411 lines
14 KiB
Plaintext
411 lines
14 KiB
Plaintext
unit imjidctfst;
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{ This file contains a fast, not so accurate integer implementation of the
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inverse DCT (Discrete Cosine Transform). In the IJG code, this routine
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must also perform dequantization of the input coefficients.
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A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
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on each row (or vice versa, but it's more convenient to emit a row at
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a time). Direct algorithms are also available, but they are much more
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complex and seem not to be any faster when reduced to code.
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This implementation is based on Arai, Agui, and Nakajima's algorithm for
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scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
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Japanese, but the algorithm is described in the Pennebaker & Mitchell
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JPEG textbook (see REFERENCES section in file README). The following code
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is based directly on figure 4-8 in P&M.
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While an 8-point DCT cannot be done in less than 11 multiplies, it is
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possible to arrange the computation so that many of the multiplies are
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simple scalings of the final outputs. These multiplies can then be
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folded into the multiplications or divisions by the JPEG quantization
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table entries. The AA&N method leaves only 5 multiplies and 29 adds
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to be done in the DCT itself.
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The primary disadvantage of this method is that with fixed-point math,
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accuracy is lost due to imprecise representation of the scaled
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quantization values. The smaller the quantization table entry, the less
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precise the scaled value, so this implementation does worse with high-
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quality-setting files than with low-quality ones. }
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{ Original : jidctfst.c ; Copyright (C) 1994-1996, Thomas G. Lane. }
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interface
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{$I imjconfig.inc}
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uses
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imjmorecfg,
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imjinclude,
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imjpeglib,
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imjdct; { Private declarations for DCT subsystem }
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{ Perform dequantization and inverse DCT on one block of coefficients. }
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{GLOBAL}
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procedure jpeg_idct_ifast (cinfo : j_decompress_ptr;
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compptr : jpeg_component_info_ptr;
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coef_block : JCOEFPTR;
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output_buf : JSAMPARRAY;
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output_col : JDIMENSION);
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implementation
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{ This module is specialized to the case DCTSIZE = 8. }
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{$ifndef DCTSIZE_IS_8}
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Sorry, this code only copes with 8x8 DCTs. { deliberate syntax err }
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{$endif}
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{ Scaling decisions are generally the same as in the LL&M algorithm;
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see jidctint.c for more details. However, we choose to descale
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(right shift) multiplication products as soon as they are formed,
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rather than carrying additional fractional bits into subsequent additions.
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This compromises accuracy slightly, but it lets us save a few shifts.
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More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
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everywhere except in the multiplications proper; this saves a good deal
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of work on 16-bit-int machines.
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The dequantized coefficients are not integers because the AA&N scaling
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factors have been incorporated. We represent them scaled up by PASS1_BITS,
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so that the first and second IDCT rounds have the same input scaling.
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For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to
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avoid a descaling shift; this compromises accuracy rather drastically
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for small quantization table entries, but it saves a lot of shifts.
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For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway,
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so we use a much larger scaling factor to preserve accuracy.
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A final compromise is to represent the multiplicative constants to only
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8 fractional bits, rather than 13. This saves some shifting work on some
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machines, and may also reduce the cost of multiplication (since there
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are fewer one-bits in the constants). }
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{$ifdef BITS_IN_JSAMPLE_IS_8}
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const
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CONST_BITS = 8;
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PASS1_BITS = 2;
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{$else}
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const
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CONST_BITS = 8;
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PASS1_BITS = 1; { lose a little precision to avoid overflow }
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{$endif}
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const
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FIX_1_082392200 = INT32(Round((INT32(1) shl CONST_BITS)*1.082392200)); {277}
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FIX_1_414213562 = INT32(Round((INT32(1) shl CONST_BITS)*1.414213562)); {362}
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FIX_1_847759065 = INT32(Round((INT32(1) shl CONST_BITS)*1.847759065)); {473}
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FIX_2_613125930 = INT32(Round((INT32(1) shl CONST_BITS)*2.613125930)); {669}
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{ Descale and correctly round an INT32 value that's scaled by N bits.
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We assume RIGHT_SHIFT rounds towards minus infinity, so adding
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the fudge factor is correct for either sign of X. }
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function DESCALE(x : INT32; n : int) : INT32;
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var
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shift_temp : INT32;
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begin
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{$ifdef USE_ACCURATE_ROUNDING}
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shift_temp := x + (INT32(1) shl (n-1));
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{$else}
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{ We can gain a little more speed, with a further compromise in accuracy,
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by omitting the addition in a descaling shift. This yields an incorrectly
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rounded result half the time... }
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shift_temp := x;
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{$endif}
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{$ifdef RIGHT_SHIFT_IS_UNSIGNED}
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if shift_temp < 0 then
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Descale := (shift_temp shr n) or ((not INT32(0)) shl (32-n))
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else
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{$endif}
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Descale := (shift_temp shr n);
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end;
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{ Multiply a DCTELEM variable by an INT32 constant, and immediately
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descale to yield a DCTELEM result. }
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{(DCTELEM( DESCALE((var) * (const), CONST_BITS))}
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function Multiply(Avar, Aconst: Integer): DCTELEM;
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begin
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Multiply := DCTELEM( Avar*INT32(Aconst) div (INT32(1) shl CONST_BITS));
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end;
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{ Dequantize a coefficient by multiplying it by the multiplier-table
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entry; produce a DCTELEM result. For 8-bit data a 16x16->16
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multiplication will do. For 12-bit data, the multiplier table is
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declared INT32, so a 32-bit multiply will be used. }
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{$ifdef BITS_IN_JSAMPLE_IS_8}
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function DEQUANTIZE(coef,quantval : int) : int;
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begin
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Dequantize := ( IFAST_MULT_TYPE(coef) * quantval);
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end;
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{$else}
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function DEQUANTIZE(coef,quantval : INT32) : int;
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begin
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Dequantize := DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS);
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end;
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{$endif}
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{ Like DESCALE, but applies to a DCTELEM and produces an int.
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We assume that int right shift is unsigned if INT32 right shift is. }
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function IDESCALE(x : DCTELEM; n : int) : int;
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{$ifdef BITS_IN_JSAMPLE_IS_8}
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const
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DCTELEMBITS = 16; { DCTELEM may be 16 or 32 bits }
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{$else}
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const
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DCTELEMBITS = 32; { DCTELEM must be 32 bits }
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{$endif}
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var
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ishift_temp : DCTELEM;
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begin
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{$ifndef USE_ACCURATE_ROUNDING}
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ishift_temp := x + (INT32(1) shl (n-1));
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{$else}
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{ We can gain a little more speed, with a further compromise in accuracy,
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by omitting the addition in a descaling shift. This yields an incorrectly
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rounded result half the time... }
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ishift_temp := x;
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{$endif}
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{$ifdef RIGHT_SHIFT_IS_UNSIGNED}
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if ishift_temp < 0 then
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IDescale := (ishift_temp shr n)
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or ((not DCTELEM(0)) shl (DCTELEMBITS-n))
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else
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{$endif}
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IDescale := (ishift_temp shr n);
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end;
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{ Perform dequantization and inverse DCT on one block of coefficients. }
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{GLOBAL}
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procedure jpeg_idct_ifast (cinfo : j_decompress_ptr;
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compptr : jpeg_component_info_ptr;
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coef_block : JCOEFPTR;
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output_buf : JSAMPARRAY;
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output_col : JDIMENSION);
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type
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PWorkspace = ^TWorkspace;
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TWorkspace = coef_bits_field; { buffers data between passes }
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var
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tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7 : DCTELEM;
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tmp10, tmp11, tmp12, tmp13 : DCTELEM;
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z5, z10, z11, z12, z13 : DCTELEM;
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inptr : JCOEFPTR;
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quantptr : IFAST_MULT_TYPE_FIELD_PTR;
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wsptr : PWorkspace;
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outptr : JSAMPROW;
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range_limit : JSAMPROW;
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ctr : int;
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workspace : TWorkspace; { buffers data between passes }
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{SHIFT_TEMPS} { for DESCALE }
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{ISHIFT_TEMPS} { for IDESCALE }
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var
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dcval : int;
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var
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dcval_ : JSAMPLE;
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begin
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{ Each IDCT routine is responsible for range-limiting its results and
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converting them to unsigned form (0..MAXJSAMPLE). The raw outputs could
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be quite far out of range if the input data is corrupt, so a bulletproof
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range-limiting step is required. We use a mask-and-table-lookup method
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to do the combined operations quickly. See the comments with
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prepare_range_limit_table (in jdmaster.c) for more info. }
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range_limit := JSAMPROW(@(cinfo^.sample_range_limit^[CENTERJSAMPLE]));
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{ Pass 1: process columns from input, store into work array. }
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inptr := coef_block;
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quantptr := IFAST_MULT_TYPE_FIELD_PTR(compptr^.dct_table);
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wsptr := @workspace;
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for ctr := pred(DCTSIZE) downto 0 do
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begin
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{ Due to quantization, we will usually find that many of the input
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coefficients are zero, especially the AC terms. We can exploit this
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by short-circuiting the IDCT calculation for any column in which all
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the AC terms are zero. In that case each output is equal to the
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DC coefficient (with scale factor as needed).
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With typical images and quantization tables, half or more of the
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column DCT calculations can be simplified this way. }
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if (inptr^[DCTSIZE*1]=0) and (inptr^[DCTSIZE*2]=0) and (inptr^[DCTSIZE*3]=0) and
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(inptr^[DCTSIZE*4]=0) and (inptr^[DCTSIZE*5]=0) and (inptr^[DCTSIZE*6]=0) and
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(inptr^[DCTSIZE*7]=0) then
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begin
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{ AC terms all zero }
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dcval := int(DEQUANTIZE(inptr^[DCTSIZE*0], quantptr^[DCTSIZE*0]));
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wsptr^[DCTSIZE*0] := dcval;
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wsptr^[DCTSIZE*1] := dcval;
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wsptr^[DCTSIZE*2] := dcval;
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wsptr^[DCTSIZE*3] := dcval;
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wsptr^[DCTSIZE*4] := dcval;
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wsptr^[DCTSIZE*5] := dcval;
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wsptr^[DCTSIZE*6] := dcval;
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wsptr^[DCTSIZE*7] := dcval;
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Inc(JCOEF_PTR(inptr)); { advance pointers to next column }
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Inc(IFAST_MULT_TYPE_PTR(quantptr));
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Inc(int_ptr(wsptr));
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continue;
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end;
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{ Even part }
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tmp0 := DEQUANTIZE(inptr^[DCTSIZE*0], quantptr^[DCTSIZE*0]);
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tmp1 := DEQUANTIZE(inptr^[DCTSIZE*2], quantptr^[DCTSIZE*2]);
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tmp2 := DEQUANTIZE(inptr^[DCTSIZE*4], quantptr^[DCTSIZE*4]);
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tmp3 := DEQUANTIZE(inptr^[DCTSIZE*6], quantptr^[DCTSIZE*6]);
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tmp10 := tmp0 + tmp2; { phase 3 }
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tmp11 := tmp0 - tmp2;
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tmp13 := tmp1 + tmp3; { phases 5-3 }
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tmp12 := MULTIPLY(tmp1 - tmp3, FIX_1_414213562) - tmp13; { 2*c4 }
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tmp0 := tmp10 + tmp13; { phase 2 }
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tmp3 := tmp10 - tmp13;
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tmp1 := tmp11 + tmp12;
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tmp2 := tmp11 - tmp12;
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{ Odd part }
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tmp4 := DEQUANTIZE(inptr^[DCTSIZE*1], quantptr^[DCTSIZE*1]);
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tmp5 := DEQUANTIZE(inptr^[DCTSIZE*3], quantptr^[DCTSIZE*3]);
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tmp6 := DEQUANTIZE(inptr^[DCTSIZE*5], quantptr^[DCTSIZE*5]);
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tmp7 := DEQUANTIZE(inptr^[DCTSIZE*7], quantptr^[DCTSIZE*7]);
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z13 := tmp6 + tmp5; { phase 6 }
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z10 := tmp6 - tmp5;
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z11 := tmp4 + tmp7;
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z12 := tmp4 - tmp7;
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tmp7 := z11 + z13; { phase 5 }
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tmp11 := MULTIPLY(z11 - z13, FIX_1_414213562); { 2*c4 }
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z5 := MULTIPLY(z10 + z12, FIX_1_847759065); { 2*c2 }
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tmp10 := MULTIPLY(z12, FIX_1_082392200) - z5; { 2*(c2-c6) }
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tmp12 := MULTIPLY(z10, - FIX_2_613125930) + z5; { -2*(c2+c6) }
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tmp6 := tmp12 - tmp7; { phase 2 }
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tmp5 := tmp11 - tmp6;
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tmp4 := tmp10 + tmp5;
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wsptr^[DCTSIZE*0] := int (tmp0 + tmp7);
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wsptr^[DCTSIZE*7] := int (tmp0 - tmp7);
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wsptr^[DCTSIZE*1] := int (tmp1 + tmp6);
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wsptr^[DCTSIZE*6] := int (tmp1 - tmp6);
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wsptr^[DCTSIZE*2] := int (tmp2 + tmp5);
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wsptr^[DCTSIZE*5] := int (tmp2 - tmp5);
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wsptr^[DCTSIZE*4] := int (tmp3 + tmp4);
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wsptr^[DCTSIZE*3] := int (tmp3 - tmp4);
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Inc(JCOEF_PTR(inptr)); { advance pointers to next column }
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Inc(IFAST_MULT_TYPE_PTR(quantptr));
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Inc(int_ptr(wsptr));
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end;
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{ Pass 2: process rows from work array, store into output array. }
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{ Note that we must descale the results by a factor of 8 == 2**3, }
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{ and also undo the PASS1_BITS scaling. }
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wsptr := @workspace;
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for ctr := 0 to pred(DCTSIZE) do
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begin
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outptr := JSAMPROW(@output_buf^[ctr]^[output_col]);
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{ Rows of zeroes can be exploited in the same way as we did with columns.
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However, the column calculation has created many nonzero AC terms, so
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the simplification applies less often (typically 5% to 10% of the time).
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On machines with very fast multiplication, it's possible that the
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test takes more time than it's worth. In that case this section
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may be commented out. }
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{$ifndef NO_ZERO_ROW_TEST}
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if (wsptr^[1]=0) and (wsptr^[2]=0) and (wsptr^[3]=0) and (wsptr^[4]=0) and
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(wsptr^[5]=0) and (wsptr^[6]=0) and (wsptr^[7]=0) then
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begin
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{ AC terms all zero }
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dcval_ := range_limit^[IDESCALE(wsptr^[0], PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[0] := dcval_;
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outptr^[1] := dcval_;
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outptr^[2] := dcval_;
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outptr^[3] := dcval_;
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outptr^[4] := dcval_;
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outptr^[5] := dcval_;
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outptr^[6] := dcval_;
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outptr^[7] := dcval_;
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Inc(int_ptr(wsptr), DCTSIZE); { advance pointer to next row }
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continue;
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end;
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{$endif}
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{ Even part }
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tmp10 := (DCTELEM(wsptr^[0]) + DCTELEM(wsptr^[4]));
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tmp11 := (DCTELEM(wsptr^[0]) - DCTELEM(wsptr^[4]));
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tmp13 := (DCTELEM(wsptr^[2]) + DCTELEM(wsptr^[6]));
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tmp12 := MULTIPLY(DCTELEM(wsptr^[2]) - DCTELEM(wsptr^[6]), FIX_1_414213562)
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- tmp13;
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tmp0 := tmp10 + tmp13;
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tmp3 := tmp10 - tmp13;
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tmp1 := tmp11 + tmp12;
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tmp2 := tmp11 - tmp12;
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{ Odd part }
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z13 := DCTELEM(wsptr^[5]) + DCTELEM(wsptr^[3]);
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z10 := DCTELEM(wsptr^[5]) - DCTELEM(wsptr^[3]);
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z11 := DCTELEM(wsptr^[1]) + DCTELEM(wsptr^[7]);
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z12 := DCTELEM(wsptr^[1]) - DCTELEM(wsptr^[7]);
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tmp7 := z11 + z13; { phase 5 }
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tmp11 := MULTIPLY(z11 - z13, FIX_1_414213562); { 2*c4 }
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z5 := MULTIPLY(z10 + z12, FIX_1_847759065); { 2*c2 }
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tmp10 := MULTIPLY(z12, FIX_1_082392200) - z5; { 2*(c2-c6) }
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tmp12 := MULTIPLY(z10, - FIX_2_613125930) + z5; { -2*(c2+c6) }
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tmp6 := tmp12 - tmp7; { phase 2 }
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tmp5 := tmp11 - tmp6;
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tmp4 := tmp10 + tmp5;
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{ Final output stage: scale down by a factor of 8 and range-limit }
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outptr^[0] := range_limit^[IDESCALE(tmp0 + tmp7, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[7] := range_limit^[IDESCALE(tmp0 - tmp7, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[1] := range_limit^[IDESCALE(tmp1 + tmp6, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[6] := range_limit^[IDESCALE(tmp1 - tmp6, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[2] := range_limit^[IDESCALE(tmp2 + tmp5, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[5] := range_limit^[IDESCALE(tmp2 - tmp5, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[4] := range_limit^[IDESCALE(tmp3 + tmp4, PASS1_BITS+3)
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and RANGE_MASK];
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outptr^[3] := range_limit^[IDESCALE(tmp3 - tmp4, PASS1_BITS+3)
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and RANGE_MASK];
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Inc(int_ptr(wsptr), DCTSIZE); { advance pointer to next row }
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end;
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end;
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end.
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