Subtraction Operator

Original Images ( Operands )

Image 1	Image 2

Details

Performs subtraction operation on two pixel intensities ( from two different images ), not difference operation i.e. absolute subtraction, so putting operand images in different order may result into different images

Image_1 - Image_2 != Image_2 - Image_1

It’s pretty evident that after application of subtraction operator on two equal sized images, resulting pixel intensity values may be out of range ( > 255 ), which can’t be stored in a 24-bit RGB image. For putting pixel intensities in [0, 255] range, we can consider either of following two ways

• All pixel intensities < 0, becomes 0 and > 255, becomes 255. [ Clipping ] ( Produces different result depending upon operand placement )
• We’ll simply apply modulus ( % ) 256, to put all pixel intensities back to range. [ Scaling ].

It doesn’t matter how you place operands, when you want to scale pixel intensities, because before scaling pixel intensity absolute value to be taken only.

• abs( 257 ) % 256 = 1
• abs( -257 ) % 256 = 1, both boils down to same value.

Resulting Image = Image_1 - Image_2

Clipped	Scaled

Resulting Image = Image_2 - Image_1

Clipped	Scaled