Choosing the Parameters

Start with the values of the first conversion (THRESH=80, TRASHNUM=3, ACCURACY=2, BDWIDTH=2000) and tune them on a small part of the image before you convert a large one.

To …	Change
get the right size in CAD	Set PIXEL to the real pixel size of the scan, and check one known dimension in the result.
keep thin lines that disappear	Move THRESH towards the background level, or lower TRASHNUM.
remove dust and noise	Raise TRASHNUM.
get smoother, more accurate edges	Use ACCURACY=2.
reduce memory use	Lower BDWIDTH.

PIXEL: the scale

ImgScan writes coordinates in millimetres. PIXEL is the size of one pixel in micrometres, so PIXEL=1.0 makes a 3000 × 3000 pixel image a 3 × 3 mm drawing. A wrong value scales the whole result; LinkCAD can measure a known feature to check it.

THRESH: artwork or background

Each pixel is compared with THRESH (0 – 255). Pixels on one side become shapes, the others background. Look at the gray levels of copper and background in an image viewer and set THRESH between them. A value too close to either side breaks thin lines or merges close shapes.

TRASHNUM: noise

Specks smaller than TRASHNUM pixels are discarded. Dust on a scan disappears with a small value such as 3; raise it for a noisy image, and lower it when real small features, such as fine text, are lost.

ACCURACY and BDWIDTH: precision and memory

ACCURACY=2 traces edges more precisely than 1. BDWIDTH sets how many pixels wide each band is, up to 2000. Wider bands need more memory but produce fewer band borders to join. Keep 2000 unless the computer runs out of memory.