Hi Jan, Am 01.05.2011 01:01, schrieb Vleeshouwers, J.M.: > Michael& Noah, > > Well, I got it to work: > $ scanimage -L > device `pie:libusb:001:005' is a PIE SF Scanner film scanner > <snip> > Thanks for the help, I'll start experimenting with it! > > Yours, > > Jan I am glad you were successful!
I have good and not so good news about our scanners. To find out something about the texp values in the dc-write command I started experimenting with an old empty, but a bit dirty slide. The first thing I saw were smooth, greyish vertical bands on a white background. I think that they are the result of not having done proper shading correction. From the genesys backend I understood that shading correction is needed to tune each ccd element between a white and a dark value in addition to setting the general gain and offset. Then I had a look at a scan of the same slide with cyberview, perfect white background. However, the corresponding snoop showed the same greyish bands. So, I guess under Windows shading correction is done in software as we assume already for gamma. In principle the 13 lines read for calibration are white but they also have vertical greyish bands. In the calibration lines the mean R, G or B value is about 55000 with roughly 2% standard deviation. Then I started changing the texp and gain values. The mean color values of the calibration lines was independent from that also when reading a second time after having changed texp or gain. The mean values varied a bit but perfectly stayed in the range of their standard deviations. I think that we have to know exactly how to calibrate as we can not see the effect before scanning the image. For shading this must be something like a simple * TARGET / SHADE_VALUE operation as we only get one value per ccd pixel. Next, I tried to find a relation between the texp values and the color values of the scanned image (300dpi, 16 bit, quality mode) still using the empty slide. My conclusions: 1) The three colors have to be treated separately. 2) The relation between texp setting and detected illumination can be described with a linear equation. 3) There is a minimal exposure time. With texp values below 0x0c00 you easily run into an undersaturation, especially in the blue channel. I do not know what this leads to. A few thoughts: From a theoretical point of view in the equation "intensity_value = a * _texp + b" both a and b should change with the extinction of an pixel. A purely white image should give us the values which we need in our calibration function. Scanning an image twice with different texp should allow for calculation of the extinction of its pixel representation. How do different resolution settings affect the coefficents when ccd pixels are binned (or not)? I guess time will show. You find my stuff, images of shades and the spread sheet, at http://wwwuser.gwdg.de/~mrickma/sane-proscan-7200/status-010511 . Regards Michael
