A picture of the drawing

The diagram as a file, in svg or png. Where it goes is two different questions, and there is a method for each:

bb.addButton("Save as PNG", () -> panel.download(GraphExportFormat.Png, "diagram.png"));

bb.addButton("Mail it", () -> panel.export(GraphExportFormat.Png, 2.0, image -> {
    File png = image.saveTo(File.createTempFile("diagram", ".png"));
    mailer.send(png);
}));

Saving, and exporting

What the handler gets

What is in the picture

Saving, and exporting

Method	What happens
`download(GraphExportFormat, String fileName)`	the browser makes the picture and hands it to the user. Nothing reaches the server
`download(GraphExportFormat, double scale, String fileName)`	the same, drawn this many times its own size
`export(GraphExportFormat, IGraphExportHandler)`	the picture is posted back here and the handler is given it
`export(GraphExportFormat, double scale, IGraphExportHandler)`	the same, at that size

download() is what a "save this drawing" button wants: the drawing is in the browser and so is the file, so nothing has to travel. export() is for the application that has to keep the thing - put it in a report, mail it, store it next to the record it belongs to.

Both are commands, like arranging: they make a picture of the drawing as it is at that moment, not of the model as the page built it.

What the handler gets

public interface IGraphExportHandler {
    void exported(GraphExport image) throws Exception;
}

GraphExport carries the file and its size: getData() the bytes, getWidth() and getHeight() the picture in pixels, getFormat() and getMimeType() what it is, and saveTo(File) writes it out. The size is worth having, because it is something the server cannot work out for itself.

The picture arrives in a request of its own, a moment after the one that asked for it - making it takes the browser as long as it takes to draw. That request is an ordinary one, so the handler may change the page like any other: show what came back, enable a button, or hand the file to the user with TempFilePart.createDownloadAction().

An exported picture travels over the ordinary page POST, and containers limit how big that may be - two megabytes for Tomcat, by default. A drawing big enough to make a picture bigger than that is one to download() instead, which has no such limit because nothing is posted at all.

What is in the picture

The whole drawing, whatever part of it is scrolled into view and whatever the user has zoomed to. It is not a copy of what is on the screen: the picture is drawn again into a document of its own, so it has none of the handles, none of the selection, and nothing of the page around it.

Svg is vector, and the one to take when the picture is to be printed, scaled or edited afterwards. The scale sets its nominal size and nothing else.
Png is pixels, at the size the scale asks for - which is why a picture meant for print is worth asking for at 2.0 - and it is drawn on white, because a transparent png is printed on whatever it lands on.

A picture carries nothing it would have to fetch: a node whose shape is an image, or a font the drawing does not itself contain, is not in the png. The shapes, lines, colours and labels of an ordinary diagram all are.