VCT Vision ELITE Series Especificaciones descargar pdf (Pagina 59)

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GEMS "PRI-CTMR-MOD12" DICOM MOD "JPEG" Encoding Information

Note: Some earlier versions of that were intended to write JPEG Lossless Process

Selection Value 14 1.2.840.10008.1.2.4.70 actually contained errors in byte

ordering of encapsulation of fragment item tags, selection of entropy coding

tables, and calculation of difference values from predictors. These errors cause

the compressed images to be unreadable by non-GEHC software. All future GEHC

LightSpeed software will read (but not write) archive media written with these

errors.

The early release product software had a JPEG encode routine that is called the

BAD_ENCODER for the following explanation.

In the BAD_ENCODER produced 'SOS' (0xFFDA) header, the entropy coding table

selector codes are 0x11. They should be 0x00. This mistake is the key that specifies the

BAD_ENCODER processing problems

The BAD_ENCODER computation of the pixel value difference to be entropy encoded is

computed as (Predictor - value) when it should be calculated as (value - Predictor)1. The

result is that the decompression via industry standard packages result in a negative of

the original image pixel value.

The BAD_ENCODER predictor value used at the beginning of each line uses the last

value of the previous line in the image. The JPEG standard says the predictor for the first

line element of each line should be the first element of the line immediately above the

current line. The first line, first element predictor should be the unsigned value that is half

the full-scale range for the "sample precision". The first line, first element predictor is

correctly determined in the BAD_ENCODER.

Explanation

This section is provided to explain what was done during Image encoding so that people who

wish and need to decode the images can modify the available JPEG codec software. These

packages are available from the WEB and were originally developed at Stanford and Cornell as

well as others in the industry.

What was done in Generation 1 DICOM MOD encoding by GEHC:

In the JPEG codec output, the JPEG 'SOS' header defines the Huffman table selector codes to

find the appropriate Huffman table (FF DA 00 08 01 00 11 01 00 00 ...). The GEHC DICOM

Access Kit coded these as 0x11. They should have been 0x00. Zero was the number assigned

in the "DHT" header. This error is a primary key that is used in the GEHC Generation 2 system

to determine that the JPEG data was encoded using the GEHC Generation 1 coding technique.

1 No specific direct confirmation of the (value - Predictor) policy was found, but the graphic on page 185 of the book "JPEG

still image data compression standard" by Pennebaker shows the predictor entering the adder as a negative element and

the value entering as positive element. Like wise, both Cornell and Stanford packages perform the computation as (value -

Predictor).

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