A few scanning tips

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Grayscale vs Line art?

If pictorial images are to be copied to the printer, then Line art (like ClipArt, which has no gray tones) is unsatisfactory photographically. Grayscale mode must be used to preserve any intermediate tones in the images.

Obviously Gray mode has 256 shades of Gray and Line art has only Black and White, but let's look at the difference very closely and see what we can learn.

Here are scans of text from a magazine ad. The page alignment in the scanner was deliberately skewed at an angle from the proper horizontal alignment to intentionally aggravate the jaggies.

This image was scanned at 300 dpi Line art. The jaggies are very pronounced on edges that are not exactly vertical or horizontal. Scanning at higher resolution, even interpolated, would really help to smooth the jaggies, but it makes a larger image.

This is the same image and orientation, but scanned at 300 dpi Grayscale. As you can see, as compared to the 300 dpi Line art scan above, the jaggies appear to be gone in grayscale.

But why isn't the grayscale image sharper?

If we zoom in on this image 6 times, the jaggies are still there, because they are a property of the resolution. But the Gray mode scan fills in the jaggies with several shades of gray pixels to smooth the edges of the curves (Jaggies are called Aliasing, and the gray fill is called anti-aliasing). While this makes the gray edge curves look smooth, it also makes the edges soft and unfocused. This smoothness is wonderful in a scan of a real photographic grayscale image, to smooth any sharp abrupt edges in such images, but it's not great for text.

So it's not the scanner that's unsharp in Grayscale mode, it's a normally desirable property of Grayscale Mode. Grayscale mode with 256 tones is intended for pictorial images, not text, and we're used to text being sharper. But for text, you could have just used Line art mode in the first place.

A rather important note:   But for this reason, you will probably prefer grayscale (smoothed with anti-aliasing) for line drawings intended for video screens where the resolution must be low for size, perhaps down around 50 to 75 dpi.   Cartoons for example. Try it both ways, but the jaggies will be pretty bad if Line art is used at this low resolution. Line art loves a 600 dpi printer. The video screen probably needs Grayscale in this situation, and probably a GIF file reduced to 8 colors (8 to retain the anti-aliasing).

As to a Grayscale scan of such drawings, or of text and images mixed, it always makes the background be a faint gray, slightly objectionable for text. Using the Histogram to move the White point left (see A Simple Way) can help greatly to change the faint gray background to near white. I have done that here, by lowering the White Point from 255 to 230 (well to the left of the histogram spikes of the large count of white and faint gray background pixels on the Histogram). This lightened the faint gray background, and made it close to white (it prints much better). The black text is also lighter in Grayscale mode, values about 40 instead of 0. In other words, we don't have the Line art Threshold to force things one way or the other, and the gray white and black were in real life NOT as pure white or black as the scanner's range can handle. So they come out as intermediate values instead of 0 or 255. Raising the Black Point could have corrected that too, but adjusting the White and Black points to improve the text might adversely affect any photographic image on the same page (presumably we used grayscale for some reason?).

So there are advantages and disadvantages, but Grayscale is NOT that good for reproducing text. It's workable if the page must also capture some photographic images, but Line art is normally much better for text. Grayscale consumes 8 times more memory than Line art, and if Grayscale is used, then we are back to our original tune, about how scanning at more than about 100 to 150 dpi is wasted effort, because the printers halftones cannot reproduce more than this.


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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