When you run the FreeCell demo it seems on the powerpc that
BitBltSimulation>>alphaSourceBlendBits16
chews a lot of cycles, say 8.7% of the total time.

Well I noticed that in
BitBltSimulation>> dither32To16: threshold:
that  it does something interesting, why it grabs the r g b bytes and   
mangles them into a 16 bit word. But really at each step we have 8 bits  
of data with 4 bits for threshold information to result in 4 bits of  
output.

{Yes, well it's late maybe my math is wrong}

Gee can you say a lookup table (4096 elements of 4 bits (however 8 will  
do)), yes you can.
{actually I'm not sure using a byte is wise, perhaps a word is faster,  
silly risc  machines}

New.
dither32To16: srcWord threshold: ditherValue
	"Dither the given 32bit word to 16 bit. Ignore alpha."
	| addThreshold  |
	self inline: true. "You bet"
	addThreshold _ ditherValue bitShift: 8.
	^((dither8Lookup at: (addThreshold+((srcWord bitShift: -16) bitAnd:  
255))) bitShift: 10) +
		((dither8Lookup at: (addThreshold+((srcWord bitShift: -8) bitAnd:  
255))) bitShift: 5) +
		(dither8Lookup at: (addThreshold+(srcWord bitAnd: 255))).

Populating the dither8Lookup (name to change)  table is left as an  
exercise for the reader.
This needless to say makes quite a difference and we shave about 20%  
off our 16 bit alpha blended redraw times, which takes cpu cycle usage  
to 4.8%.

I'll need to clean this up a bit, and of course if anyone wants to  
test, please let me know.

Original.
dither32To16: srcWord threshold: ditherValue
	"Dither the given 32bit word to 16 bit. Ignore alpha."
	| pv threshold value out |
	self inline: true. "You bet"
	pv _ srcWord bitAnd: 255.
	threshold _ ditherThresholds16 at: (pv bitAnd: 7).
	value _ ditherValues16 at: (pv bitShift: -3).
	ditherValue < threshold
		ifTrue:[out _ value + 1]
		ifFalse:[out _ value].
	pv _ (srcWord bitShift: -8) bitAnd: 255.
	threshold _ ditherThresholds16 at: (pv bitAnd: 7).
	value _ ditherValues16 at: (pv bitShift: -3).
	ditherValue < threshold
		ifTrue:[out _ out bitOr: (value+1 bitShift:5)]
		ifFalse:[out _ out bitOr: (value bitShift: 5)].
	pv _ (srcWord bitShift: -16) bitAnd: 255.
	threshold _ ditherThresholds16 at: (pv bitAnd: 7).
	value _ ditherValues16 at: (pv bitShift: -3).
	ditherValue < threshold
		ifTrue:[out _ out bitOr: (value+1 bitShift:10)]
		ifFalse:[out _ out bitOr: (value bitShift: 10)].
	^out
--
======================================================================== 
===
John M. McIntosh <johnmci at smalltalkconsulting.com> 1-800-477-2659
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
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