CRT simulator

What you are looking at

The picture below is drawn by a script at the tiny size of 320 by 200 dots and then stretched to the size of the frame. It contains eight bars of colour, a strip of grey steps, the word ERACADE, a thin square that bounces around the lower half, and a line of text that scrolls along the bottom. Everything else is the tube: the dark lines between rows, the soft glow around bright shapes, the bowed edges and the fine pattern of stripes laid over the top.

Why 1980s pixels were soft

A cathode-ray tube makes a picture by firing a beam of electrons at the back of the glass. The beam sweeps across a row, drops down a little, and sweeps across the next one. Where it lands, a coating of phosphor glows, and it keeps glowing for a moment after the beam has moved on. The beam is a smudge of light with soft edges, not a razor. That alone rounds off every square corner.

A colour tube has three coatings, red, green and blue, laid down as thousands of tiny dots or stripes. Just behind the glass sits a thin metal sheet with a pattern of holes or slits. It is the mask. Three beams come from slightly different angles, and the holes let each beam land only on its own colour. A mask of holes is called a shadow mask. A mask made of fine vertical wires is an aperture grille, which lets more light through and gives a brighter picture with visible stripes. A mask of staggered slots sits between the two. The mask is why a close look at an old screen shows a fine pattern instead of a smooth surface.

Broadcast television added another wrinkle called interlacing. A full picture was drawn in two passes, first the odd rows and then the even ones, so that the screen refreshed quickly enough to avoid flicker while carrying fine detail. A one-pixel-high line that lived on only one row blinked at half the rate. Game hardware often skipped the second pass and drew the same set of rows every time, which leaves a thin dark gap between the lines. Those gaps are the scanlines. Artists drew sprites knowing the gaps were there, and the gaps made the dots look rounder.

The cable carrying the picture could not change brightness instantly, either. A sharp edge from white to black smeared slightly to the right, so one pixel leaked into the next. Programmers used that on purpose: two colours placed in a checkerboard blended into a third. That blur is the colour bleed slider. Finally, the glass was curved, bright areas bloomed into their surroundings, and the corners fell into shadow. Every one of those flaws is a control below.