Preview mode: Why it is not used while scanning - jareff-g/ALT-Scann8 GitHub Wiki
PiCamera2 Preview mode
The first attempt at porting T-Scann 8 to PiCamera2 was using preview mode during the scanning, same as the original version did with PiCamera legacy. However, it became instantly apparent that it would not work: PiCamera 2 uses specific, dedicated configurations for each preview and capture, and the context switch between the two takes too long as to be usable in a project like this; speed is a relevant factor when you are scanning thousands of images in a row.
Because of this, I decided to take a different approach, and display the captured image instead of the live preview. The main advantage is that it works fine, without any added delay, and also that what you see on screen is the image as captured. The disadvantage, if it matters, is that you don't 'see' the film moving forward.
Despite this, the real preview of PiCamera 2 is still retained and used for one important reason: Focus. When focusing the camera you need to have real time feedback to determine when the focus is good. So for this reason, 'real' preview mode can be enabled via a dedicated button in the UI. Sicne the fucusing process is quite sensitive, and it is not easy to move back and forth the camera support with the precision required to make a fine adjustment, you can implement the hardware modification I created to facilitate this process, and that is published in Thingiverse.