Adjusting a grayscale scan so it’s black and white

MH
Posted By
Marlene_Hochberg
Jul 2, 2008
Views
885
Replies
2
Status
Closed
A client sent me a grayscale scan of a line drawing with small text. The lines and text, which should be black, are gray. And the background, which should be white, is a light gray.

The lines should be 100% black, but I’ll take what I can get. I fiddled with curves and contrast and stuff, but when I’d get the background adjusted so it was completely white, then the lines weren’t dark enough.

And of course converting to a bi-level bitmap made the lines and text all pixelly and horrible.

FWIW, this will be offset printed, so it needs to be high resolution.

Suggestions?

TIA,

Marlene

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BH
BILL_HUNT
Jul 3, 2008
Look at applying Image>Adjustment>Threshold. Play with the slider and monitor the results.

Hunt
JM
J_Maloney
Jul 3, 2008
Up the resolution 2 x using bicubic smoother (if your version of PS has it, otherwise just bicubic).

Then, I usually blur the drawing slightly (maybe .5 – 1 pixel) so that I can add gain to the lines if they are too fine. You might want to avoid the blur step as you have text; test it and see. Also, blurring helps smooth out pixel jaggies if the original image is too low a resolution.

Then use image.. adjust.. levels to make a "bitmap" by pulling together the black and white sliders of the input levels. You don’t want the sliders to touch (as Bill’s threshold will do), you just want them close together so you’re controlling where the bitmap will "break". Make sure to zoom in and inspect your image. You might use a levels adjustment layer for easier tweaking.

You’re shooting for a bitmap image of between 800 and 1200 ppi/dpi at print size, so do the math and up-res again (using bicubic smoother) to the correct resolution. Then make the image a bi-level bitmap (image.. mode.. bitmap).

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