A sheet of glass that reads handwriting
Light goes in as a digit and the glass bends it onto one of ten sensors. All the page computes is the light: the etched plate and the printed slides were trained once, and then they just sit there, like glass would.
Draw a digit
Or pick one from the test set it never saw in training.
Try to break it
The light path. A green laser shines through the digit. A lens takes the Fourier transform of the light, and the etched glass delays each point of that transform by a different amount. A second lens transforms it back, which turns the plate into one fixed convolution over the whole image. The plate was started as a splitter, so that convolution has 16 channels on one sheet of glass.
A dye sheet then glows with the brightness of the light, which squares the field. That's the only nonlinearity. Ten small lenses each image the glowing sheet through their own printed slide, onto their own sensor, which makes a linear layer with weights between 0 and 1. The brightest sensor is the answer.