Fax preview: see your page as the receiving machine will print it

What does a fax machine actually print?

A Group 3 fax page is a grid of dots, and every dot is either black or white. ITU-T Recommendation T.4, the standard every fax machine and fax service implements, fixes the scan line at 1,728 picture elements across 215 mm, which is 8 pels per millimetre or about 204 dots per inch. Vertically, T.4 defines two everyday resolutions: standard, at 3.85 lines per millimetre (about 98 lines per inch), and fine, at 7.7 lines per millimetre (about 196 per inch). A superfine mode at 16 × 15.4 per millimetre is optional and rare. There is no grey level in any of them. A pel is on or off, and a page of A4 at fine resolution is 1,728 by roughly 2,287 of them.

That grid is what this tool draws. It takes the photo you drop in, scales it to the 1,728-pel line, works out how many scan lines the page has at the resolution you chose, converts each pel to a brightness using the Rec. 601 luminance weights, and then makes the one decision a fax machine makes for every dot: black or white. The result is shown at the proportions of the printed page, because a standard scan line is twice as tall as a fine one and the receiving machine prints it that way.

Why does a photo of a page look different after faxing?

Three things happen to a page on its way through a fax, and all three are visible in the preview. First, resolution drops. A phone camera captures a letter-size page at several thousand pixels across; the fax line carries 1,728, and at standard resolution only 98 lines per inch of height, so the vertical edges of letters become a staircase and the counters of small type start to close. Second, colour and grey disappear. Every dot is judged against a threshold, so a blue signature becomes a black one, a light pencil stroke is judged white and vanishes, and a yellow highlighter over black text produces a region whose average darkness lands near the threshold, which is why highlighted words so often arrive as a solid black bar. Third, the image is compressed with the run-length codes defined in T.4 and T.6, Modified Huffman, Modified READ and Modified Modified READ, which encode runs of identical dots. A page of clean text compresses well and transmits in under a minute; a dithered photograph is thousands of alternating runs and can take several times longer, which is the subject of the transmission time calculator.

The photo itself adds problems the fax did not cause. A page photographed at an angle is scaled unevenly; a shadow across one half of the page pushes that half under the threshold and turns it black; a JPEG saved at low quality carries blocking around every letter that the dithering faithfully reproduces as noise. The contrast slider in the tool exists for the shadow case, and the document camera in the Faxvio app exists for the angle case: it finds the page edges and flattens the shot before any of this happens.

Standard or fine: which resolution should I choose?

Fine, whenever the machine at the other end supports it, and most machines built since the 1990s do. The horizontal resolution is the same 8 pels per millimetre in both modes; the difference is entirely vertical, 3.85 lines per millimetre against 7.7, and the figure below shows what that does to one square millimetre of a letter’s edge. At standard, each scan line is printed about a quarter of a millimetre tall, so a horizontal stroke in 8-point type can be a single line and a diagonal edge steps in visible jumps. At fine, the dots are close to square and small type keeps its shape.

Rows are 297 mm × lines per mm, rounded to the nearest scan line. Superfine is optional in T.4 and is negotiated only when both machines support it.

The resolution is not yours to set alone. During the handshake defined in ITU-T T.30, the receiving machine declares what it can accept and the sender picks the highest mode both share, so a modern pair transmits at fine and a pair with an old machine or a poor line drops to standard. If you do not know what is at the other end, check the page at standard: a form that reads at standard reads everywhere.

Dither or threshold: which rendering is right?

A threshold is the simplest way to reach one bit: every pel brighter than mid-grey is white, every darker one is black. For text, rules and line art it is the right choice, because those things are already black on white and a threshold keeps their edges crisp. Its failure is anything grey: a light pencil note is judged white and disappears entirely, and a shaded area is judged black and becomes a block.

Dithering trades resolution for tone. The Floyd–Steinberg algorithm, published by Robert Floyd and Louis Steinberg in 1976, still decides each pel as black or white, but it carries the rounding error forward to the neighbouring pels, seven sixteenths to the right and the rest to the row below, so that a mid-grey area comes out as a checkerboard whose average darkness is right even though no single dot is grey. Photographs, stamps, grey signatures and the faint parts of a pencil note survive as dot patterns. The cost is that text edges pick up a little noise and, because a dithered area is thousands of short runs, the page compresses worse and takes longer to send. Most fax machines and services dither by default; this tool does too, and offers the threshold for pages that are pure text.

How to prepare a page that survives fax

• Black ink on white paper. Blue ink survives; pencil, light grey and coloured ink often do not. The sample page shows a pencil note vanishing at standard resolution.
• Type of 10 points or larger. At 98 lines per inch, an 8-point “e” has two lines for its counter. Eleven or twelve points reads at either resolution.
• Flat, shadow-free, straight-on. Put the page on a table, photograph it from directly above with the light behind you, or use a document scanner mode that flattens the page. A shadow across the page is the most common reason a fax arrives half black.
• High contrast, no photos. A photograph on the page becomes a dot pattern at best; if it must be sent, print it large. Grey backgrounds and watermarks become noise or blocks.
• No highlighter. It renders black and takes the text with it. Tick the margin instead.
• Leave the top of the page clear. The sending service prints the header line, date, time, sender number and page count, across the top few millimetres of every page, and a title that sits there is overprinted.
• Check it at standard. If the page reads at 98 lines per inch it reads on any machine.

What the machines add to your page

Two things you did not draw arrive with the page. The sending side stamps a header line across the top of every page, carrying the date and time, the sending number and the page count, in the style 06-SEP-2026 09:29 FROM: +1 719 771 0496 P.1; in the United States that mark is required of every sender by federal rule, and fax services print it whether or not the law applies. Some receiving machines add a second stamp of their own at the foot, with the time and the line they received on. And if the two machines negotiated error correction mode, a T.30 option in which the page is sent in blocks and any block the receiver could not decode is retransmitted, the page arrives exactly as sent; without it, a noisy line can leave a scan line or two of garbage that the receiving machine prints as a streak.

How this preview relates to the Faxvio app

The Faxvio app does the same conversion, on every page, before you send. When you scan a page or import a PDF, the app renders it as a one-bit image at fax resolution and shows you that image, labelled “as received”, on the way to the number screen, so what you approve is what the machine at the other end prints and a faint signature is caught while it still costs nothing. This browser tool runs the same idea on a single photo so you can check a page before you open the app, or check a page you intend to send some other way. Details of what the app does and does not do are on the home page.

Qualitative results from rendering typical pages with a mid-grey threshold and Floyd–Steinberg dithering; a shadowed or skewed photo does worse in every row.

Bottom line

A fax is a grid of black and white dots, 1,728 across the scan line at 8 per millimetre under ITU-T T.4, with 3.85 lines per millimetre at standard resolution or 7.7 at fine, and nothing in between black and white. Anything grey, coloured or small on the page is decided dot by dot at the threshold, which is why pencil disappears, highlighter turns black and 8-point type fills in at standard. Preview the page at the resolution the other machine is likely to use, prepare it in black ink at 10 points or more, photographed flat and without shadows, and leave the top clear for the sender’s header line. The Faxvio app shows every page this way before a page is spent.