I am unable to shoot manually with my camera, when gphoto2 is also running and in control.
OK, so there's not much we can do with that camera over USB. An EyeFi card might be the only option here (they're about $50 -- cheaper than a new camera if you just have to have some kind of tethred shooting). (See below about using the dummy API with this.)
tkr wrote:Qs 1: I just found a used Canon A590-IS, listed on my local Craiglist for $60. Will I be able to use Scanmanager to get live-view functionality as well remote control ?
Hmm...it's not supported out of the box but CHDK is available for it. I don't know if live view is available in standard CHDK builds yet or if live view or remote shooting via CHDK work with libgphoto2 at all. So the answer is maybe, with some setup, but its untested. Having said that, it would be great if someone could test gphoto (and ScanManager) with CHDK and exactly this sort of camera (i.e. one with no remote support out of the box but supported via CHDK). It would vastly increase the number of cheap compacts you could run tethered if we knew this worked.
tkr wrote:Qs 2: Am I using the 'dummy API' in the manner it which it was intended ? If I'm right, I should be able to use this method to also import some shots that I had taken earlier, and run the calibration/rotation/cropping on them
Well, not really. ScanTailor does the post-processing job brilliantly -- much better than I could do it (and there are other packages discussed here too -- e.g. BookScanWizard -- that will also do a very good job for batch processing).
The dummy API is really designed for something like the EyeFi card (people have used it with your camera with some success, e.g. here http://forums.eye.fi/viewtopic.php?f=8&t=1100), not for importing existing pictures. With an EyeFi card you can use something like ScanManager with any camera. Just point it (using the Dummy API) at your EyeFi network share and shoot manually. As soon as a new shot gets written to the SD card in ScanManager will see it and move it to the PC, showing you a nice zoomable view of the shot on screen so you can check it as you turn pages (you can also reorder shots by dragging the thumbnails around and delete a bad shot by right-clicking on the thumbnail so one mistake doesn't mean you have to open up scantailor in interactive mode and start fiddling with the pages).
tkr wrote:Qs 3: Most cameras have a grid superimposed on the viewfinder, which helps in lining up the edges of the book. Would that be something that your live-preview would (optionally ?) be able to incorporate - don't know what is involved or how much work it is. The advantage would be that it would also save a rotation step in software - the step is currently needed when the book edge is a bit tilted.
A grid is easly enough to do. I'll probably include one when I add the cropping code.