Transfer syntaxes
A transfer syntax is how an image is encoded: uncompressed, or compressed with JPEG 2000, JPEG-LS, RLE or JPEG. Routes can accept what senders offer and send each destination what it wants, converting on the way.
What sources accept
Section titled “What sources accept”When a sender opens an association it proposes, for each kind of image, the syntaxes it can send. The default rule is in Configuration › Settings: accept any syntax the router knows, as the sender prefers. A source can set its own:
- a list of syntaxes to accept, chosen by the sender’s order of preference or by ours;
- when the sender proposes none of them, accept its first choice anyway, or refuse that kind of image;
- overrides for particular kinds of images (ultrasound, mammography…) or SOP class UIDs;
- presets: anything, uncompressed only, lossless only, prefer JPEG-LS.
A sender whose proposals are all refused cannot send that kind of image (most then offer uncompressed, if they can). The node log lists what was refused.
What destinations are sent
Section titled “What destinations are sent”A destination lists the syntaxes it receives, most preferred first, such as JPEG 2000 Lossless › As received › Uncompressed for an archive. The default list is in Settings; each destination can have its own.
| Choice | Lossless |
|---|---|
| As received (no conversion) | |
| Uncompressed (explicit or implicit VR) | yes |
| JPEG 2000 Lossless, JPEG-LS Lossless, RLE Lossless | yes |
| JPEG 2000 (lossy), JPEG Baseline (lossy, 8-bit) | no |
- Each syntax is proposed separately, so the destination’s own preference cannot change the order. An instance goes out in the first one the destination accepts, converted if needed.
- Overrides give chosen kinds of images their own list: ultrasound as JPEG Baseline, say.
- Lossy syntaxes must be allowed explicitly for each list, and lossy images are never compressed again with another lossy method. JPEG Baseline quality is set per destination (default 90).
- If a conversion fails (a 16-bit image cannot become 8-bit JPEG Baseline), the next syntax is tried. An instance the destination accepts in none of the listed syntaxes is dead-lettered with the reason, so lists usually end with As received.
- The history shows what each delivery was sent as, and what it was converted from.
Test syntaxes, on the destinations list, proposes the destination’s list for one kind of each image from the central server, and shows what it accepts and what each kind would be sent as.
Converting costs CPU
Section titled “Converting costs CPU”Compressing to JPEG 2000 is the most expensive thing a node does. A node that sends most instances as received needs about half the CPUs of one that compresses everything; see Plan and size.