DEBUG, "Using channel coupling since sample stddev {} is better than {}", correlated_costs.standard_deviation(), normal_costs.standard_deviation());
channel_type = FlacFrameChannelType::MidSideStereo;
subframe_samples[0] = move(mid_channel);
subframe_samples[1] = move(side_channel);
}
}
return write_frame_for(subframe_samples, channel_type);
}
ErrorOr<void> FlacWriter::write_frame_for(ReadonlySpan<Vector<i64, block_size>> subblock, FlacFrameChannelType channel_type)
{
auto sample_count = subblock.first().size();
FlacFrameHeader header {
.sample_rate = m_sample_rate,
.sample_count = static_cast<u16>(frame_samples.size()),
.sample_count = static_cast<u16>(sample_count),
.sample_or_frame_index = static_cast<u32>(m_current_frame),
.blocking_strategy = BlockingStrategy::Fixed,
// FIXME: We should brute-force channel coupling for stereo.
.channels = static_cast<FlacFrameChannelType>(m_num_channels - 1),
.channels = channel_type,
.bit_depth = static_cast<u8>(m_bits_per_sample),
// Calculated for us during header write.
.checksum = 0,
@ -333,8 +373,17 @@ ErrorOr<void> FlacWriter::write_frame()
TRY(frame_stream.write_value(header));
BigEndianOutputBitStream bit_stream { MaybeOwned<Stream> { frame_stream } };
for (auto const& subframe : subframe_samples)
TRY(write_subframe(subframe.span(), bit_stream));
for (auto i = 0u; i < subblock.size(); ++i) {
auto const& subframe = subblock[i];
auto bits_per_sample = m_bits_per_sample;
// Side channels need an extra bit per sample.
if ((i == 1 && (channel_type == FlacFrameChannelType::LeftSideStereo || channel_type == FlacFrameChannelType::MidSideStereo))
|| (i == 0 && channel_type == FlacFrameChannelType::RightSideStereo)) {
bits_per_sample++;
}
TRY(write_subframe(subframe.span(), bit_stream, bits_per_sample));
}
TRY(bit_stream.align_to_byte_boundary());
auto frame_crc = frame_stream.digest();
@ -347,12 +396,12 @@ ErrorOr<void> FlacWriter::write_frame()
m_min_frame_size = min(m_min_frame_size, frame_size);
m_current_frame++;
m_sample_count += frame_samples.size();
m_sample_count += sample_count;
return {};
}
ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream)
ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample)
{
// The current subframe encoding strategy is as follows:
// - Check if the subframe is constant; use constant encoding in this case.
@ -376,11 +425,11 @@ ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOu
TRY(bit_stream.write_bits(1u, 0));
TRY(bit_stream.write_bits(to_underlying(FlacSubframeType::Constant), 6));
TRY(bit_stream.write_bits(1u, 0));
TRY(bit_stream.write_bits(bit_cast<u64>(constant_value), m_bits_per_sample));
TRY(bit_stream.write_bits(bit_cast<u64>(constant_value), bits_per_sample));
return {};
}
auto verbatim_cost_bits = subframe.size() * m_bits_per_sample;
auto verbatim_cost_bits = subframe.size() * bits_per_sample;
Optional<FlacLPCEncodedSubframe> best_lpc_subframe;
auto current_min_cost = verbatim_cost_bits;
@ -389,7 +438,7 @@ ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOu
if (to_underlying(order) > subframe.size())
continue;
auto encode_result = TRY(encode_fixed_lpc(order, subframe, current_min_cost));
auto encode_result = TRY(encode_fixed_lpc(order, subframe, current_min_cost, bits_per_sample));
if (encode_result.has_value() && encode_result.value().residual_cost_bits < current_min_cost) {
current_min_cost = encode_result.value().residual_cost_bits;
best_lpc_subframe = encode_result.release_value();
@ -399,23 +448,23 @@ ErrorOr<void> FlacWriter::write_subframe(ReadonlySpan<i64> subframe, BigEndianOu
// No LPC encoding was better than verbatim.
if (!best_lpc_subframe.has_value()) {
dbgln_if(FLAC_ENCODER_DEBUG, "Best subframe type was Verbatim; encoding {} samples at {} bps = {} bits", subframe.size(), m_bits_per_sample, verbatim_cost_bits);
TRY(write_verbatim_subframe(subframe, bit_stream));
TRY(write_verbatim_subframe(subframe, bit_stream, bits_per_sample));
} else {
dbgln_if(FLAC_ENCODER_DEBUG, "Best subframe type was Fixed LPC order {} (estimated cost {} bits); encoding {} samples", to_underlying(best_lpc_subframe->coefficients.get<FlacFixedLPC>()), best_lpc_subframe->residual_cost_bits, subframe.size());
TRY(write_lpc_subframe(best_lpc_subframe.release_value(), bit_stream));
TRY(write_lpc_subframe(best_lpc_subframe.release_value(), bit_stream, bits_per_sample));
}
return {};
}
ErrorOr<Optional<FlacLPCEncodedSubframe>> FlacWriter::encode_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> subframe, size_t current_min_cost)
ErrorOr<Optional<FlacLPCEncodedSubframe>> FlacWriter::encode_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> subframe, size_t current_min_cost, u8 bits_per_sample)
{
FlacLPCEncodedSubframe lpc {
.warm_up_samples = Vector<i64> { subframe.trim(to_underlying(order)) },
.coefficients = order,
.residuals {},
// Warm-up sample cost.
.residual_cost_bits = to_underlying(order) * m_bits_per_sample,
.residual_cost_bits = to_underlying(order) * bits_per_sample,
.single_partition_optimal_order {},
};
TRY(lpc.residuals.try_ensure_capacity(subframe.size() - to_underlying(order)));
@ -532,19 +581,19 @@ void predict_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> samples, Span<i64>
}
// https://www.ietf.org/archive/id/draft-ietf-cellar-flac-08.html#name-verbatim-subframe
ErrorOr<void> FlacWriter::write_verbatim_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream)
ErrorOr<void> FlacWriter::write_verbatim_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample)
{
TRY(bit_stream.write_bits(0u, 1));
TRY(bit_stream.write_bits(to_underlying(FlacSubframeType::Verbatim), 6));
TRY(bit_stream.write_bits(0u, 1));
for (auto const& sample : subframe)
TRY(bit_stream.write_bits(bit_cast<u64>(sample), m_bits_per_sample));
TRY(bit_stream.write_bits(bit_cast<u64>(sample), bits_per_sample));
return {};
}
// https://www.ietf.org/archive/id/draft-ietf-cellar-flac-08.html#name-fixed-predictor-subframe
ErrorOr<void> FlacWriter::write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe, BigEndianOutputBitStream& bit_stream)
ErrorOr<void> FlacWriter::write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample)
{
// Reserved.
TRY(bit_stream.write_bits(0u, 1));
@ -560,7 +609,7 @@ ErrorOr<void> FlacWriter::write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe
TRY(bit_stream.write_bits(0u, 1));
for (auto const& warm_up_sample : lpc_subframe.warm_up_samples)
TRY(bit_stream.write_bits(bit_cast<u64>(warm_up_sample), m_bits_per_sample));
TRY(bit_stream.write_bits(bit_cast<u64>(warm_up_sample), bits_per_sample));
// 4-bit Rice parameters.
TRY(bit_stream.write_bits(0b00u, 2));

9
Userland/Libraries/LibAudio/FlacWriter.h
View file

@ -87,15 +87,16 @@ private:
ErrorOr<void> write_header();
ErrorOr<void> write_frame();
ErrorOr<void> write_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream);
ErrorOr<void> write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe, BigEndianOutputBitStream& bit_stream);
ErrorOr<void> write_verbatim_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream);
ErrorOr<void> write_frame_for(ReadonlySpan<Vector<i64, block_size>> subblock, FlacFrameChannelType channel_type);
ErrorOr<void> write_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample);
ErrorOr<void> write_lpc_subframe(FlacLPCEncodedSubframe lpc_subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample);
ErrorOr<void> write_verbatim_subframe(ReadonlySpan<i64> subframe, BigEndianOutputBitStream& bit_stream, u8 bits_per_sample);
// Assumes 4-bit k for now.
ErrorOr<void> write_rice_partition(u8 k, ReadonlySpan<i64> residuals, BigEndianOutputBitStream& bit_stream);
// Aborts encoding once the costs exceed the previous minimum, thereby speeding up the encoder's parameter search.
// In this case, an empty Optional is returned.
ErrorOr<Optional<FlacLPCEncodedSubframe>> encode_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> subframe, size_t current_min_cost);
ErrorOr<Optional<FlacLPCEncodedSubframe>> encode_fixed_lpc(FlacFixedLPC order, ReadonlySpan<i64> subframe, size_t current_min_cost, u8 bits_per_sample);
NonnullOwnPtr<SeekableStream> m_stream;
WriteState m_state { WriteState::HeaderUnwritten };