/* | |

* IDCT implementation using the MIPS DSP ASE (little endian version) | |

* | |

* jidctfst.c | |

* | |

* Copyright (C) 1994-1998, Thomas G. Lane. | |

* This file is part of the Independent JPEG Group's software. | |

* For conditions of distribution and use, see the accompanying README file. | |

* | |

* This file contains a fast, not so accurate integer implementation of the | |

* inverse DCT (Discrete Cosine Transform). In the IJG code, this routine | |

* must also perform dequantization of the input coefficients. | |

* | |

* 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 fixed-point math, | |

* accuracy is lost due to imprecise representation of the scaled | |

* quantization values. The smaller the quantization table entry, the less | |

* precise the scaled value, so this implementation does worse with high- | |

* quality-setting files than with low-quality ones. | |

*/ | |

#define JPEG_INTERNALS | |

#include "jinclude.h" | |

#include "jpeglib.h" | |

#include "jdct.h" /* Private declarations for DCT subsystem */ | |

#ifdef DCT_IFAST_SUPPORTED | |

/* | |

* This module is specialized to the case DCTSIZE = 8. | |

*/ | |

#if DCTSIZE != 8 | |

Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ | |

#endif | |

/* Scaling decisions are generally the same as in the LL&M algorithm; | |

* see jidctint.c for more details. However, we choose to descale | |

* (right shift) multiplication products as soon as they are formed, | |

* rather than carrying additional fractional bits into subsequent additions. | |

* This compromises accuracy slightly, but it lets us save a few shifts. | |

* More importantly, 16-bit arithmetic is then adequate (for 8-bit samples) | |

* everywhere except in the multiplications proper; this saves a good deal | |

* of work on 16-bit-int machines. | |

* | |

* The dequantized coefficients are not integers because the AA&N scaling | |

* factors have been incorporated. We represent them scaled up by PASS1_BITS, | |

* so that the first and second IDCT rounds have the same input scaling. | |

* For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to | |

* avoid a descaling shift; this compromises accuracy rather drastically | |

* for small quantization table entries, but it saves a lot of shifts. | |

* For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway, | |

* so we use a much larger scaling factor to preserve accuracy. | |

* | |

* A final compromise is to represent the multiplicative constants to only | |

* 8 fractional bits, rather than 13. This saves some shifting work on some | |

* machines, and may also reduce the cost of multiplication (since there | |

* are fewer one-bits in the constants). | |

*/ | |

#if BITS_IN_JSAMPLE == 8 | |

#define CONST_BITS 8 | |

#define PASS1_BITS 2 | |

#else | |

#define CONST_BITS 8 | |

#define PASS1_BITS 1 /* lose a little precision to avoid overflow */ | |

#endif | |

/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus | |

* causing a lot of useless floating-point operations at run time. | |

* To get around this we use the following pre-calculated constants. | |

* If you change CONST_BITS you may want to add appropriate values. | |

* (With a reasonable C compiler, you can just rely on the FIX() macro...) | |

*/ | |

#if CONST_BITS == 8 | |

#define FIX_1_082392200 ((INT32) 277) /* FIX(1.082392200) */ | |

#define FIX_1_414213562 ((INT32) 362) /* FIX(1.414213562) */ | |

#define FIX_1_847759065 ((INT32) 473) /* FIX(1.847759065) */ | |

#define FIX_2_613125930 ((INT32) 669) /* FIX(2.613125930) */ | |

#else | |

#define FIX_1_082392200 FIX(1.082392200) | |

#define FIX_1_414213562 FIX(1.414213562) | |

#define FIX_1_847759065 FIX(1.847759065) | |

#define FIX_2_613125930 FIX(2.613125930) | |

#endif | |

/* We can gain a little more speed, with a further compromise in accuracy, | |

* by omitting the addition in a descaling shift. This yields an incorrectly | |

* rounded result half the time... | |

*/ | |

#ifndef USE_ACCURATE_ROUNDING | |

#undef DESCALE | |

#define DESCALE(x,n) RIGHT_SHIFT(x, n) | |

#endif | |

/* Multiply a DCTELEM variable by an INT32 constant, and immediately | |

* descale to yield a DCTELEM result. | |

*/ | |

#define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS)) | |

/* Dequantize a coefficient by multiplying it by the multiplier-table | |

* entry; produce a DCTELEM result. For 8-bit data a 16x16->16 | |

* multiplication will do. For 12-bit data, the multiplier table is | |

* declared INT32, so a 32-bit multiply will be used. | |

*/ | |

#if BITS_IN_JSAMPLE == 8 | |

#define DEQUANTIZE(coef,quantval) (((IFAST_MULT_TYPE) (coef)) * (quantval)) | |

#else | |

#define DEQUANTIZE(coef,quantval) \ | |

DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS) | |

#endif | |

/* Like DESCALE, but applies to a DCTELEM and produces an int. | |

* We assume that int right shift is unsigned if INT32 right shift is. | |

*/ | |

#ifdef RIGHT_SHIFT_IS_UNSIGNED | |

#define ISHIFT_TEMPS DCTELEM ishift_temp; | |

#if BITS_IN_JSAMPLE == 8 | |

#define DCTELEMBITS 16 /* DCTELEM may be 16 or 32 bits */ | |

#else | |

#define DCTELEMBITS 32 /* DCTELEM must be 32 bits */ | |

#endif | |

#define IRIGHT_SHIFT(x,shft) \ | |

((ishift_temp = (x)) < 0 ? \ | |

(ishift_temp >> (shft)) | ((~((DCTELEM) 0)) << (DCTELEMBITS-(shft))) : \ | |

(ishift_temp >> (shft))) | |

#else | |

#define ISHIFT_TEMPS | |

#define IRIGHT_SHIFT(x,shft) ((x) >> (shft)) | |

#endif | |

#ifdef USE_ACCURATE_ROUNDING | |

#define IDESCALE(x,n) ((int) IRIGHT_SHIFT((x) + (1 << ((n)-1)), n)) | |

#else | |

#define IDESCALE(x,n) ((int) IRIGHT_SHIFT(x, n)) | |

#endif | |

// this table of constants has been moved from mips_idct_le/_be.s to | |

// avoid having to make the assembler code position independent | |

static const int mips_idct_coefs[4] = { | |

0x45464546, // FIX( 1.082392200 / 2) = 17734 = 0x4546 | |

0x5A825A82, // FIX( 1.414213562 / 2) = 23170 = 0x5A82 | |

0x76427642, // FIX( 1.847759065 / 2) = 30274 = 0x7642 | |

0xAC61AC61 // FIX(-2.613125930 / 4) = -21407 = 0xAC61 | |

}; | |

void mips_idct_columns(JCOEF * inptr, IFAST_MULT_TYPE * quantptr, | |

DCTELEM * wsptr, const int * mips_idct_coefs); | |

void mips_idct_rows(DCTELEM * wsptr, JSAMPARRAY output_buf, | |

JDIMENSION output_col, const int * mips_idct_coefs); | |

/* | |

* Perform dequantization and inverse DCT on one block of coefficients. | |

*/ | |

GLOBAL(void) | |

jpeg_idct_mips (j_decompress_ptr cinfo, jpeg_component_info * compptr, | |

JCOEFPTR coef_block, | |

JSAMPARRAY output_buf, JDIMENSION output_col) | |

{ | |

JCOEFPTR inptr; | |

IFAST_MULT_TYPE * quantptr; | |

DCTELEM workspace[DCTSIZE2]; /* buffers data between passes */ | |

/* Pass 1: process columns from input, store into work array. */ | |

inptr = coef_block; | |

quantptr = (IFAST_MULT_TYPE *) compptr->dct_table; | |

mips_idct_columns(inptr, quantptr, workspace, mips_idct_coefs); | |

/* 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, */ | |

/* and also undo the PASS1_BITS scaling. */ | |

mips_idct_rows(workspace, output_buf, output_col, mips_idct_coefs); | |

} | |

#endif /* DCT_IFAST_SUPPORTED */ |