[PyQt] Efficiency under nested loop

Sergi Blanch i Torné sbt at ya.com
Thu Apr 24 15:05:36 BST 2008


In the discussion about the QListWidget[1] I saw something that happens to me 
also with the QImage conversion from 12 bits to 8. Remember that I have a 
problem showing grayscaled images[2] I need to say that the data needs to be 
process and it means the solution cannot be simply acquire the pictures in 8 

Right now I write a loop for the list and then convert to the array of 
unsigned chars need by the QImage. It means touch each element two times. The 
code is:
	#input: values list of the image
	#output: array of unsigned chars for the QImage
	def convertfrom(self,image,bits):
		if self._debug:
			print "[PyImageViewer]convertfrom():"+str(int(bits*8))
		if depth == 1: # 8 bit grayscale
			barfoo = image
		elif depth == 1.5: # 12 bits unpacket
			bar,barfoo = image,[]
			for i in range(len(bar)):
				barfoo.append(bar[i].__lshift__(4))#shift to loss 4 less significant bits
			#barfoo = bar
		elif depth == 2: # 16 bits
			bar,barfoo = image,[]
			for i in range(len(bar)):
				barfoo.append(bar[i].__lshift__(8))#shift to loss 4 less significant bits
			#barfoo = bar
			raise "Not supported depth"
		return array.array('B',barfoo).tostring()

I am not sure but this tostring() call at the end, maybe means another third 
touch of all the elements to make it worst.

Sano kaj liber!


[1] http://www.riverbankcomputing.com/pipermail/pyqt/2008-April/019142.html 
[2] http://www.riverbankcomputing.com/pipermail/pyqt/2008-April/019130.html

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