Producing images of line drawings for screen display

[Mike Dallwitz]

                                                              27 February 1995

> From: "Marie-Claude Lariviere" <LariviereM at landcare.cri.nz>
> To: md at ento.csiro.au (Mike Dallwitz)
>
> I have run into a few problems while preparing images for use in INTKEY.
>
> The procedure I used is as follows:
>
> 1. Originally my master images were scanned as line art, 600 DPI and saved
> as .pcx. At that time I wasn't using DELTA and the images were prepared to
> produce a high resolution camera-ready hard copy for publication.
>
> 2. For INTKEY, I decided to use the above images and convert them to the
> required .gif format.
>
> 3. For this, the above .pcx files were converted to greyscale images,
> imported in Corel DRAW 4.0 where text, symbols etc were added, and exported
> as .gif files (640X480, 256 colours, 150 DPI).
>
> The results are a bit disappointing, i.e., they appear much coarser than the
> images provided in the beetle-larvae CD-ROM. Did you use a special technique
> to obtain those wonderful smooth images?

The following extract from my Introduction to Computer Images (Taxacom ftp
server, file pub/delta/graphics/images.txt, modified today) partly answers
your questions.

   Colour images should be scanned with 24 bits per pixel, and line drawings
   with 8 bits per pixel (i.e. 256 grey levels, NOT as `line art' with 1 bit
   per pixel).

   The scaling method built into scanning software is often very poor. It is
   best to do no scaling at this stage (`100%' scaling), or restrict it to
   integer factors (25%, 50%, 200%; not 33%). Note that there may be an
   interaction with the dots-per-inch (dpi) setting, which should be set to
   the `native' setting of the scanner (usually 300dpi or 600dpi). If you want
   to check the quality of scaling, use line drawings with fine lines; poor
   scaling usually shows up as discontinuities in the lines.

   Line drawings can be stored compactly as GIF or compressed TIFF files with
   16 or 256 grey levels. Using 16 or 256 grey levels gives smoother lines
   than 2 levels (black and white). Using 256 levels does not give noticeably
   better results than 16 levels, and produces larger files. However, it is
   best to use 256 levels until any touching up, such as cleaning up the
   background and increasing the contrast, has been done.

   Line drawings tend to become greyer when scaled to reduce the image size,
   particularly when the lines are fine. The appearance of the scaled versions
   can be improved by darkening the lines. This can be done by changing the
   `gamma' of the image (provided the background of the original image has
   been made pure white, which should be done routinely). For example, the
   following Image Alchemy command will convert all GIF file in the current
   directory to a maximum size of 640x480 and 16 grey levels, with moderate
   darkening, and place the output files in the subdirectory `scaled'.
      alchemy -g -d -b -c16 -+ -Xb640 -Yb480 -Gi1.0 -Go0.7 -o *.gif scaled
   Lower values of `Go' (`gamma output') give more darkening.

Scaling will make your `line art' (black and white) images look smooth,
provided there is a substantial reduction (say 50%) and a good scaling method
is used.

It's often difficult to know exactly what high-level graphics programs like
CorelDraw are doing to the images. You may be able to get better results by
experimentation, or by getting advice from an expert in that program. You may
get more predictable results by using an image editor, where you are editing
the bitmap images directly, and don't go through the `export' step. There is
an image editor in the Corel package, but it seems to be less powerful than
editors such as Photoshop and Picture Publisher. The beetle-larvae images were
mainly done with Photoshop.

Mike Dallwitz                                  Internet md at ento.csiro.au
CSIRO Division of Entomology                   Fax +61 6 246 4000
GPO Box 1700, Canberra ACT 2601, Australia     Phone +61 6 246 4075