photocraft/docs/printing.md

# Printing

File › Print renders the flattened document onto one PDF page (paper size and orientation,
position and scale, crop and registration marks, labels, colour handling) and hands that page
to a printer. The rendering is `print_cmds.rs` in `photocraft-engine`, shared by the dialog, the
batch commands and automation (`file.print`, `file.printOneCopy`); this page is about the hand-off.

## Routes to the printer

| Route | When | What the user sees |
|---|---|---|
| **`craft-print`** (`lp` on CUPS, the Windows spooler) | Whenever `lp` is on the path (macOS, Linux outside a sandbox) or on Windows | `Sent to the printer (request id is Office-42)` |
| **CUPS over IPP** (`photocraft-ipp`) | No `lp` on the path (a Flatpak) and a print server answers: `CUPS_SERVER`, then `/run/cups/cups.sock`, `/var/run/cups/cups.sock`, `/private/var/run/cupsd`, then `localhost:631`; always for a `printServer` named in the request | `Sent to the printer (job 42 on Office)` |
| **Print portal** (`org.freedesktop.portal.Print`, desktop app on Linux) | No print server reachable or no printer set up, or always with `PHOTOCRAFT_PRINT_PORTAL=1` | The system print dialog, seeded with the copies and paper; the outcome goes to the log |
| **Print to PDF** | `output` given (the Output field of the dialog) | The file; nothing is spooled unless `send` is true |

A route that exists but fails (an unknown printer, a job the queue refuses) is an error; only a
missing route (no `lp`, no print server, no printer) moves on to the next one.

The IPP client is in-process, pure Rust (RFC 8010/8011: `CUPS-Get-Printers`, `CUPS-Get-Classes`,
`CUPS-Get-Default`, `Print-Job` with the PDF as the document), so no `lp` binary is needed. That
is what makes printing work in the Flatpak: the manifest grants `--socket=cups`, which mounts the
host's CUPS socket, and the sandbox has no `lp`. On a system without CUPS, or in a sandbox
without the socket, the app falls back to the print portal, whose dialog runs outside the sandbox.

## Which printer

The dialog's Printer field, or `"printer"` in the command, names a queue (case-insensitively; a
CUPS class counts, as with `lp -d`).
Left empty, the server's default printer is used; without a default, the only printer; with
several and no default, the error names them. With no printer at all, the portal takes over when
it is available, otherwise the error says to add one in the system settings or print to PDF.

`file.printers` lists what the print server knows: `{"printServer": "host[:port]|socket"}`
is optional and defaults to the system's CUPS; the result is
`{server, default, printers: [{name, info, state, acceptingJobs, default, class}]}`, or an empty list
with a `note` when no server answers. `"printServer"` on `file.print` sends one job to a named
server; `"dryRun": true` renders the page and reports the equivalent `lp` command without
spooling.

## Results

`file.print` returns `sent`, `backend` (`lp`, `windows`, `ipp`, `portal`), `spooler` (where the
job went) and `pdf` (the page's path, only when `output` named one).

## Environment

| Variable | Effect |
|---|---|
| `CUPS_SERVER` | The print server, as CUPS itself reads it: `host[:port]` or a socket path |
| `PHOTOCRAFT_PRINT_PORTAL=1` | Always use the system print dialog (the portal) instead of `lp` or CUPS directly |
| `PHOTOCRAFT_PRINT_PORTAL=0` | Never use the portal; a missing print server is then an error |

## Verification

- `crates/ipp/src/tests.rs`: the client against a fake CUPS over TCP and a Unix socket:
  printer listing with state and default, the default printer and its absence, `Print-Job`
  with the document and the job id, chunked and content-length replies, HTTP and IPP error
  statuses, broken replies.
- `crates/engine/src/print_cmds/tests.rs`: printer resolution (requested, default, only one,
  several without a default, unknown, none); a complete `file.print` against a fake CUPS
  (printer URI, copies, job name, document format, the PDF itself) and `file.printers`;
  `lp` taking the job when it is on the path; the IPP route when it is not (a child process
  with an empty path); the app's spooler when preferred, as the fallback without a printer, and
  skipped when a server is named; print to PDF untouched.
- `apps/photocraft/src/print_portal.rs`: the request path rule and the paper/copies seeding of
  the dialog (PWG names for the standard papers, millimetres for custom sizes).