I need to print some pages scanned from a school workbook. The pages are off-white, and show up as dark grayish regions on the scanned page. Is there a way I can prevent those parts from printing? Can I preview what parts of the page are going to be printed before actually printing so that I can verify I'm not going to waste ink?
Using photoshop, you can adjust the "levels" and white-out the 10% grey, to full 100% white. We run into the same problems here, put a book on the flatbed, and the areas that do not get light the same brightness, will be shades of grey.
By using photoshops "levels" you can see an average of the pixels in the "histogram" the graph lines on the right side are the brighter pixels, the graph on the left side is the darker pixels. (sounds reversed, because I adjust the High side first)
In levels you get 2 sliders (3 actually).
I find that it is best to have zoomed up on the text , and to move the view of the picture to the breech area of the book where the problem shows up most. By zooming up on the text you can also see how that effects the "anti-alias" (smoothing using grey) parts of the text , in the lower resolution scans.
If there is still the noise in the non-text part of the breech, because it simply cannot be adjusted far enough, then the last bit is just "cut to background" or whited out. To do that, it is best to have a very straight original scan, or to rotate the picture prior to cutting, white brushing, or blocking (rectangle tool) out that last area.
The printers can also be adjusted. the problem with that is you have to print, and lose a lot of ink/toner trying to find the right settings. In the printers settings (often in advanced , color , or manuel) Increasing the brightness, will push the grey to white (the text can also be grey). Increasing the contrast, will expand the differance between the light and dark pixels. The effect of "contrast" on printers varies a lot, but brightness acts similar in most of them.
If the scans are all the same, the same levels settings can be used over and over again. Or a script "action" could be made to repeat the same actions.
Gimp will do the same thing, I have done all the same things in gimp just as quickly as doing it in photoshop. Xnview also has levels and can be fastest still, but does not have a lot of paint features (enough to paint a block white though). Also think about OCR , if only reading or printing it is the issue. Book text (vrses written) is a good candidate for OCR (optical charachter recognition) , and the ocr can work with a greyscale image easily.
Other methods could include creating a gradient greyscale overlay picture, and a horizontal flipped one, that would be applied as a "screen" or "overlay" using layers, to quickly try and get a similar brightness going on the whole page prior to adjusting. I think most situations that would be unessisary, but that is what the alogrythm would be like if they ever made a Magic fixer for this problem.