'From Squeak4.1beta of 15 April 2010 [latest update: #9972] on 15 April 2010 at 6:23:23 pm'!

!PNGReadWriter methodsFor: 'pixel copies' stamp: 'ul 4/15/2010 18:19'!
copyPixelsGray: y 
	"Handle non-interlaced grayscale color mode (colorType = 0)"

	| base bits |
	bitsPerChannel = 16 ifTrue: [
		"Warning: This is extremely slow. Besides we are downsampling to 8 bits!!"
		| blitter |
		blitter := BitBlt current bitPokerToForm: form.
		0 to: width - 1 do: [ :x |
			blitter pixelAt: x @ y put: 255 - (thisScanline at: x * 2 + 1) ].
			^self ].

	"Just copy the bits"
	"This interesting technique (By Andreas Raab) is faster for very large images, but might be slower for small ones"
	"^self copyPixelsGrayWeirdBitBltHack: y ".

	"This interesting technique  (By Yoshiki Ohshima) is faster for very large images, but might be slower for small ones"
	"form bits copyFromByteArray2: thisScanline to: y * (form width* bitsPerChannel // 32)".

	"This Smalltalk version might be easier to understand"
	base := y * form width * bitsPerChannel // 32 + 1.
	bits := form bits.
	0 to: thisScanline size - 1 // 4 do: [ :i |
		| ii word |
		ii := i * 4.
		"This somewhat weird mixture of (#* and #+) with (#bitShift: and #bitOr:) 
		is to make use of faster arithmetic bytecodes, but not of slow largeintegers."
		word :=
			(((thisScanline at: ii + 1) * 256 + 
			(thisScanline at: ii + 2) * 256 + 
			(thisScanline at: ii + 3)) bitShift: 8) bitOr: 
			(thisScanline at: ii + 4).
		bits at: base + i put: word ].! !