Hi, On Thu, Dec 3, 2020 at 1:34 PM Charles Lindsey <[email protected]> wrote:
> I have an Epson Perfection 1250/Photo which uses the plustek backend. It > works > fine for color and gray modes, but for lineart, every time you scan a page > it > introduces more black noise along the right hand edge of the page. It > turned out > to be the result of calibration. I still don't know why repeated > calibration > causes that problem, but that is how I cured it. > > There was a problem with the calibration routine which I fixed some time in the past, you may remember. It's not impossible that there may be other issues :D > Obviously, on startup (more specifically on warmup), any scanner has to be > calibrated; in fact if the warmup is set to 'auto' it repeats the > calibration > until the result remains unchanged three times. Examination of the code > for the > Plustek Backend shows that it does much more; indeed every time you start > a new > scan, it does a new calibration (except for automatic document feeders and > the > like). This wastes time (typically six times until the obligatory three > identical results), and it causes my black noise in lineart mode (for > reasons > yet unknown). Yeah, I know. It's annoying. Some of the older scanners seem to take *forever* to do calibration. Other scanners that I have used do regular calibrations, but they do not do it on every scan. I would assume that other parameters like temperature might affect calibration (as the machine warms up with use perhaps) so it would be reasonable to do at least a quick calibration to ensure that the scanner was still within spec every now and then. It is for you guys to decide whether this is worth making permanent. I > understand Ralph has a similar Epson scanner, so perhaps he could check > whether > that has this lineart problem, and give this patch a try. > > I will have a look over the weekend if I get a chance. > Just a few more oddities that I encountered on the way. > > Even after the patch, it sits for 13 seconds in > usb_IsDataAvailableInDRAM() > before the scan data starts to become available, even though after that it > comes > out steadily even if several buffers-full have to be downloaded. I think > this is > a problem with the scanner hardware which is too lazy about getting the > carriage > out of its garage. > :D Cheers, Ralph
