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Scott's Schematic Scanning Tips

Since I started posting these manuals on line, I have received a lot of e-mail from people wanting to know how I scanned the pages. I am not a scanning "expert", but I have done enough of these now to put a workable procedure into written form. The procedure I used is really quite simple.

REQUIREMENTS FOR SCANNING

First of all, you will need:

1. A reasonably fast computer, with a lot of memory, a decent video card, and a well-focused video display.
2. A good scanner, capable of rendering 256 grayscale shades at a minimum.
3. A good graphics manipulation program such as Paint Shop Pro or Photoshop.
4. A decent printer.
5. A good pair of eyes.

What's a reasonably fast computer? Probably Pentium II or above, with at least 128 MB of memory. Although you could get by with less speed and memory, It really slows things down if you do. My current system is a Pentium III 800 Mhz syatem with 128 MB of memory, and it seems to work pretty well for this purpose. A well-focused display is a must!

What's a good scanner? One that's reasonably fast, rugged, scales the size of documents well, produces nice bright images and clear readable text, and has versatile, well-behaved TWAIN software with it. These requirements seem to eliminate about 90% of the low-buck parallel port scanners out there. I would prefer SCSI or USB interfaces for the scanner because of the higher speeds possible with these interfaces. You do NOT need the latest, greatest scanner to do schematics- I have two rather geriatric scanners, an HP ScanJet IIcx that is almost 10 years old, and a ScanJet 4c that is about 5 years old. I have used both of these scanners to produce the online Beitman books, and they both work great and are built like tanks.

What's a good graphics manipulation program? It will be one that works with your scanner, allows you to view an image in extreme close-up, can adjust the brightness and contrast of an image, has an easy way to convert greyscale to 2-color black and white, and one that you are comfortable working with. If you have Photoshop, by all means use it; If not, Paint Shop Pro is a good choice-It does everything that needs to be done when scanning schematics at about 1/5 the cost of Photoshop.

A decent printer? Anything 300 dots per inch resolution or above will do the trick. The ability to render grayscale or color images accurately is a real plus.

THE SCANNING PROCESS

When a beginner first scans a schematic, the first thought is usually to scan it in black and white (2 color) mode. After all, It's a black and white drawing on paper, so why not scan it that way?

Well, It's not quite that simple. Sometimes, a black-and-white scan will come out just fine, but often, there will be a loss of detail-component values and numbers may not be readable, there may be broken lines, or there are black patches in the scan in areas that should be white.

The reasons for the unsatisfactory results lie in the characteristics of the paper and ink used to print the original schematics. Some batches or types of paper discolor more over time than others, some absorb ink better than others, and some inks behave differently than others. So, instead of merely having a purely black and white original schematic as it appears to the naked eye, you actually have a drawing that has a very subtle greyish fringe around everything that is printed on the page. I have found that these subtleties must be captured in the initial scan in order to obtain good results later on in the process.

The fastest, most memory efficient to do this is to scan the drawing in grey-scale mode. Scanning it this way allows all sorts of subtle tweaking to be done to improve the quality of the finished product that is impossible with a two color image, as we will see later.

Now, we will scan a test image. Fire up your graphics program of choice , put a schematic on the scanner glass and preview-scan it. Almost all scanner software has a "preview" mode, where a test scan is done quickly at a very low resolution so the user can crop the scan, adjust color, brightness, contrast, resolution, etc. (The preview window of HP's scanner software is shown in the picture below, your software may be different in appearance). You will want to set the software's options to scan in "grey scale" mode, with 256 shades of grey, with a resolution of 300 dots per inch (dpi). Do NOT confuse grey-scale with half-tone mode; they are entirely different. The preview mode usually performs an analysis of the image being scanned and will automatically set the brightness and contrast settings to what it thinks are the best values. You will need to bring the brightness down a few notches and the contrast up a few notches before scanning in the final drawing. When everything is set up properly, do the final scan. The resulting scan will most likely be somewhat grey in appearance, rather than white-this is normal.

Now, use the magnifying glass tool in your graphics program to display the scan in 1:1 mode. This will allow you to view only a small portion of the scan in extreme close-up, as shown in the picture below.

You will want to pay particular attention to how clearly small but important details, such as component values and tube pin numbers appear in this close-up mode. If these small details show up clearly, then you can go on to the next step. If they appear to bleed together a bit, you will need to reduce the contrast setting and/or increase the brightness and do another final scan. If the details appear to be missing or broken up, then you need to increase the contrast and reduce the brightness some more and scan again.

Now, you will want to reduce the number of colors to 2-black and white. This will greatly reduce the space required to save the drawing to disk and will make it much easier to download if it is to be put on the internet. Paint Shop Pro has a function for this, called "Decrease Color Depth", which is shown below.

 

Photoshop's function for this is called "Image mode", and needs to have "Bitmap" mode selected, which is shown below..

Note the settings shown in the above pictures. These are the settings you should use when reducing the drawing from gray scale to 2-color mode. Set your graphics program to these settings and click OK. In a few seconds you will have the final result-A good looking, clear, black and white schematic.

Save the drawing to your disk and you are done. I prefer saving the scans in TIFF format; They seem to be a little easier to work with in that format. You can also save them in GIF format as well. Do NOT save them as JPEG files. JPEG is a "lossy" format and will attempt to compress the files in a way that will result in a substantial loss of detail.

You will probably notice a minor loss of detail after the colors are reduced; This is normal and the schematic will still print out nicely on your printer. It won't be quite as good as the original, but it will be close.

SCANNING PRINTED PHOTOS

Sometimes, you will encounter schematics and other printed material that has black and white or color photos showing component locations, alignment points and other information. Attempting to scan these printed photos often results in a weird, objectionable symmetical pattern in the scanned image that looks as if you are looking at the picture through a screen door, as shown in the picture below, which was scanned directly from a magazine:

The original magazine image looks good and the scanner can scan ordinary photos just fine, so what is going on here?

In order to understand why this occurs, it's necessary to understand how these printed photos are constructed, and how they differ from ordinary photos.

In an ordinary photograph, the photo is printed by selectively exposing to light a sheet of paper that has been coated with a substance containing lots of light senstive particles, and then developing the photo in a darkroom to make the light-exposed particles visible. Due to the nature of the process used to manufacture the light-sensitive paper, the particles are pretty randomly spaced-there is no real discernable pattern to their placement on the paper.

A photo that is printed in a magazine or other publication is constructed quite differently. It consists of a set grid of thousands of rows of extremely small, precisely placed colored dots, each placed in such a way that almost any color or shade of grey can be represented. The dots are so small and so numerous that the naked eye perceives the series of dots as one continuous photo. The number of dots in each row is determined by the quality demands of the publisher, and the limitations of the printer used to render the picture.

A scanned image is somewhat similar to a magazine photo, in that it also consists of thousands of rows of dots. The exact of number of dots in each row is determined by the scanner's resolution settings, usually represented in dots-per-inch (dpi)

So, when you are scanning a magazine photo into a computer using a scanner, you are in essence building an image out of a pattern of dots with a fixed number of dots per row, that is based on an image that is ALREADY BUILT out of a pattern of dots with a fixed number of dots per row.

And therein lies the problem......Unless the resolution your scanner is set at precisely matches the number of dots-per-inch in the magazine photo you are trying to scan, there are going to be portions of the resulting scan where some dots in the two images will nearly intersect, and because the number of dots per row is fixed in each image, they will repeatedly intersect in the same place, line after line. It is this repeated intersecting throughout the image that causes the weird patterns.

So, what can be done about it?

Well, the TWAIN software that comes with some scanners has a "descreen" button or checkbox that attempts to eliminate this problem as you are scanning the document. This feature, on the scanners I've seen that has it, seems to work but the results are a bit generic and inconsistent. Sometimes, the option works OK, but many times the descreening is not enough to eliminate the weird patterns completely. In some cases there is an unacceptable loss of detail along with the de-screening.

My scanner's software does not have the de-screening option, so I had to find another way to do it. Basically, what needs to be done is to have the resolution of the initial scan match the resolution of the magazine photo as closely as possible, and then make the dots in the scanned image more random, like a photograph.

The way I have found to do this that works best is to scan the picture several times at various resolutions, each one being at least double the resolution you intend for the finished product. View a portion of each one at 1:1 size; you will most likely have one of them that shows less of the weird pattern than the rest. Keep that one and discard all the rest, as shown below:

Next, soften the image using your graphics program. In Paint Shop Pro, the function is called "Soften", in Photoshop it is called "Blur". Apply the softening function twice, and there will be a less coarse dot pattern in the scanned image as shown below:

Now, reduce the image size by 50%, as shown below:

Now, use the "Sharpen" function in your graphics program two or three times until you get an acceptable degree of sharpness:

NOTE: You may notice as you are sharpening the image that the weird pattern starts to show up. In this case, you will need to back off the sharpening a notch.

Now, reduce the image size to whatever resolution you want for the finished product. If it needs further sharpening as a result of the size reduction, do it and then save your final image:

You are done! Now, doesn't the picture above look a lot better than that first scan?

What if you can't get the sharpness that you want and still eliminate the weird pattern? I have found that scanning the magazine image cock-eyed and then using the "Rotate Image" function in the graphics program to make it straight somewhere in the above process sometimes helps to eliminate the weird pattern in really stubborn photos. Try it if you can't get acceptable results any other way.

SCANNING LARGE SCHEMATICS

Sometimes you might encounter a schematic that is bigger than your scanner's glass. One way to get around this problem is to scan the drawing into two or more image files, print the files, and then stick the pieces together with scotch tape. This will work, but it's kind of inconvenient.

A better way is to scan the drawing into two or more image files as described above, then piece them back together using your graphics program. The procedure is fairly straightforward.

First of all, scan in all of the pieces in such a way that they all overlap a bit when put together, as shown in the pictures below:

Then, use your graphics program to make the canvas size of one of them equal to the size of the whole schematic:

 

Now find the point on this portion of the schematic where the drawing matches the edge of the other portion of the schematic. Zoom in to that point so that you have about a 1:2 view of the schematic, then copy and paste the other portion of the schematic and move it into the place where the two portions join as seamlessly as possible, as shown in the pictures below:

Not quite there yet.......

Perfect!!

Once you have done this, you will have to examine the entire length of the "seam" where the portions of the schematic join to make sure it lines up perfectly everywhere. If one portion of the drawing was scanned at a slightly different angle realitive to the other portion(s), It will NOT line up perfectly. The easiest way out of this is to rotate the pasted portion of the drawing a little at a time until everything lines up just about right.

Now, reduce the image size so that it will fit on a single sheet of paper, and print it out. Most likely, the text and graphics on the drawing will be small, but still readable. If they are not, then you will need to Increase the image size, and the size of the paper, until it is readable.

ELIMINATING BLEED-THROUGH

Many schematics taken from books (such as the Beitman books) will have text and graphics printed on both sides of the paper they are printed on. If the paper used is thin and cheap, as is often the case, the information on the side of the page you aren't scanning will "bleed" through to the side being scanned.

Adjusting the brightness and contrast of the image will reduce the effects of the bleed-through, but tends to make the results look a little washed out. Fortunately, there is an easier and better way to do it.

Get a black piece of paper or cardboard, as black and as thick as you can find, and place it on top of the page being scanned, so that it completely covers the printing on the other side, then scan it as you normally would. This will usually eliminate at least 90% of the bleed-through effect.

If there is still enough bleed-through to be concerned about, you can use the brightness and contrast settings in your graphics software to eliminate the remainder of it.