Ring Light and HDR Sketch Scanning

I don’t know how many sketchers there are out there, but I thought this might interest some photographers and imaging geeks. A while ago, I posted this question on Ask MetaFilter:

Why do flatbed scanners seem to have so little dynamic range for sketches? I draw for fun, usually with a 0.5mm mechanical pencil. I do not put a lot of pressure onto the pen because it allows for more details/subtle shading. When I scan the images, the “light” details are always blown out and no amount of fiddling xsane’s settings (gamma, brightness, contrast) seems to reveal them. […]

The best I could do with a scanner was to ensure I scanned in 24-bit color before converting to grayscale (instead of scanning in grayscale directly). The resulting image was slightly better, but not that much: a lot of detail was still missing. I guess generic flatbed scanners are made to scan text and photographs, not fine lineart. A powerful cold-cathode light at 1 mm of the sensor is not exactly subtle.

Your average photo camera doesn’t have this problem, because it has a better dynamic range and you can control the lighting. However, sketches require even lighting. Achieving this is hard. You can’t set-up your gear quickly and effortlessly.

Enter the ring light.

I used the same method as this tutorial, albeit slightly modified:

  • I made holes on the sides of the “plastic lampholders” so that the wires could come through the sides, instead of having the wires come through the plywood. Safer, more robust, and easier to rearrange if I change my mind.
  • Since I didn’t want to make a custom tripod mount, I simply stuck the ring between two notches of the tripod mount and let the tripod rise to full height.

The results are stunning.

On the left: scanned with a flatbed scanner. On the right: shot with a camera and ring light and post-processed with GIMP (cropped, converted to grayscale and levels correction). It takes longer, as you still have to set-up your tripods, take the picture and post-process in GIMP (instead of simply hitting the Scan button), but you get a much richer output.

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5 Replies to “Ring Light and HDR Sketch Scanning”

  1. This depends on the dynamic range of the particular scanner I think, as well as the software you use to drive it. I have an Epson V700 for scanning film, and it scans high-contrast documents (like black and white prints) without any clipping at either end. My wife is a graphics designer and uses an older Canon to scan her drawings and sketches, again without any clipping.

    Also, you can typically not just use scanning software right out of the box if you want good results. It may be not your scanner, but your software, that is tripping you up. Most software seems to assume you want the maximum dynamic range and clip the top and bottom, values, giving you clipped highlights and blocked blacks.

    What you want is for your software to do _nothing_ – a straight, linear 24 bit scan, with just the scanner illumination level set beforehand (if your scanner supports that). You’ll get a flat, dark dull grey image. Then use other tools to transform your file into what you want (I tend to use Imagemagick scripts for my film scans).

  2. @pitiviviver: Good catch. It was an error in the configuration of moonmoon (the pitivi planet aggregator) which was pointing to an incorrect feed URL. Should be fixed now.

    @Janne: Well the problem is finding such a scanner that 1) is well supported on Linux 2) doesn’t cost an arm and leg. Three different consumer flatbed scanners (from HP, Canon, Brother) I have tried all have poor dynamic range and no way to configure it.

  3. Janne is right. Just get a better scanner. I had a crappy one that cost me $50 and had the same problems. Then I happened to be at my girlfriend’s one day and used her, much nicer, scanner and it was night and day. Most of my drawings in my portfolio (from the last couple years at least) on myopica.org are scanned with her scanner, from my laptop running Ubuntu using Gimp. I do the bare minimum of cropping and as little other processing as I can get away with.

    Eg, this was scanned with her scanner: http://myopica.org/myopica-2009/704/

    And this one, using the same light blue pencil was scanned with my old scanner: http://myopica.org/colored-pencil/588/

    Someday, I’ll get around to re-scanning my old work with better equipment.

    I don’t know offhand what model she has, but it’s an Epson. I’m pretty sure it cost her a pretty penny (she’s a pro photographer), but, having lit and shot paintings for years as well, I can say that it’s a million times easier to scan than light and shoot and process. I’ll check what model it is when I get home. It definitely worked out of the box with Ubuntu and Xsane/Gimp. No compatability issues.

  4. As I wrote way back on that original metafilter thread, Epson scanners will let you set a custom gamma correction (for the internal 16->8 bit conversion) which lets you avoid the threshold filter that’s eliminating much of the fine detail in your drawings.

    Scanning in 24-bit colour isn’t helping you much, because there’s probably still some kind of threshold being applied there too. It’s still only 8 bits / channel after all.

    Janne is right: you want to get access to the high bit depth scan data & postprocess it yourself (apply gamma correction, thresholding, despeckle etc etc) for the best possible image. The Canon-pixma & Epson SANE drivers will let you do this IIRC.

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