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CentrED/Imaging/JpegLib/imjidctflt.pas

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unit imjidctflt;
{ This file contains a floating-point implementation of the
inverse DCT (Discrete Cosine Transform). In the IJG code, this routine
must also perform dequantization of the input coefficients.
This implementation should be more accurate than either of the integer
IDCT implementations. However, it may not give the same results on all
machines because of differences in roundoff behavior. Speed will depend
on the hardware's floating point capacity.
A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
on each row (or vice versa, but it's more convenient to emit a row at
a time). Direct algorithms are also available, but they are much more
complex and seem not to be any faster when reduced to code.
This implementation is based on Arai, Agui, and Nakajima's algorithm for
scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
Japanese, but the algorithm is described in the Pennebaker & Mitchell
JPEG textbook (see REFERENCES section in file README). The following code
is based directly on figure 4-8 in P&M.
While an 8-point DCT cannot be done in less than 11 multiplies, it is
possible to arrange the computation so that many of the multiplies are
simple scalings of the final outputs. These multiplies can then be
folded into the multiplications or divisions by the JPEG quantization
table entries. The AA&N method leaves only 5 multiplies and 29 adds
to be done in the DCT itself.
The primary disadvantage of this method is that with a fixed-point
implementation, accuracy is lost due to imprecise representation of the
scaled quantization values. However, that problem does not arise if
we use floating point arithmetic. }
{ Original: jidctflt.c ; Copyright (C) 1994-1996, Thomas G. Lane. }
interface
{$I imjconfig.inc}
uses
imjmorecfg,
imjinclude,
imjpeglib,
imjdct; { Private declarations for DCT subsystem }
{ Perform dequantization and inverse DCT on one block of coefficients. }
{GLOBAL}
procedure jpeg_idct_float (cinfo : j_decompress_ptr;
compptr : jpeg_component_info_ptr;
coef_block : JCOEFPTR;
output_buf : JSAMPARRAY;
output_col : JDIMENSION);
implementation
{ This module is specialized to the case DCTSIZE = 8. }
{$ifndef DCTSIZE_IS_8}
Sorry, this code only copes with 8x8 DCTs. { deliberate syntax err }
{$endif}
{ Dequantize a coefficient by multiplying it by the multiplier-table
entry; produce a float result. }
function DEQUANTIZE(coef : int; quantval : FAST_FLOAT) : FAST_FLOAT;
begin
Dequantize := ( (coef) * quantval);
end;
{ Descale and correctly round an INT32 value that's scaled by N bits.
We assume RIGHT_SHIFT rounds towards minus infinity, so adding
the fudge factor is correct for either sign of X. }
function DESCALE(x : INT32; n : int) : INT32;
var
shift_temp : INT32;
begin
{$ifdef RIGHT_SHIFT_IS_UNSIGNED}
shift_temp := x + (INT32(1) shl (n-1));
if shift_temp < 0 then
Descale := (shift_temp shr n) or ((not INT32(0)) shl (32-n))
else
Descale := (shift_temp shr n);
{$else}
Descale := (x + (INT32(1) shl (n-1)) shr n;
{$endif}
end;
{ Perform dequantization and inverse DCT on one block of coefficients. }
{GLOBAL}
procedure jpeg_idct_float (cinfo : j_decompress_ptr;
compptr : jpeg_component_info_ptr;
coef_block : JCOEFPTR;
output_buf : JSAMPARRAY;
output_col : JDIMENSION);
type
PWorkspace = ^TWorkspace;
TWorkspace = array[0..DCTSIZE2-1] of FAST_FLOAT;
var
tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7 : FAST_FLOAT;
tmp10, tmp11, tmp12, tmp13 : FAST_FLOAT;
z5, z10, z11, z12, z13 : FAST_FLOAT;
inptr : JCOEFPTR;
quantptr : FLOAT_MULT_TYPE_FIELD_PTR;
wsptr : PWorkSpace;
outptr : JSAMPROW;
range_limit : JSAMPROW;
ctr : int;
workspace : TWorkspace; { buffers data between passes }
{SHIFT_TEMPS}
var
dcval : FAST_FLOAT;
begin
{ Each IDCT routine is responsible for range-limiting its results and
converting them to unsigned form (0..MAXJSAMPLE). The raw outputs could
be quite far out of range if the input data is corrupt, so a bulletproof
range-limiting step is required. We use a mask-and-table-lookup method
to do the combined operations quickly. See the comments with
prepare_range_limit_table (in jdmaster.c) for more info. }
range_limit := JSAMPROW(@(cinfo^.sample_range_limit^[CENTERJSAMPLE]));
{ Pass 1: process columns from input, store into work array. }
inptr := coef_block;
quantptr := FLOAT_MULT_TYPE_FIELD_PTR (compptr^.dct_table);
wsptr := @workspace;
for ctr := pred(DCTSIZE) downto 0 do
begin
{ Due to quantization, we will usually find that many of the input
coefficients are zero, especially the AC terms. We can exploit this
by short-circuiting the IDCT calculation for any column in which all
the AC terms are zero. In that case each output is equal to the
DC coefficient (with scale factor as needed).
With typical images and quantization tables, half or more of the
column DCT calculations can be simplified this way. }
if (inptr^[DCTSIZE*1]=0) and (inptr^[DCTSIZE*2]=0) and
(inptr^[DCTSIZE*3]=0) and (inptr^[DCTSIZE*4]=0) and
(inptr^[DCTSIZE*5]=0) and (inptr^[DCTSIZE*6]=0) and
(inptr^[DCTSIZE*7]=0) then
begin
{ AC terms all zero }
FAST_FLOAT(dcval) := DEQUANTIZE(inptr^[DCTSIZE*0], quantptr^[DCTSIZE*0]);
wsptr^[DCTSIZE*0] := dcval;
wsptr^[DCTSIZE*1] := dcval;
wsptr^[DCTSIZE*2] := dcval;
wsptr^[DCTSIZE*3] := dcval;
wsptr^[DCTSIZE*4] := dcval;
wsptr^[DCTSIZE*5] := dcval;
wsptr^[DCTSIZE*6] := dcval;
wsptr^[DCTSIZE*7] := dcval;
Inc(JCOEF_PTR(inptr)); { advance pointers to next column }
Inc(FLOAT_MULT_TYPE_PTR(quantptr));
Inc(FAST_FLOAT_PTR(wsptr));
continue;
end;
{ Even part }
tmp0 := DEQUANTIZE(inptr^[DCTSIZE*0], quantptr^[DCTSIZE*0]);
tmp1 := DEQUANTIZE(inptr^[DCTSIZE*2], quantptr^[DCTSIZE*2]);
tmp2 := DEQUANTIZE(inptr^[DCTSIZE*4], quantptr^[DCTSIZE*4]);
tmp3 := DEQUANTIZE(inptr^[DCTSIZE*6], quantptr^[DCTSIZE*6]);
tmp10 := tmp0 + tmp2; { phase 3 }
tmp11 := tmp0 - tmp2;
tmp13 := tmp1 + tmp3; { phases 5-3 }
tmp12 := (tmp1 - tmp3) * ({FAST_FLOAT}(1.414213562)) - tmp13; { 2*c4 }
tmp0 := tmp10 + tmp13; { phase 2 }
tmp3 := tmp10 - tmp13;
tmp1 := tmp11 + tmp12;
tmp2 := tmp11 - tmp12;
{ Odd part }
tmp4 := DEQUANTIZE(inptr^[DCTSIZE*1], quantptr^[DCTSIZE*1]);
tmp5 := DEQUANTIZE(inptr^[DCTSIZE*3], quantptr^[DCTSIZE*3]);
tmp6 := DEQUANTIZE(inptr^[DCTSIZE*5], quantptr^[DCTSIZE*5]);
tmp7 := DEQUANTIZE(inptr^[DCTSIZE*7], quantptr^[DCTSIZE*7]);
z13 := tmp6 + tmp5; { phase 6 }
z10 := tmp6 - tmp5;
z11 := tmp4 + tmp7;
z12 := tmp4 - tmp7;
tmp7 := z11 + z13; { phase 5 }
tmp11 := (z11 - z13) * ({FAST_FLOAT}(1.414213562)); { 2*c4 }
z5 := (z10 + z12) * ({FAST_FLOAT}(1.847759065)); { 2*c2 }
tmp10 := ({FAST_FLOAT}(1.082392200)) * z12 - z5; { 2*(c2-c6) }
tmp12 := ({FAST_FLOAT}(-2.613125930)) * z10 + z5; { -2*(c2+c6) }
tmp6 := tmp12 - tmp7; { phase 2 }
tmp5 := tmp11 - tmp6;
tmp4 := tmp10 + tmp5;
wsptr^[DCTSIZE*0] := tmp0 + tmp7;
wsptr^[DCTSIZE*7] := tmp0 - tmp7;
wsptr^[DCTSIZE*1] := tmp1 + tmp6;
wsptr^[DCTSIZE*6] := tmp1 - tmp6;
wsptr^[DCTSIZE*2] := tmp2 + tmp5;
wsptr^[DCTSIZE*5] := tmp2 - tmp5;
wsptr^[DCTSIZE*4] := tmp3 + tmp4;
wsptr^[DCTSIZE*3] := tmp3 - tmp4;
Inc(JCOEF_PTR(inptr)); { advance pointers to next column }
Inc(FLOAT_MULT_TYPE_PTR(quantptr));
Inc(FAST_FLOAT_PTR(wsptr));
end;
{ Pass 2: process rows from work array, store into output array. }
{ Note that we must descale the results by a factor of 8 = 2**3. }
wsptr := @workspace;
for ctr := 0 to pred(DCTSIZE) do
begin
outptr := JSAMPROW(@(output_buf^[ctr]^[output_col]));
{ Rows of zeroes can be exploited in the same way as we did with columns.
However, the column calculation has created many nonzero AC terms, so
the simplification applies less often (typically 5% to 10% of the time).
And testing floats for zero is relatively expensive, so we don't bother. }
{ Even part }
tmp10 := wsptr^[0] + wsptr^[4];
tmp11 := wsptr^[0] - wsptr^[4];
tmp13 := wsptr^[2] + wsptr^[6];
tmp12 := (wsptr^[2] - wsptr^[6]) * ({FAST_FLOAT}(1.414213562)) - tmp13;
tmp0 := tmp10 + tmp13;
tmp3 := tmp10 - tmp13;
tmp1 := tmp11 + tmp12;
tmp2 := tmp11 - tmp12;
{ Odd part }
z13 := wsptr^[5] + wsptr^[3];
z10 := wsptr^[5] - wsptr^[3];
z11 := wsptr^[1] + wsptr^[7];
z12 := wsptr^[1] - wsptr^[7];
tmp7 := z11 + z13;
tmp11 := (z11 - z13) * ({FAST_FLOAT}(1.414213562));
z5 := (z10 + z12) * ({FAST_FLOAT}(1.847759065)); { 2*c2 }
tmp10 := ({FAST_FLOAT}(1.082392200)) * z12 - z5; { 2*(c2-c6) }
tmp12 := ({FAST_FLOAT}(-2.613125930)) * z10 + z5; { -2*(c2+c6) }
tmp6 := tmp12 - tmp7;
tmp5 := tmp11 - tmp6;
tmp4 := tmp10 + tmp5;
{ Final output stage: scale down by a factor of 8 and range-limit }
outptr^[0] := range_limit^[ int(DESCALE( INT32(Round((tmp0 + tmp7))), 3))
and RANGE_MASK];
outptr^[7] := range_limit^[ int(DESCALE( INT32(Round((tmp0 - tmp7))), 3))
and RANGE_MASK];
outptr^[1] := range_limit^[ int(DESCALE( INT32(Round((tmp1 + tmp6))), 3))
and RANGE_MASK];
outptr^[6] := range_limit^[ int(DESCALE( INT32(Round((tmp1 - tmp6))), 3))
and RANGE_MASK];
outptr^[2] := range_limit^[ int(DESCALE( INT32(Round((tmp2 + tmp5))), 3))
and RANGE_MASK];
outptr^[5] := range_limit^[ int(DESCALE( INT32(Round((tmp2 - tmp5))), 3))
and RANGE_MASK];
outptr^[4] := range_limit^[ int(DESCALE( INT32(Round((tmp3 + tmp4))), 3))
and RANGE_MASK];
outptr^[3] := range_limit^[ int(DESCALE( INT32(Round((tmp3 - tmp4))), 3))
and RANGE_MASK];
Inc(FAST_FLOAT_PTR(wsptr), DCTSIZE); { advance pointer to next row }
end;
end;
end.
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