OK, I'm almost there! The version I'm working with now (including you're wonderful suggestions!) displays exactly ONE frame of the B/W image (Woo Hoo!) but then blows up. This is the code:

initializeProcess

   | colorMap bwFrame |
   frame _ Form extent: self decoderSize depth: Display depth.	
   process _ [[true] whileTrue: [
	self decoderRequestCapture.
	frameSemaphore wait.
	self decoderCopyLastCapturedFrameInto: frame.
	colorMap _ ColorMap shifts: #(0 0 -5 0)
		masks: #(0 0 2r000001111100000 0)
		colors: ((1 to: 32) collect: [:i | 
		i < 16 ifTrue: [1] ifFalse: [0]]).
	bwFrame _ Form extent: frame extent depth:1.
	(BitBlt toForm: bwFrame)
		copyForm: frame to: 0 at 0 rule: Form over colorMap: colorMap.

	"Here's the problem: the class I am extending wants to display 
	frame (which MUST be of depth 16 or greater), not bwFrame 
	(which is of depth 1), so I need to get bwFrame's revised 
	colorMap back to depth 16 before I make frame equal to 
	bwFrame. This is why it is blowing up. Any ideas?"

	frame _ bwFrame.
	]] forkAt: self decoderProcessPriority.

-----Original Message-----
From: squeak-dev-bounces at lists.squeakfoundation.org [mailto:squeak-dev-bounces at lists.squeakfoundation.org] On Behalf Of Bert Freudenberg
Sent: Thursday, July 22, 2004 4:38 AM
To: The general-purpose Squeak developers list
Subject: Re: Using video input in Squeak


Am 22.07.2004 um 03:58 schrieb Dean_Swan at Mitel.COM:

>
> Hi Rob, Bert, Jack:
>
> Form asGrayScale actually does extract the green component from 32 bit 
> forms, or converts a lower depth Form to a 32 bit form and then 
> extracts the green component.  It is NOT however, a single BitBlt 
> call.  It does a copyBits for each column, and some fancy form/bitmap 
> substituion to treat the 32 bit form as if it were an 8 bit form four 
> times as wide.

I think the asGrayscale method is older than the ColorMap class, which 
is a rather recent extension to the original BitBlt. For performance, 
it should be rewritten now.

> How would you accomplish this with only a "single BitBlt call"?

==== snip ====
| orgForm bwForm |
"Make a dark shape on light ground"
Display getCanvas
	fillRectangle: (0 at 0 extent: 400 at 300) color: Color yellow;
	fillOval: (100 at 100 extent: 30 at 50) color: Color blue.
orgForm := Form fromUser asFormOfDepth: 16.
"setup colormap to extract green component and map to 0 or 1"
colorMap := ColorMap shifts: #(0 0 -5 0)
	masks: #(0 0 2r000001111100000 0)
	colors: ((1 to: 32) collect: [:i | i < 16 ifTrue: [1] ifFalse: [0]]). 
"16 = threshold of 0.5"
"This should also be allocated only once "
bwForm := Form extent: orgForm extent depth: 1.
"Do this every frame"
(BitBlt toForm: bwForm)
	copyForm: orgForm to: 0 at 0 rule: Form over colorMap: colorMap.
"Done"
bwForm display
==== snap ====

For 32 bpp, you of course need to tweak the shifts, masks, and colors:

ColorMap shifts: #(0 0 -8 0)
	masks: #(0 0 16r00FF00 0)
	colors: ((1 to: 256) collect: [:i | i < 128 ifTrue: [1] ifFalse: 
[0]]).  "128 = threshold of 0.5"

I guess the 16 bpp version will be faster, but you should use whatever 
the plugin delivers fastest. You should avoid the conversion to another 
depth unless it is really necessary. Also, all constant calculations 
should be moved out of the inner loop (the one that fetches images from 
the camera and processes them).

Also, if your destination bitmap is not of depth 1, the colors in the 
ColorMap of course need other PixelValues:

String streamContents: [:s | #(1 8 16 32) do: [:d |
	s print: d; nextPutAll: ': '.
	{Color black. Color white} do: [:c |
		s nextPutAll: (c pixelValueForDepth: d) hex; space].
	s cr]]

1: 16r1 16r0
8: 16r1 16rFF
16: 16r1 16r7FFF
32: 16rFF000001 16rFFFFFFFF

> Rob, just a caveat using Form>>asGrayScale - It is an approximation. 
>  I tried it using a workspace as the test bitmap, and the selected 
> text didn't come out as selected because with default colors, selected 
> text is black on a bright green and the background is bright white. 
>  This leads to what looks like no selected text in the grayscale 
> version, where the "properly" calculated Form>>yComponent method does 
> show the selected text as black on medium gray.

You just have to adjust the threshold ...

- Bert -