How Do I Use my Computer's Scanner to Change Plans?

There are several things to consider. What are you trying to accomplish?
Do you want to change the scale of an image?
Or, are you trying to clean up a drawing and remove blemishes (for publication)?

First of all, you need to determine which of your tools (if any) are up to what you're trying to accomplish. A scanner normally comes with a set of software tools; if you don't have these tools, you can get a nice one called IrfanView32 - a freeware tool that can do a lot (including running a scanner). Many of the tools try to be "helpful" - they'll print an image to fill the page, rather than at a uniform scale! So, the first thing you'll need to do is scan something and print it out, and then compare the printed image with the original - if they're not the same size, you've got a problem you'll have to resolve!

An image has two independent values that are important - the "Dots-per-inch" (DPI) and the image size (in pixels).

DPI is primarily an attribute for printing.

Image size primarily affects what you see on your screen (and, to a lesser extent, the resolution you'll see on you printer).

Here are two images, each one 210 x 425 pixels. The one on the left is at 200 DPI, the one at the right is at 100 DPI.

Save each and import the image into your image program (e.g., PhotoShop), and then print out the resulting image! You'll see the right image PRINTS half as big as the left one, even though each is the same SCREEN size!

(At least it's supposed to - if it doesn't, you'll either have to change some settings, or use a different program!)

Your scanner has a couple of different modes of operation. If you scan the plans as a "B/W Photo" (left), you'll get an image of the original, including all stains and discolorations of the paper, etc.; the lines, however, will appear reasonably smooth.

If you instead scan the plans as a "B/W Document" (right), you'll lose all shades of gray; every pixel will be either black or white.

You'll get rid of many of the "blots", but your lines will go all "jaggey". Notice that the "smudges" appear "sharper"; there are no grey pixels bordering them to make a smoother transition.

The most important thing you can do is to get the drawing lined up with the scanner! Here are two images, each scanned at 72 dpi. The one on the left is not aligned - notice how the baseline and centerline vary in apparent width (another case of jaggies). The one on the right has been aligned so the base scans as a single line; notice how the centerline appears much cleaner.

Scaling the image

An image can be made to print larger or smaller in one of two ways:

Example:


As an example, we'll use this early 19th century whaleboat drawing (scanned at 150 DPI) from a book. Let's say that we want to rescale the drawing to 1:48 (1/4 inch = 1 foot) to go with a model of a whaleship of the period, already at that scale.

The drawing has a scale, and that scale is divided into 9 feet. At 1:48 scale, 9 feet will be 2 1/4 inches = 2.250 inches long.

To begin, I cropped the scanned scale (cropped dimensions = 242w x 40h; DPI = 150) and printed it out (note: it's better to scale from the printed image than from the original - your scanner and or printer may introduce a bias); Using a decimal ruler (to avoid having to work with fractions), I determined that the scale printed out with a length of 1.342 inches. The drawing should be printed bigger!

There are now two choices of method:

Changing the Image Dimensions:

Increase the dimension of 242 pixels by 2.250/1.340 = 406 pixels. (Note that this image file consumes 3.82Kb of storage.)

Changing the DPI

Decrease the DPI of 150 by 1.340/2.250 = 89 DPI (note that this image, whileappearing smaller on the screen, still prints at the correct scale (and only takes 2.17Kb of storage). Caution: some printers will refuse to print the image properly if they consider the DPI is too small - verify this with your own printer; the solution is to perform the initial scan at a higher DPI!

Finally, I applied my choice of method to the original drawing. Take your choice!

Steps to perform

  1. Seriously consider using "B/W Document" mode for scanning if the drawing is in poor condition.
  2. At low resolution (DPI), adjust brightness and contrast and other scanning parameters to minimize blemishes and defects.
    (if you get a moire pattern of blemishes (say, due to a regular pattern such as cloth weave in drawings on linen), consider using the descreening feature of the scanner.
  3. Switch to a higher resolution (DPI) for the actual scanning process.
    (If you're planning to enlarge the drawing, allow enough DPI so that the enlarged drawing will still have at least 300 DPI).
  4. Incrementally rotate the drawing until reference lines (e.g., baseline, centerline) are smooth and have no more jaggies.
    This step is especially important if the drawing is so large that you will later have to join ("stitch") several successive scan images together.
  5. After one last preview, make the actual scanned image.
  6. If the final result will require stitching, repeat the last two steps as necessary.
  7. Convert each of these scans from two-level (B/W) to 256-level gray scale.
    (Remember that the recorded DPI is likely to change in this copy process)
    If you're going to splice scans together, save each of these copies.
  8. Perform any splicing needed. Save the result.
  9. Using a drawing program (e.g., MsPaint), clean up the image by removing extra black areas and repairing "lost" lines. Hint: Straight edges are easier to clean up if you use the line-drawing tool and "erase" by drawing a white line where the edges need to be sharpened.
  10. Scale the resulting image - you can adjust DPI or the actual image dimensions (or both):
  11. Use a "Blur" or "Smooth" tool to de-emphasize jaggies in diagonal lines.
  12. Convert the image to the desired final file format. Hint: JPG images are preferred for shaded images (e.g., photographs); the JPG compression is very efficient when there are no sharp edges. However, drawings encoded as GIF images are likely to be smaller than the corresponding JPG encoding. This may be significant if you intend to share the result over the Internet.
    If you use GIF encoding and intend to use the resulting image for a web site, make sure that you have "Transparent Color" disabled when you save the image, or all of your lines may display in the color of your web-page's background!

Example

Here are a series of scans of a simple object (the scale that often appears on most plans; in this case it allows the scaling of the drawing in English, French, and Swedish feet! Note: All image scanning and manipulation was performed as TIFF files; I then converted them to JPG and GIF files in order to display them here.

This one was scanned at 100 DPI as a "True Color, RGB" image. Note that the paper is brown (that doesn't make it any easier - it adds a lot of "noise" to the scanned image!)

This one was scanned at 100 DPI as a "B/W Photo" image. Note how gray the colored paper appears.

This one was scanned at 100 DPI as a "B/W Document" image. Note that the background is now white, but there's a lot of "noise" (black dots) (Note - I had to change the image for this - and subsequent - scans from Black & White to Index 256 Gray so that I could display it here. Doing that also changed to DPI value recorded to 96; I had to then change that back too!)

This one was also scanned at 100 DPI as a "B/W Document" image; I rotated the drawing until the jaggies disappeared.

The same scanned at 200 DPI; also cropped for subsequent changes.

I next ran the same image through MsPaint and manually erased superfluous dots...

...and then filled some white dots within black areas.

Finally, I chose a Blur (sometimes called Smooth) filter to smooth off the edges and minimize the jaggies (in this case, this added 3 shades of gray to the previous Blank and White).

The image was finally converted to a GIF format and saved for use here.

{John O. Kopf}
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