QuantFwd8x8

Performs quantization of an 8x8 block of DCT coefficients.

Syntax

IppStatus ippiQuantFwd8x8_JPEG_16s_C1I(Ipp16s* pSrcDst, const Ipp16u* pQuantFwdTable);

Parameters

pSrcDst	Pointer to the 8x8 block of DCT coefficients.
pQuantFwdTable	Pointer to the quantization table for the encoder.

Description

The function ippiQuantFwd8x8_JPEG is declared in the ippj.h file. This function performs quantization of computed DCT coefficients for an 8x8 block. Quantization is defined in [ISO10918], Annex A.3.4, DCT Coefficient Quantization and Dequantization, as

Sq(u,v) = round(S(u,v)/Q(u,v)),

where Sq(u,v) is the quantized DCT coefficient, S(u,v) is a computed DCT coefficient, and Q(u,v) is a quantization table element.

Rounding is to the nearest integer. To avoid division, quantization table elements are scaled up by 2¹⁵. After that, division operation is replaced by multiplication of DCT coefficients by scaled quantization table elements. Then the products are rescaled back by 15 bits.

Example “Using Intel IPP for Quantization of 8x8 Block of DCT Coefficients” shows how to use the function ippiQuantFwd8x8_JPEG_16s_C1I.

Return Values

ippStsNoErr	Indicates no error.
ippStsNullPtrErr	Indicates an error condition if one or both specified pointers are NULL.

Using Intel IPP for Quantization of 8x8 Block of DCT Coefficients

Ipp8u pQuantRawTable[64]= {16,  11,  12,  14,  12,  10,  16,  14,

                            13,  14,  18,  17,  16,  19,  24,  40,

                            26,  24,  22,  22,  24,  49,  35,  37,

                            29,  40,  58,  51,  61,  60,  57,  51,

                            56,  55,  64,  72,  92,  78,  64,  68,

                            87,  69,  55,  56,  80, 109,  81,  87,

                            95,  98, 103, 104, 103,  62,  77, 113,

                           121, 112, 100, 120, 92,  101, 103,  99 };

Ipp16s dstFwd[8*8] = { 4,  4, 4,  4,  4,  4,  4, 4,

                       4,  3, 3,  3, -3, 3, -3, 4,

                       4, -3, 2,  0,  0,  2,  3, 4,

                       4,  3, 0,  1, -1,  0, -3, 4,

                       4, -3, 0, -1,  1,  0,  3, 4,

                       4,  3, 2,  0,  0,  2, -3, 4,

                       4, -3, 3, -3,  3, -3,  3, 4,

                       4,  4, 4,  4,  4,  4,  4, 4};

Ipp16u pQuantFwdTable[64];

int quality = 75;

ippiQuantFwdRawTableInit_JPEG_8u ( pQuantRawTable, quality );

ippiQuantFwdTableInit_JPEG_8u16u ( pQuantRawTable, pQuantFwdTable );

ippiQuantFwd8x8_JPEG_16s_C1I ( dstFwd, pQuantFwdTable );

result:

pQuantRawTable (with quality)

8 6 6 7 6 5 8 7

7 7 9 9 8 10 12 20

13 12 11 11 12 25 18 19

15 20 29 26 31 30 29 26

28 28 32 36 46 39 32 34

44 35 28 28 40 55 41 44

48 49 52 52 52 31 39 57

61 56 50 60 46 51 52 50

pQuantFwdTable

4096 5461 6554 4096 2731 1638 1260 1057

5461 5461 4681 3277 2521 1130 1092 1170

4681 4681 4096 2731 1638 1130  936 1170

4681 3641 2979 2185 1260  745  819 1057

3641 2979 1725 1170  964  596  630  840

2731 1820 1170 1024  799  630  575  712

1311 1024  840  745  630  537  546  643

 910  712  683  669  585  655  630  655

dstFwd (after pQuantFwdTable)

0 1 1 0 0 0 0 0

1 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0

1 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0