I wrote this article to give more visibility and integrate the ideas generated in a corresponding forum discussion, which has aroused great interest.
It is possible with FidoCadJ, Davide Bucci's (aka DarwinNE ) electronic-but-not-only CAD, to trace images using two programs that make semi-transparent the active window or a PNG image superimposed on the screen: Peek Through for Windows and CThruView for operating systems supporting the Java Virtual Machine.
Trace with Peek Through
Peek Through is provided for free by Luke Payne.
Once downloaded, unzipped, installed and started, you can adjust the transparency of the active window, and turn it on and off with a key combination. The default combination is: Windows key + A.
An usage example will be worth more than a thousand words.
If we want to trace the scissors in this picture:
we proceed, if we have not already done so, to start Peek Through. Then we open the FidoCadJ window above the displayed image and we make it semi-transparent with the appropriate key combination set during Peek Through's initialization (default: Windows + A):
Using Lines, Ellipses and Bézier 's curves to follow the contours the result is:
Using Curves instead of Bézier 's curves the difficulty of following the contours is greater, and you have to get the hang of it. But the Curves (as well as the Rectangles, the Polygons and the Ellipses) can be filled with the color of the corresponding layer, by double-clicking the object and selecting Filled.
Using different overlapping Layers and changing the colors (options View -> Layers -> Modify -> RGB) you can color the figures as you wish:
Anyone wishing to color the figures with the original colors under Windows can get the RGB, HSB and CMYK components of any displayed pixel using the small free utility ColorPix:
You can add black edges to a figure by duplicating it, disabling the Filled option in the duplicate, giving it a black and high priority layer (e.g. layer 12) and finally overlaying it to the original.
In order to have sufficient resolution it is advisable to work with a fairly small zoom (100% or 150%) and, if necessary, disable the "Snap to grid" button.
Trace with CThruView
For those who do not use Windows, there is CThruView, written in Java, which is also free. It allows to make PNG images semi-transparent and view them above another window. In this case you have to enable the click through option to work on the window below.
You must use the Java Virtual Machine 1.6. With newer versions the click through option and perhaps other things do not work (as happened to me). Although the JVM 1.6 can co-exist with newer versions I did not want to install it, so I have not finished trying this program.
If anyone wants to try it, I'll be happy to integrate his impressions and images in this article.
Thanks for reading and see you next time!