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235 lines
8.1 KiB
235 lines
8.1 KiB
/* |
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* jidctflt.c |
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* |
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* Copyright (C) 1994-1998, Thomas G. Lane. |
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* Modified 2010 by Guido Vollbeding. |
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* This file is part of the Independent JPEG Group's software. |
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* For conditions of distribution and use, see the accompanying README file. |
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* |
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* This file contains a floating-point 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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* |
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* This implementation should be more accurate than either of the integer |
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* IDCT implementations. However, it may not give the same results on all |
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* machines because of differences in roundoff behavior. Speed will depend |
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* on the hardware's floating point capacity. |
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* |
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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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* |
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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 a fixed-point |
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* implementation, accuracy is lost due to imprecise representation of the |
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* scaled quantization values. However, that problem does not arise if |
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* we use floating point arithmetic. |
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*/ |
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#define JPEG_INTERNALS |
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#include "jinclude.h" |
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#include "jpeglib.h" |
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#include "jdct.h" /* Private declarations for DCT subsystem */ |
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#ifdef DCT_FLOAT_SUPPORTED |
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/* |
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* This module is specialized to the case DCTSIZE = 8. |
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*/ |
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#if DCTSIZE != 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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/* Dequantize a coefficient by multiplying it by the multiplier-table |
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* entry; produce a float result. |
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*/ |
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#define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval)) |
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/* |
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* Perform dequantization and inverse DCT on one block of coefficients. |
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*/ |
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GLOBAL(void) |
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jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr, |
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JCOEFPTR coef_block, |
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JSAMPARRAY output_buf, JDIMENSION output_col) |
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{ |
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FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; |
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FAST_FLOAT tmp10, tmp11, tmp12, tmp13; |
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FAST_FLOAT z5, z10, z11, z12, z13; |
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JCOEFPTR inptr; |
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FLOAT_MULT_TYPE * quantptr; |
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FAST_FLOAT * wsptr; |
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JSAMPROW outptr; |
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JSAMPLE *range_limit = cinfo->sample_range_limit; |
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int ctr; |
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FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */ |
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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 = (FLOAT_MULT_TYPE *) compptr->dct_table; |
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wsptr = workspace; |
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for (ctr = DCTSIZE; ctr > 0; ctr--) { |
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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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*/ |
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if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 && |
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inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 && |
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inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 && |
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inptr[DCTSIZE*7] == 0) { |
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/* AC terms all zero */ |
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FAST_FLOAT dcval = 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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inptr++; /* advance pointers to next column */ |
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quantptr++; |
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wsptr++; |
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continue; |
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} |
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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 = (tmp1 - tmp3) * ((FAST_FLOAT) 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 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); /* 2*c4 */ |
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z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ |
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tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */ |
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tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 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] = tmp0 + tmp7; |
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wsptr[DCTSIZE*7] = tmp0 - tmp7; |
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wsptr[DCTSIZE*1] = tmp1 + tmp6; |
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wsptr[DCTSIZE*6] = tmp1 - tmp6; |
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wsptr[DCTSIZE*2] = tmp2 + tmp5; |
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wsptr[DCTSIZE*5] = tmp2 - tmp5; |
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wsptr[DCTSIZE*3] = tmp3 + tmp4; |
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wsptr[DCTSIZE*4] = tmp3 - tmp4; |
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inptr++; /* advance pointers to next column */ |
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quantptr++; |
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wsptr++; |
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} |
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/* Pass 2: process rows from work array, store into output array. */ |
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wsptr = workspace; |
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for (ctr = 0; ctr < DCTSIZE; ctr++) { |
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outptr = 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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* And testing floats for zero is relatively expensive, so we don't bother. |
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*/ |
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/* Even part */ |
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/* Apply signed->unsigned and prepare float->int conversion */ |
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z5 = wsptr[0] + ((FAST_FLOAT) CENTERJSAMPLE + (FAST_FLOAT) 0.5); |
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tmp10 = z5 + wsptr[4]; |
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tmp11 = z5 - wsptr[4]; |
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tmp13 = wsptr[2] + wsptr[6]; |
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tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - 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 = wsptr[5] + wsptr[3]; |
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z10 = wsptr[5] - wsptr[3]; |
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z11 = wsptr[1] + wsptr[7]; |
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z12 = wsptr[1] - wsptr[7]; |
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tmp7 = z11 + z13; |
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tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); |
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z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */ |
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tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */ |
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tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */ |
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tmp6 = tmp12 - tmp7; |
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tmp5 = tmp11 - tmp6; |
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tmp4 = tmp10 - tmp5; |
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/* Final output stage: float->int conversion and range-limit */ |
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outptr[0] = range_limit[((int) (tmp0 + tmp7)) & RANGE_MASK]; |
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outptr[7] = range_limit[((int) (tmp0 - tmp7)) & RANGE_MASK]; |
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outptr[1] = range_limit[((int) (tmp1 + tmp6)) & RANGE_MASK]; |
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outptr[6] = range_limit[((int) (tmp1 - tmp6)) & RANGE_MASK]; |
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outptr[2] = range_limit[((int) (tmp2 + tmp5)) & RANGE_MASK]; |
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outptr[5] = range_limit[((int) (tmp2 - tmp5)) & RANGE_MASK]; |
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outptr[3] = range_limit[((int) (tmp3 + tmp4)) & RANGE_MASK]; |
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outptr[4] = range_limit[((int) (tmp3 - tmp4)) & RANGE_MASK]; |
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wsptr += DCTSIZE; /* advance pointer to next row */ |
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} |
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} |
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#endif /* DCT_FLOAT_SUPPORTED */
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