// * jidctflt.c // * // * Copyright (C) 1994, 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 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. #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 // Dequantize a coefficient by multiplying it by the multiplier-table // * entry; produce a float result. #define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval)) // * Perform dequantization and inverse DCT on one block of coefficients. GLOBAL void jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr, JCOEFPTR coef_block, JSAMPARRAY output_buf, JDIMENSION output_col) { FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; FAST_FLOAT tmp10, tmp11, tmp12, tmp13; FAST_FLOAT z5, z10, z11, z12, z13; JCOEFPTR inptr; FLOAT_MULT_TYPE * quantptr; FAST_FLOAT * wsptr; JSAMPROW outptr; JSAMPLE *range_limit = IDCT_range_limit(cinfo); int ctr; FAST_FLOAT workspace[DCTSIZE2]; // buffers data between passes SHIFT_TEMPS // Pass 1: process columns from input, store into work array. inptr = coef_block; quantptr = (FLOAT_MULT_TYPE *) compptr->dct_table; wsptr = workspace; for (ctr = DCTSIZE; ctr > 0; ctr--) { // 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] | inptr[DCTSIZE*2] | inptr[DCTSIZE*3] | inptr[DCTSIZE*4] | inptr[DCTSIZE*5] | inptr[DCTSIZE*6] | inptr[DCTSIZE*7]) == 0) { // 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; inptr++; // advance pointers to next column quantptr++; wsptr++; continue; } // 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; inptr++; // advance pointers to next column quantptr++; wsptr++; } // 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; ctr < DCTSIZE; ctr++) { outptr = 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) (tmp0 + tmp7), 3) & RANGE_MASK]; outptr[7] = range_limit[(int) DESCALE((INT32) (tmp0 - tmp7), 3) & RANGE_MASK]; outptr[1] = range_limit[(int) DESCALE((INT32) (tmp1 + tmp6), 3) & RANGE_MASK]; outptr[6] = range_limit[(int) DESCALE((INT32) (tmp1 - tmp6), 3) & RANGE_MASK]; outptr[2] = range_limit[(int) DESCALE((INT32) (tmp2 + tmp5), 3) & RANGE_MASK]; outptr[5] = range_limit[(int) DESCALE((INT32) (tmp2 - tmp5), 3) & RANGE_MASK]; outptr[4] = range_limit[(int) DESCALE((INT32) (tmp3 + tmp4), 3) & RANGE_MASK]; outptr[3] = range_limit[(int) DESCALE((INT32) (tmp3 - tmp4), 3) & RANGE_MASK]; wsptr += DCTSIZE; // advance pointer to next row } } #endif // DCT_FLOAT_SUPPORTED