Colours that mean the same thing on every device

In plain words

Written for anybody, with nothing taken for granted. It says what the rule is for and what it changes on the document you end up with.

A colour inside a PDF is a set of numbers. On their own they mean nothing precise: the same numbers are one red on a screen, another on glossy paper and another again on newsprint. What turns them into a colour is a small file describing a device — a reference screen, a press, a paper. That file is called a colour profile, and a document kept for the long term carries the one its colours were chosen against.

For a document meant to be read on a screen there is nothing to decide: the engine writes the profile of the ordinary screen by itself, and the document is complete. For a document going to a press, hand it the profile of that press, and the same numbers then mean the ink that press lays down.

For a developer

The same thing without the detour: the objects, the constraints, and where the line falls between what is produced and what is declared.

The statement is the output intent: an embedded profile, the short name its condition is known by, and a sentence a person reads. It is written whenever the document claims the standard, so an archival file never leaves without one.

No profile file is shipped and none is fetched. Where the caller states none, the engine builds the ordinary screen profile itself from the published colorimetry — the white point, the three primaries adapted to the reference illuminant, and a tone curve — which is what keeps somebody else's licence out of every customer document. A profile the caller does supply is embedded once, however many times it is also painted through.

A profile is accepted as an output intent when it describes a printer or a screen; one describing a scanner or a camera is declined at the call rather than at the write. Twenty-six reference conditions have been put through the whole chain and handed to a validator, and the page on colour profiles lists which came back accepted.

Clause by clause

The published text behind each requirement, what the engine measures the document against, and what it leaves to a validator.

The engine states no clause here, because it never leaves the statement out: there is nothing to report. What it does not do is compare the paint to the statement. A page painted in four inks under an ordinary screen profile is written without complaint, and a validator declines it. That match is yours to keep, and it is the single most common reason an otherwise sound archival file comes back.

What the engine does on its own

Nothing on this list has to be asked for. It comes out of a document that claims the standard, whatever else the calling program says.

Writes an output intent into every document that claims the standard, whether or not one was stated.
Builds the ordinary screen profile itself, from the published figures, so nothing has to be downloaded or licensed.
Embeds one copy of a profile however many times the document also paints through it.
Declines, at the call, a profile that describes neither a printer nor a screen.

What you supply

Nothing on this list can be guessed. The engine states what it was given and declines to invent the rest, which is why the document is refused rather than written with a value nobody chose.

The profile of the device the colours were actually chosen for, whenever that is not the ordinary screen.
The short name the condition is known by, and the sentence a person reads to tell one condition from another.
Agreement between the paint and the statement: four inks on the page go with a press profile.

The other families of requirements

Which part of the archiving standard to claim Every letter the document draws travels inside it A page that comes out the same for everyone The document says what its marks stand for A document that carries other files inside it What the file says about itself Checking the claim, before and after

Back to what the archiving standard asks for

See the prices See the examples

Glossary

standard: A rule argued out in committee, published under a number anybody may buy and read, and identical for every firm claiming it. A claim to follow one can therefore be checked against the text. A way of working that merely spread because it worked is a habit of the trade: useful, widespread, and answerable to no text at all.
veraPDF: A free program that checks whether a file really is what it claims to be, against the published rules. It is the tool archives use, so it is the one used here: a document is not called compliant because we say so, but because veraPDF passed it.
colour profile: A file that says what a colour actually looks like. Without one, the same red is one red on your screen and another on a press. A document meant for print carries the profile its colours were chosen against.
output intent: The colour profile written into a document to say which press or which screen its colours were chosen for. A printer reads it to know what the numbers in the file are meant to look like, and the rules on documents kept for the long term ask for one.
CMYK: The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.

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