Hi @Skien, as far as I am aware, the iPad Pro doesn’t support Java so getting ImageJ to run on it would be… difficult at best. As @imagejan describes, Android seems to be being more actively investigated - but it still sounds like it would be a big job.
I also tried a graphics tablet with a built-in screen for making such annotations, attached to a regular PC/Mac. So this should already be possible with ImageJ. In the end I went back to using a mouse… but maybe I just didn’t stick with it long enough.
In any case, I do my annotations using QuPath (http://qupath.github.io). This was written using JavaFX, and should work with touch screens (but not iPad/Android); you can also customize which touch gestures to turn on, since not all really help*. QuPath also incorporates JPen, which should enable pressure-sensitive drawing if connected to a compatible graphics tablet.
But the most useful feature I find for annotation is the ability to quickly switch between tools in QuPath using shortcut keys (polygon [p], brush [b], wand [w]…), and use additional keys to switch between modes (e.g. hold down the ‘Alt’ key to turn either the brush or wand into an eraser). The brush and wand also adapt according to magnification, so by zooming in or out it’s possible to draw either very detailed or very large and coarse regions using the same tools - and both edit or lock the annotations later as required. Personally, I find that lets me work effectively with a mouse and not really need anything else.
If QuPath supports the kind of files you’re working with, you might want to try it out. It should also be possible to extract the QuPath annotations and put them into an ImageJ-friendly
Roi format if needed at the end.
*-When using a Mac and a multitouch mouse or trackpad, I turn on scroll gestures so that I can use scrolling on the mouse to pan around a large image, while scrolling with the Shift key down is used to zoom in and out. But that’s not the default setting, since for a ‘regular’ mouse I think it’s usually more intuitive to link scrolling to zooming directly, and activate a different tool to pan around the image.