PCX files How to read ??

 BBS: Inland Empire Archive
Date: 11-16-92 (08:21)             Number: 303
From: HARRY GISH                   Refer#: NONE
  To: BOB OEHRLE                    Recvd: NO  
Subj: PCX files How to read ??       Conf: (2) Quik_Bas
 BO> Would like to find out how Pcx files are organized
 BO> so that I can then
 BO> use BLOAD to plot on the screen. The files that I
 BO> am interested in viewing
 BO> are 43830 bytes long. I have copied these files
Why use BLOAD? Just do a PCX reader. It's easier in this case since you are
limiting yourself to a specific situation (when you're
doing a general PCX reader you have to allow for 3 distinct
formats, 2, 16 and 256 color files). All you really need to
do is double-check the absolute size , which is located in
bytes 8-11 from the beginning of the file in the 128 byte
header. The 16 color definitions (which you restore and set
using PALETTE) are in the 48 bytes starting at byte 17, and
are the red, green, and blue intensities respectively.

The actual encoding starts at byte 128 and works this way : If the byte is
greater than 191 it is a multiplier otherwise it is data
{be VERY careful in logicing this out. The multiplier can
be between 1 and 63 only AND the only way to encode a data
value greater than 191 is with a multiplier first. Thus a
one code of data 243 would have to be encoded as a 1
multiplier of 243. This is what makes a 320X200X256 color
PCX, which should be 64000 maximum ... plus resolution and
palette header info ... often baloon up to 70-80K). Anyway,
the data is encoded into groups of 8 pixel values and in 4
"scans". Thus you actually read an X line 4 times,
accumulating the binary values to range from 0 to 15 for each pixel.

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