Colours that mean the same thing on every device
Numbers on a page become a precise colour only once the device they were chosen for is named. An archival document carries that device's profile inside it.
Summary
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.
Technically
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.
The engine contains no profile file and fetches none. Where the customer 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 customer 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 refused at the call rather than at the write. The team has put twenty-six reference conditions through the whole chain: three were refused at the call, before a byte was written, and a checker accepted all twenty-three that were written. The page on colour profiles lists them.
Clause by clause
The engine always writes that statement, so a missing one has no clause to report. What it does check is that the paint agrees with the statement. Every colour a page gives as bare numbers is read: in its contents, in the pictures, gradients, patterns and letters it places, in the group it is blended in, and in the appearance of its annotations. Grey agrees with any statement. The three screen colours need a screen profile, and the four inks need a press profile.
Two other things give bare numbers their meaning. A default colour space of the same kind, named in the resources the colour is painted with, is read in place of the statement. Under part 4 alone, a page may state an output intent of its own, read in place of the document's, and a page or a form blended in a calibrated or profiled space of the same kind needs nothing more.
A colour nothing gives a meaning is reported page by page, naming the page and which of the three it is: in the report asked for before writing, and in the check of a file read from elsewhere, where a part of a page that cannot be read is named as not checked. A document that claims the standard is refused rather than written, and the refusal names the first colour found, not the page it is on. A page painted in the four inks under the ordinary screen profile, with nothing else saying what the inks mean, is therefore refused.
What the engine does on its own
The engine does all of this as soon as a document claims the standard, whatever else the calling program asks for.
- 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.
- Reads every colour a page paints against the statement, and refuses the document when a page paints in screen colours or in the four inks with nothing to say what they mean.
What you supply
The engine states what it was given and invents nothing else: without those values it refuses the document rather than choosing one for you.
- 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.
The published texts behind these rules
| Published text | What it asks for |
|---|---|
ISO 19005-1 6.2.3.3 |
A colour given as bare numbers has a meaning: a default colour space of the same kind, or an output intent describing that kind of device — any output intent, for grey. |
ISO 19005-2 6.2.4.3 |
The same, for part 2. |
ISO 19005-3 6.2.4.3 |
The same, for part 3. |
ISO 19005-4 6.2.4.3 |
The same, for the part built on PDF 2.0, where an output intent the page states for itself, and a calibrated or profiled space the page or a form is blended in, count as well. |
The other families of requirements
Which part of the archiving standard to claim Every font the document uses is embedded in the file 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