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https://github.com/RGBCube/serenity
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df9dd8ec69
I think the context normally changes for every bit. But this here is enough to correctly decode the test bitstream in Annex H.2 in the spec, which seems like a good checkpoint. The internals of the decoder use spec naming, to make the code look virtually identical to what's in the spec. (Even so, I managed to put in several typos that took a while to track down.)
96 lines
2.5 KiB
C++
96 lines
2.5 KiB
C++
/*
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* Copyright (c) 2024, Nico Weber <thakis@chromium.org>
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*
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* SPDX-License-Identifier: BSD-2-Clause
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*/
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#pragma once
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#include <AK/MemoryStream.h>
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#include <AK/OwnPtr.h>
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#include <LibGfx/ImageFormats/ImageDecoder.h>
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namespace Gfx {
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struct JBIG2LoadingContext;
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namespace JBIG2 {
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// E.3 Arithmetic decoding procedure, but with the changes described in
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// Annex G Arithmetic decoding procedure (software conventions).
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// Exposed for testing.
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class ArithmeticDecoder {
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public:
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struct Context {
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u16 I; // Index I stored for context CX (E.2.4)
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u8 is_mps; // "More probable symbol" (E.1.1). 0 or 1.
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};
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static ErrorOr<ArithmeticDecoder> initialize(ReadonlyBytes data, Context context);
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bool get_next_bit();
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private:
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ArithmeticDecoder(ReadonlyBytes data)
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: m_data(data)
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{
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}
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ReadonlyBytes m_data;
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// The code below uses names from the spec, so that the algorithms look exactly like the flowcharts in the spec.
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// Abbreviations:
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// "CX": "Context" (E.1)
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// "D": "Decision" (as in "encoder input" / "decoder output") (E.1)
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// "I(CX)": "Index I stored for context CX" (E.2.4)
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// "MPS": "More probable symbol" (E.1.1)
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// "LPS": "Less probable symbol" (E.1.1)
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void INITDEC();
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u8 DECODE(); // Returns a single decoded bit.
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u8 MPS_EXCHANGE();
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u8 LPS_EXCHANGE();
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void RENORMD();
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void BYTEIN();
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u8 B(size_t offset = 0) const; // Byte pointed to by BP.
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size_t BP; // Pointer into compressed data.
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// E.3.1 Decoder code register conventions
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u32 C; // Consists of u16 C_high, C_low.
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u16 A; // Current value of the fraction. Fixed precision; 0x8000 is equivalent to 0.75.
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u8 CT; // Count of the number of bits in C.
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Context CX;
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static u16& I(Context& cx) { return cx.I; }
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static u8& MPS(Context& cx) { return cx.is_mps; }
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static u16 Qe(u16);
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static u8 NMPS(u16);
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static u8 NLPS(u16);
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static u8 SWITCH(u16);
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};
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}
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class JBIG2ImageDecoderPlugin : public ImageDecoderPlugin {
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public:
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static bool sniff(ReadonlyBytes);
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static ErrorOr<NonnullOwnPtr<ImageDecoderPlugin>> create(ReadonlyBytes);
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virtual ~JBIG2ImageDecoderPlugin() override = default;
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virtual IntSize size() override;
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virtual ErrorOr<ImageFrameDescriptor> frame(size_t index, Optional<IntSize> ideal_size = {}) override;
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static ErrorOr<ByteBuffer> decode_embedded(Vector<ReadonlyBytes>);
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private:
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JBIG2ImageDecoderPlugin();
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OwnPtr<JBIG2LoadingContext> m_context;
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};
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}
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