Multiple images
Having lots of images open at the same time isn't really encouraged with QuPath. Because it is intended mainly for whole slide data, this can put substantial demands on the computer's available memory. Therefore QuPath was initially designed to work only with one image at a time.
Nevertheless, it became clear after a while that having multiple images open is sometimes needed - and so long as they are not all filled with millions of detected objects, the memory requirements can be kept modest.
The way to view multiple images together is to turn the main viewer into a grid containing multiple viewers.
Do this by right-clicking on the viewer, and choosing Multi-view → Add row or Multi-view → Add column.
You can then click on any of the viewers, and this will then become the active viewer, i.e. the one that receives all the commands that are sent. So if you click on a new, empty viewer and open an image, the image will appear in the viewer you clicked.
The Multi-view → Synchronize viewers option allows you to turn on or off viewer synchronization, which essentially makes sure that any panning and zooming operations you apply to one viewer are also applied to the others.
This can be particularly useful with TMA slides. If you have dearrayed TMA slides in multiple viewers, then you can double-click to select a core on one viewer and then use the arrow keys to move around the TMA grid. If synchronization is on, then cores at the corresponding grid locations will be shown for all other open slides.
To close a viewer, right-click on it and choose Multi-view → Close viewer. If all the viewers for a row or column are closed, you can also choose Multi-view → Remove row or Multi-view → Remove column to remove it and reclaim the space for whatever viewers are left.