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					Originally Posted by  ownedbycats
					 
				 
				I was mostly speculating, perhaps transparency could be used in place of a custom bg colour in the letterboxing. 
			
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 The cover generator in calibre uses a function to resize the image which will do the letterboxing and takes a parameter for the colour to be used. It then calls a another function to handle the transparency. That takes a parameter to for the background colour. Unfortunately, the overall function, only takes one parameter for these colours and it is only used for the transparency. I have to add the other and pass it through.
Personally, I just let the device do it. That is completely predictable as to what is happening. The black letterboxing it uses works with the black surrounds of the device.