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LibGfx/JBIG2: Add arithmetic coding decoder

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.)
This commit is contained in:
Nico Weber 2024-03-11 22:28:20 -04:00 committed by Andrew Kaster
parent c4be9318a2
commit df9dd8ec69
3 changed files with 294 additions and 0 deletions

View file

@ -14,6 +14,66 @@ namespace Gfx {
struct JBIG2LoadingContext;
namespace JBIG2 {
// E.3 Arithmetic decoding procedure, but with the changes described in
// Annex G Arithmetic decoding procedure (software conventions).
// Exposed for testing.
class ArithmeticDecoder {
public:
struct Context {
u16 I; // Index I stored for context CX (E.2.4)
u8 is_mps; // "More probable symbol" (E.1.1). 0 or 1.
};
static ErrorOr<ArithmeticDecoder> initialize(ReadonlyBytes data, Context context);
bool get_next_bit();
private:
ArithmeticDecoder(ReadonlyBytes data)
: m_data(data)
{
}
ReadonlyBytes m_data;
// The code below uses names from the spec, so that the algorithms look exactly like the flowcharts in the spec.
// Abbreviations:
// "CX": "Context" (E.1)
// "D": "Decision" (as in "encoder input" / "decoder output") (E.1)
// "I(CX)": "Index I stored for context CX" (E.2.4)
// "MPS": "More probable symbol" (E.1.1)
// "LPS": "Less probable symbol" (E.1.1)
void INITDEC();
u8 DECODE(); // Returns a single decoded bit.
u8 MPS_EXCHANGE();
u8 LPS_EXCHANGE();
void RENORMD();
void BYTEIN();
u8 B(size_t offset = 0) const; // Byte pointed to by BP.
size_t BP; // Pointer into compressed data.
// E.3.1 Decoder code register conventions
u32 C; // Consists of u16 C_high, C_low.
u16 A; // Current value of the fraction. Fixed precision; 0x8000 is equivalent to 0.75.
u8 CT; // Count of the number of bits in C.
Context CX;
static u16& I(Context& cx) { return cx.I; }
static u8& MPS(Context& cx) { return cx.is_mps; }
static u16 Qe(u16);
static u8 NMPS(u16);
static u8 NLPS(u16);
static u8 SWITCH(u16);
};
}
class JBIG2ImageDecoderPlugin : public ImageDecoderPlugin {
public:
static bool sniff(ReadonlyBytes);