ZR36060PQC datasheet(6/46 Pages) ETC

6 / 46 page

Integrated JPEG CODEC

cients. The encoding methods used for DC and AC coefficients

differ in their details, and this requires two Huffman tables to be

specified, one for DC and one for AC. And since the statistics of

the luminance and chrominance components are generally quite

different, separate Huffman tables are required for luminance

and chrominance, for a total of four tables, two DC and two AC.

The ZR36060 supports this configuration.

Baseline decompression essentially consists of the inverses of

each of the stages used in compression, in reverse order:

Huffman decoding, dequantization, inverse DCT, and conver-

sion of the blocks back to raster order.

1.2.1.1 The Minimum Coded Unit

If the compressed image data is interleaved, as is the case in the

ZR36060, the compression is performed in units of a Minimum

Coded Unit, or MCU, which contains one or more blocks of each

color components. For the 4:2:2 pixel format used by the

ZR36060, where the chrominance (U and V) components are

decimated by 2:1 horizontally relative to the luminance (Y), the

MCU consists of 2 blocks of Y followed by one block each of U

and V.

1.2.1.2 Restart Intervals

The ZR36060 supports compression and decompression of

JPEG data that includes restart intervals. A restart interval is

defined as an integral number of MCUs, which are processed as

an “independent sequence”, meaning that it is possible to

identify and decode a restart interval within a JPEG data

sequence, without the need to decode whatever data precedes

it. In the context of baseline compression, this has significance

because the DC coefficients of the DCT are differentially

encoded. Note that the use of restarts is optional; it is acceptable

(and very common) to use no restart markers and encode the

whole image as a single sequence.

1.2.2 JPEG markers

JPEG defines three data formats for the compressed bitstream,

all of which are supported by the ZR36060:

• The

interchange format, which contains the specifications of

all the tables required to decode the image.

• the

abbreviated format for compressed data, which can con-

tain some or none of the tables, under the assumption that

the remaining tables are known to the decoder and are al-

ready loaded in the decoder or can be loaded. This is

commonly used for motion video, in order to save the time

otherwise required to decode the tables from their

specifications.

• the abbreviated

tables-only format, which contains no com-

pressed data but only tables. It is one means by which it is

possible to load tables into the decoder; in the ZR36060 the

other means is by specifying the tables to the device and is-

suing an explicit Load command.

In all three of the formats, the tables and the parameters required

for decoding the image and/or the tables are contained in

marker

segments, which are sequences of bytes that start with special

two-byte codes called

markers or marker codes. The two bytes

that follow the marker specify the length of the marker segment

in bytes, including the two length bytes but not including the

marker code itself. There are two special stand-alone markers

that are not associated with marker segments, to mark the start-

of-image (SOI) and end-of-image (EOI). The code values are

0xFFD8 for SOI, and 0xFFD9 for EOI.

The first byte of every marker is 0xFF. A marker may be prefixed

by an arbitrary number of 0xFF bytes which are discarded by the

decoder. The second byte of a marker has defined values,

except for 0x00, which is used as follows. In order to permit a

decoder to identify the restart markers, if they exist, and the EOI

marker, the encoder stuffs a 0x00 byte after every 0xFF byte that

results from the Huffman encoding. Note that this “byte stuffing”

is an essential part of the JPEG standard, and there is no defini-

tion in the standard of a bitstream that does not include the byte

stuffing. The ZR36060 always produces image bitstreams with

byte stuffing, and requires the byte stuffing to be present in order

to decode a JPEG bitstream.

The JPEG standard also does not define any sort of “markerless”

bitstream data format. Certain markers and marker segments

are defined in the standard to be “required”, and others, such as

the restart markers and the table marker segments, are optional.

The ZR36060 always includes the required markers when it

produces a compressed bitstream, and can be programmed to

include certain optional markers. To be decompressed by the

ZR36060, an image bitstream

must include the required

markers. All markers included in the bitstream, required and

optional, are handled automatically, without host intervention, by

the ZR36060 in decompression.

1.2.2.1 Required markers and marker segments

The required markers for baseline JPEG are:

• Start-of-image, SOI (0xFFD8). This is the first marker in a

JPEG image bitstream.

• Start-of-frame marker segment, SOF0 (0xFFC0), followed

by a variable number of bytes depending on the number of

color components. For the ZR36060, there are always three

components and the segment has a length of 17 bytes. The

SOF segment is used to specify which quantization table to

use for each color component, and the number of blocks of

each color component in the MCU.

• Start-of-scan marker segment, SOS (0xFFDA), followed by a

variable number of bytes depending on the number of color

components. The Huffman coded data follows immediately

after the last byte of the SOS segment. In the case of the

ZR36060, the length of the SOS segment is always 12 bytes.

The SOS segment is used to specify which Huffman table to

use for each color component.

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