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May I ask how you did that, what program did you use? Or in newer and up to date terminology, what app did you use?
I don't use apps, too small.
For this task I used Corel Paint Shop Pro X. It’s an older version that I’ve become accustom, but you can use Photoshop and many others as well, virtually any paint or photo manipulation program that has a “color replacement tool.” I basically use the “suction” tool and right click to capture the background gray area. I then move the suction tool over to the color pallet and choose white with a left click. You’re now set to use the “color replacement tool” on the image. To cover the image faster, use a large brush size. Both Density and Thickness setting are at 100. The Tolerance setting is critical. Too much Tolerance and it will start to erase or eat into the schematic. Too little and it won’t eliminate the background entirely. You will need to experiment with different Tolerance setting. In this particular case I used a setting of 18. Now that you background is all white, the last step is to add a little more black content to the schematic. Paint Shop Pro has a selection called “Fade Correction.” In this particular case a setting of 20 was good.
Instead of going though all those steps, I quick and dirty way is by simply experimenting with the contrast selection, but it’s almost impossible to achieve a clutter free background without some adverse effect on the schematic. The trouble is, some of the colors you see in the background, are also contained at the pixel level within the letters and lines on the schematic.
Another old version program I use a lot is ACDSee v 3.1. The newer versions have way too many unwanted functions with multiple windows. ACDSee is basically a file manager / graphic converter with photo manipulation features. It will let you control the white level and black level separately, as well as contrast, brightness, and gama correction. It’s a perfect program for this task as well.
I also use a number of other paint and drawing programs, so if you’re more familiar with something different, I might be able to help you with those, except forget Microsoft Paint..It’s worthless as tits on a bull…