Index colours CAD

Black screens, white paper and colours. BricsCAD and AutoCAD rely on 255 indexed colours. Many people like to work with a black background. Staring at a white screen during a long working day is ‘less relaxing’ for the brain.

So people draw yellow lines on a black screen and find out that they cannot be seen when printed on white paper. Similarly, deep blue on black is visible on white paper, but barely visible on a black screen.

Index colours

Processing and converting colour information is crucial for optimal display in PDFs or on paper. Information can be used to change colours in CTB files (colour tables) for printing. The question is whether you want to use different colours for paper than the ones you use for your screen. In any case, it doesn’t really get any easier.

So we are looking for an approach to optimise the visibility of index colours. To reduce complexity: RGB “Truecolor” is not discussed. RGB we do need to determine visibility.

More specifically, we are talking about a mapping of the 256 (1-256) indexed screen colours, paper colours.

Another approach: Let’s create a table where visibility can be set to both black and white – so we don’t need a CTB file.

The index colour table backgrounds

• There used to be a colorwheel drawing. Here it is: http://images.autodesk.com/adsk/files/colorwh.dwg (cache here).
• A proper guide can be found here: http://www.niwa.nu/2013/05/math-behind-colorspace-conversions-rgb-hsl/
  • However, 255,0,0 and 255,255,0 both results in hue is 0 values, some work to be done here… So:
• The subject of Hue can be found here: https://en.wikipedia.org/wiki/Hue#Defining_hue_in_terms_of_RGB
• CTB-files are compressed stuff and so far I am not able to properly write data to CTB. A link for uncompressing data can be found here: http://www.noliturbare.com/plot-print/print-ctbs-and-more
  • So far, no method is found to compress data back to CTB files.
• It is pretty hard to obtain a list of RGB values of all indexed colours. A spreadsheet is here: index-colors.ods
  • How to calculate things can be seen in the formulas, just open the sheet.
  • A print from that sheet with calculated HSL (Hue Saturation Lightness) values is written below. Sorry for the missing Crayola, coloured entities on coloured backgrounds.
• The L or Lightness value may be helpful to eliminate the “hard to see colours”.
  • High L values on white is bad
  • Low L values on black is bad
  • So simply sort the L values in the spreadsheet to eliminate bad colours and use only in-between values.

Lisp for colours

We can use index colours to find out RGB values. Suppose we want to find out the colour of index 122. In that case, you can enter this on the command line:

(vle-aci2rgb 122)

This returns the following RGB information:

(0 204 153)

So we need some code to figure out all the colours. The following can be pasted line by line on the command line:

(setq n 2 m (list 1))
(while (<= n 256) (setq m (append m (list n))) (setq n (1+ n)))
(mapcar '(lambda (k) (cons k (vle-aci2rgb k))) m)

This produces a long list of values that you can convert to a spreadsheet using a text editor.

Table Index sorted

Note: I wrote this somewhere around 2018 on the NedCAD wiki. I was then, and still am, using BricsCAD and I don’t remember how I got the values of column 2:4, the RGB values. Now I used the (vle-aci2rgb …) method and the results are below.

END OF TABLE.

Table Lightness sorted

Just download the spreadsheet (link above) and look at the L and L2 column for Lightness. They are calculated differently where I personally prefer the L (instead of L2) approach, not because of its simplicity but because of the resulting lightness perceived.