source/jfdctflt.c (LibJPEG)

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/* * jfdctflt.c * * Copyright (C) 1994-1996, Thomas G. Lane. * Modified 2003-2009 by Guido Vollbeding. * 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 floating-point implementation of the * forward DCT (Discrete Cosine Transform). * * This implementation should be more accurate than either of the integer * DCT 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 DCT can be done by 1-D DCT on each row followed by 1-D DCT * on each column. 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. */ #define JPEG_INTERNALS #include "jinclude.h" #include "jpeglib.h" #include "jdct.h" /* Private declarations for DCT subsystem */ #ifdef DCT_FLOAT_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 /* * Perform the forward DCT on one block of samples. */ GLOBAL(void) jpeg_fdct_float (FAST_FLOAT * data, JSAMPARRAY sample_data, JDIMENSION start_col) { FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; FAST_FLOAT tmp10, tmp11, tmp12, tmp13; FAST_FLOAT z1, z2, z3, z4, z5, z11, z13; FAST_FLOAT *dataptr; JSAMPROW elemptr; int ctr; /* Pass 1: process rows. */ dataptr = data; for (ctr = 0; ctr < DCTSIZE; ctr++) { elemptr = sample_data[ctr] + start_col; /* Load data into workspace */ tmp0 = (FAST_FLOAT) (GETJSAMPLE(elemptr[0]) + GETJSAMPLE(elemptr[7])); tmp7 = (FAST_FLOAT) (GETJSAMPLE(elemptr[0]) - GETJSAMPLE(elemptr[7])); tmp1 = (FAST_FLOAT) (GETJSAMPLE(elemptr[1]) + GETJSAMPLE(elemptr[6])); tmp6 = (FAST_FLOAT) (GETJSAMPLE(elemptr[1]) - GETJSAMPLE(elemptr[6])); tmp2 = (FAST_FLOAT) (GETJSAMPLE(elemptr[2]) + GETJSAMPLE(elemptr[5])); tmp5 = (FAST_FLOAT) (GETJSAMPLE(elemptr[2]) - GETJSAMPLE(elemptr[5])); tmp3 = (FAST_FLOAT) (GETJSAMPLE(elemptr[3]) + GETJSAMPLE(elemptr[4])); tmp4 = (FAST_FLOAT) (GETJSAMPLE(elemptr[3]) - GETJSAMPLE(elemptr[4])); /* Even part */ tmp10 = tmp0 + tmp3; /* phase 2 */ tmp13 = tmp0 - tmp3; tmp11 = tmp1 + tmp2; tmp12 = tmp1 - tmp2; /* Apply unsigned->signed conversion */ dataptr[0] = tmp10 + tmp11 - 8 * CENTERJSAMPLE; /* phase 3 */ dataptr[4] = tmp10 - tmp11; z1 = (tmp12 + tmp13) * ((FAST_FLOAT) 0.707106781); /* c4 */ dataptr[2] = tmp13 + z1; /* phase 5 */ dataptr[6] = tmp13 - z1; /* Odd part */ tmp10 = tmp4 + tmp5; /* phase 2 */ tmp11 = tmp5 + tmp6; tmp12 = tmp6 + tmp7; /* The rotator is modified from fig 4-8 to avoid extra negations. */ z5 = (tmp10 - tmp12) * ((FAST_FLOAT) 0.382683433); /* c6 */ z2 = ((FAST_FLOAT) 0.541196100) * tmp10 + z5; /* c2-c6 */ z4 = ((FAST_FLOAT) 1.306562965) * tmp12 + z5; /* c2+c6 */ z3 = tmp11 * ((FAST_FLOAT) 0.707106781); /* c4 */ z11 = tmp7 + z3; /* phase 5 */ z13 = tmp7 - z3; dataptr[5] = z13 + z2; /* phase 6 */ dataptr[3] = z13 - z2; dataptr[1] = z11 + z4; dataptr[7] = z11 - z4; dataptr += DCTSIZE; /* advance pointer to next row */ } /* Pass 2: process columns. */ dataptr = data; for (ctr = DCTSIZE-1; ctr >= 0; ctr--) { tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7]; tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7]; tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6]; tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6]; tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5]; tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5]; tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4]; tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4]; /* Even part */ tmp10 = tmp0 + tmp3; /* phase 2 */ tmp13 = tmp0 - tmp3; tmp11 = tmp1 + tmp2; tmp12 = tmp1 - tmp2; dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */ dataptr[DCTSIZE*4] = tmp10 - tmp11; z1 = (tmp12 + tmp13) * ((FAST_FLOAT) 0.707106781); /* c4 */ dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */ dataptr[DCTSIZE*6] = tmp13 - z1; /* Odd part */ tmp10 = tmp4 + tmp5; /* phase 2 */ tmp11 = tmp5 + tmp6; tmp12 = tmp6 + tmp7; /* The rotator is modified from fig 4-8 to avoid extra negations. */ z5 = (tmp10 - tmp12) * ((FAST_FLOAT) 0.382683433); /* c6 */ z2 = ((FAST_FLOAT) 0.541196100) * tmp10 + z5; /* c2-c6 */ z4 = ((FAST_FLOAT) 1.306562965) * tmp12 + z5; /* c2+c6 */ z3 = tmp11 * ((FAST_FLOAT) 0.707106781); /* c4 */ z11 = tmp7 + z3; /* phase 5 */ z13 = tmp7 - z3; dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */ dataptr[DCTSIZE*3] = z13 - z2; dataptr[DCTSIZE*1] = z11 + z4; dataptr[DCTSIZE*7] = z11 - z4; dataptr++; /* advance pointer to next column */ } } #endif /* DCT_FLOAT_SUPPORTED */