Cypress DCT-1D Podręcznik Użytkownika Pobierz pdf (Strona 51)

4.6. QUANTIZATION

0 0.5 1 1.5 2 2.5

x 10

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

dmadct(N)

read(N) Q(N) huff(N)

ACC

CPU

clockcycles

Figure 4.3: Timestamps for JPEG compression pipeline. Each color coded patch rep-

resents the processing of an 8 ×8-block. Colorcodes: DMA+DCT red, readout green,

quantization blue and Huffman encoding yellow.

4.6 Quantization

4.6.1 General

The standard JPEG quantization table for the luminance channel is given by

[u, v] =







16 11 10 16 24 40 51 61

12 12 14 19 26 58 60 55

14 13 16 24 40 57 69 56

14 17 22 29 51 87 80 62

18 22 37 56 68 109 103 77

24 35 55 64 81 104 113 92

49 64 78 87 103 121 120 101

72 92 95 98 112 100 103 99







. (4.1)

To get higher image quality this table is divided by 2. Reciprocals are then computed

by the formula

R[u, v] =

[u, v]/2

, (4.2)

which is the table in the ﬁle jcdctmgr.c.

As an example we use A[u, v] from (3.6). Before quantizing A[u, v] we subtract

64 × 128 = 8192 from the DC value A[0, 0], which corresponds to taking the DCT of

a[x, y] − 128. This procedure is meant to give a smaller value on the average, which

however is false in this particular case.

Finally we compensate for the scale factor 8 introduced in the DCT2 step by right-

1 2 ... 46 47 48 49 50 51 52 53 54 55 56 ... 81 82

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