Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!mailrus!ames!lll-winken!uunet!opusys!rlkd
From: rlkd@opusys.UUCP (R.L.K.Dattatri)
Newsgroups: comp.windows.x
Subject: color speedups
Keywords: color speed PFILL
Message-ID: <365@opusys.UUCP>
Date: 28 Apr 89 22:37:25 GMT
Organization: Opus Systems, Cupertino, CA
Lines: 61


We recently implemented a color version of X (R3) on a EGA and VGA
for a System V UNIX. Since the color version was pretty slow to say
the least (that too on a MC88000) we went about some speed ups.
The most important (I feel) speed up is in the file cfbmskbits.h
and cfbmskbits.c. There is a macro PFILL that shifts and ORs
a pattern to propagate a bit pattern to all of the bytes in a pixel.
If it is a 4 bit deep display it will have to propagate the pattern
into 4 bytes and for a 8 bit deep display the propagation is to 8 bytes.

For a 4 bit display the possible values for the bit pattern are only 16
and for the 8 bit display it is 256.

Instead of doing the shifts and ORs, I have a look up table of 16 and 256
entries. This table contains the complete word pattern that results
from the shifts and ORs. So for any value of the bit pattern I just use
the pattern value as an index into the table and use the value their as
the result of PFILL.

The following code fragment illustrates the point:
This is for a 4 bit display. 

/* cfbmskbits.h */

long PFILLtab[] =
	{ 0x00000000,
	  0x11111111,
	  0x22222222,
	  0x33333333,
	  0x44444444,
	  0x55555555,
	  0x66666666,
	  0x77777777,
	  0x88888888,
	  0x99999999,
	  0xaaaaaaaa,
	  0xbbbbbbbb,
	  0xcccccccc,
	  0xdddddddd,
	  0xeeeeeeee,
	  0xffffffff
	};

/* now redefine PFILL macro as follows
   PMSK is the pixel index mask already defined
   and is 0x0F for a 4 bit display
*/ 
#define PFILL(p) (PFILLtab[(p) & PMSK])

For a 8 bit (n bit) display you will need a 256 entry table (2**n).
If it is hard to fill this table by hand, then use the code in PFILL
(one that is already there in cfbmskbits.h) and write a small loop to do the
rest.

This macro is heavily used in all pixel operations.

ANYBODY HAVE SIMILAR SPEED UP EXPERIENCE ?

R.L.K. Dattatri
Opus Systems
rlkd!opusys