I have a scan of an old anatomic atlas; it's grayscale and not really bright. I suppose it's a copy of an engraving. My question is: is it possible to remove this background relatively easy? Or magnet lasso will be the only option?
Seems pretty straightforward with Photoshop CS6. I used the dropper tool to select the general background and did cleanup with the quick selection tool.
If the selection of the background ends up breaking due to the "grid pattern", you can work around it, like I did, by duplicating the later and applying a blur filter to smooth it out, then applying the magic wand selection. Switch back to the other later and fine tune with the quick selection tool.
All told, it took about 5-10 minutes to do the selection and rough cleanup. Can probably do a better job if you spend more time and then apply some filters to improve the contrast of the image.
Some tweaks to bring out the details of the original.
Here's my take on this:
I made this in GIMP, but the general steps should be similar in Photoshop:
First, I used the Descreen plugin to remove the screen pattern. There's a similar plugin available for Photoshop. I left the settings at their default values (sensitivity = 72, selection growth = 16, middle preservation = 5), but unchecked the "despeckle" checkbox.
Next, I used the Levels tool to make the background pure white and to darken the dark parts a little. I also used the middle slider to slightly darken the midtones for improved contrast.
Then I cleaned up the remaining not-quite-white spots in the background by first selecting the background using the Magic Wand tool with threshold 0, then enlarging the selection by one pixel and shrinking it by the same amount, and filling the resulting selection by one pixel.
Finally, just for a finishing touch, I decided to make the background transparent. (View the picture on a non-white background to see the effect!) To do that, I selected the background (including the gaps between the toes) again, enlarged the selection by two pixels and feathered it by one to put the selection boundary just inside the edge of the drawing, and then used Color to Alpha to replace white with transparency in the selected area. I did have to first tweak the selection manually at the tip of one toe, though, since there was a gap in the drawing's edge there.
Increase the pixel count aggressively (e.g. ~1530px wide to 10,000px), apply a strong blur filter, and then reduce the image back to the original pixel count. This will get rid of (most of) the halftone dither.
Use the magic wand tool with a setting around 20-30% and "contiguous" enabled. Select the outer grey area, invert the selection and then apply this selection as a layer mask. Tweak the layer mask as needed.
Place a layer below the image and flood it with white.
Note that you can do the masking at the higher resolution and the imperfections will be lessened after reducing back to original size.
Whenever there is a rasterized scan our automatic selection tools fail. I therefore remove the raster with a despeckle tool first. This will also considerably reduce the image size after compression.
We can then quickly adapt the image color levels as done here:
These are the three steps I had to take using Gimp 2.8:
This method will of course also remove the raster from the object but is is the fastest way I know of. Without fine tuning it could be done in less than a minute.