Before computers, to blend two pictures into one, I'd imagine they'd have used some sort of mirror setup to reflect the one picture onto a mirror containing the reflection of the other picture. Does that sound right?

What about different blending modes that Photoshop has, like "Difference," "Pin Light," etc. What are the pre-computer origins of these?

![](https://image.slidesharecdn.com/blendmodesforopengl-140213092122-phpapp02/95/blend-modes-for-opengl-3-638.jpg)

[Example of blending modes](https://image.slidesharecdn.com/blendmodesforopengl-140213092122-phpapp02/95/blend-modes-for-opengl-3-638.jpg?cb=1392283377)