The “add” operation is a mathematical way of combining images together. Think of each pixel in the image as a number value (black and white for the sake of explanation):
- A pixel on image 1 (bottom image) is grey, so a value of 0.5.
- The corresponding pixel on image 2 (top image) is a darker grey, a value of 0.25.
- When you ADD them together you get a lighter grey value of 0.75
So even when you use a JPEG with no alpha info, any pixel value below 1 (white) will appear as some level of transparent when using the Add operation.
