1973

Double Buffer

A display does not show you a picture. It sweeps, one scanline at a time, reading whatever is in the buffer at the moment it arrives. If something rewrites that buffer while the sweep is halfway down, the frame the display emits is the top of one picture and the bottom of another, with a seam between them. Richard Shoup's SuperPaint at Xerox PARC displayed its first picture in early April 1973, and a second buffer is not a thing you can have until you have one.

The frame you are looking at is not the one being drawn

1 One buffer, and a beam that is already halfway down it

A display does not show a picture. It sweeps, one scanline at a time, reading whatever is in the buffer when it arrives. Drag the beam to see which line it is on.

The beam is at scanline 12

2 A write that lands mid-sweep, and the frame the display emits

Now write a new picture into the buffer while the beam is partway down. The lines it had already passed still carry the old picture; the rest carry the new one. The frame the display emits is both, with a seam where the beam was.

The write lands at 50% of the way through the refresh

3 A second buffer, and the seam that stops appearing

Give the writer its own buffer and swap them only between frames. Every scanline of an emitted frame now comes from one picture, at the same write time that tore the frame above.

4 What it cost: a whole frame of latency, measured

The seam is gone and something was paid for it. The picture on screen finished being drawn before the last swap, so it is older than the one the writer has already finished. At sixty hertz a refresh is about sixteen and a half milliseconds, and the wait is the part of one that was left when the write landed.

One buffer against two, at this write time
buffers	pictures in a frame	seam	wait before it is seen

These ran in this browser when the page loaded. Each claim, whether it held, and the number behind it.

Each claim, whether it held, and the values behind it
claim	held	measured
A 640 by 480 frame 8 bits deep is 307,200 bytes, which is the figure in the paper	yes	640 x 480 x 1 = 307,200
The seam's scanline is the same whether it is divided for or counted to	yes	agreed at all 1,000 write times tested
A write halfway down a single-buffered frame emits two pictures at once	yes	the emitted frame carries frame 1 above frame 2, seam at scanline 12 of 24
With a second buffer every scanline of a frame comes from one picture	yes	one picture across all 24 scanlines
A write that lands before the sweep starts is not a tear	yes	seam at scanline 0
and a frame nobody wrote during is not a tear either	yes	no write, no seam
The second buffer costs a wait for the swap, and the earlier the write the longer it waits	yes	a write a quarter of the way down waits 75% of a refresh; three quarters down waits 25%; with one buffer it waits none of it and tears instead

What is real here, and what is not

This page cannot tear, so it models tearing

A browser composites, which is precisely why no page on the internet shows you a torn frame. Nothing here is your display failing. What is drawn is a model of the scanout: a beam sweeping down the raster at a stated rate, a writer changing the buffer at a stated moment, and the frame those two produce together. The seam's position is computed from those two numbers rather than drawn where it looks good, and the page prints the scanline it lands on.

The frame buffer's numbers are the paper's, and the arithmetic is checked

Shoup writes that a video image minimally contained 640 pixels horizontally, 480 vertically and 8 bits in depth, for a total of 307,200 bytes. The page checks that multiplication rather than repeating the total, because a number quoted from a source and a number derived from its own inputs are different claims and only one of them can be wrong quietly.

The date is the author's own sentence

SuperPaint displayed its first picture in early April 1973, which Shoup says in the first person in the IEEE Annals account. The chronology takes that rather than a secondary summary of it.

What is not modelled

No vertical blanking interval of any real duration, no vsync in hardware, no triple buffering, no GPU, no compositor. The refresh rate and the write time are yours to set and are not measured from your display. A real tear also depends on where the two frames differ, and this shows the worst case, where they differ everywhere.

The latency is the honest half of the story

Double buffering does not make anything faster. It costs a frame: the picture you are shown finished being drawn before the last flip, so at sixty hertz it is up to sixteen and a bit milliseconds old, and the page measures that rather than mentioning it. A page that showed only the seam disappearing would be an advertisement.

No sound

A tear is a spatial artifact. There is nothing to hear.

Sources

Logical Art builds interactive explanations of difficult machines.

The machines:

1935-1991 RangekeeperThe answer is on both sides of the equation, so it had to be a machine.
1964-1990s AbendThe loop you did not write, and the day it cost to get it wrong.
1971-1995 Line ModeYou could telnet to port 80 and be the browser, and then you could not.
1975 Front PanelA working 1975 Altair 8800 you program by flipping switches.
1980s 300 BaudA real dial-up modem and the BBS on the other end of it.
1994 WinsockThe two seconds after the modem stopped screeching, before anything worked.
2020s LLM LabTrain a tiny language model and watch every number as it learns.

Smaller pieces, on the studio itself: Morse, Baudot, Flip-Flop, Punch Cards, Interlace, Bitwise, The Adder, Stored Program, Two's Complement, Trigger, Bit, Nibble, Byte, Transistor, The Middle Square, Score Reel, Hamming, Huffman, Core Memory, Hash Table, The Interrupt, CORDIC, Garbage, CRC, The Pipeline, Round Robin, Page Fault, Context Switch, ASCII, Packet Switching, Bresenham, CPU Cache, Race, Fifteen Planes, B-tree, Double Buffer, Backoff, Checkout, Diffie-Hellman, Zero Address, Fringes, LZ77, Signature, Z-Characters, Merkle Proof, Branch Prediction, Write-Ahead Log, Interleave, Eight Sprites, DNS, Floating Point, Slow Start, RAID Parity, Longest Chain, JPEG, UTF-8, Masking, PageRank, AES, Merge Collapse, Merkle DAG, Count Distinct.

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