xy_to_index_test.py

class Frame():

	def __init__( self, width, factor ):
		self.width = width
		self.factor = factor

	def xy_to_index( self, x, y ):
		row_offset = ( y << self.factor )
		# two pixels per byte; index origin = 0
		index = ( row_offset - self.width ) + ( x >> 1 ) + ( x & 1 ) - 1
		if ( index < 0 ): index = 0 
		return index

frame = Frame( 4, 2 )
print ( frame.xy_to_index( 8,1 ) )