Software Prep

A multicolored bird is difficult to prepare because:

 (a) The combination of dark (black head) and light (pale yellow tail feathers) colors makes it mathematically more challenging to represent the full color range with just a single 8-bit color plane after posterizing. 

(b) The bird has a variety of shades even within a base color.

(c) The intricate detail and complex shapes of a biological subject pose challenges with respect to boundaries.

The main variables to change in Gimp are:

Brightness: decrease it to give faint areas a chance to etch.

Contrast: increase it to help with boundaries and offset the lower brightness so that you have a true zero for the white background (no etch) around the bird.

Number of posterizing levels:  a trade off between good boundaries and smooth color shade transitions.

The main variables to change in RDWorks are:

% power: a trade off between etching all the colors and losing detail.

Scan gap interval: a trade off of run time versus quality. Scan gaps on rotated media need to be a factor of 3 to 8 smaller than for flat media.

Speed: a 3 way trade off between waiting a long time for the etch to complete, the depth of the etch and the etch quality.

Warning: Make sure to change any brightness and contrast settings in Gimp BEFORE posterizing.

I will set up four g-code .rd files for permutations of 13% vs 17% power against 0.008 mm and 0.016 mm scan gap intervals running the 2 inch focal length laser on a cylindrical 7 inch drinking glass. I chose these parameters based a 2 hours worth of trial and see test cases on Gimp and RDworks simulations.

Finally, The gimp posterization process was independently verified in Octave, a shareware scientific and numerical programming development environment that can dissect image files down to the basic byte color values in all dimensions. For example, posterizing a gray scale image of values from 0 to 255 yields 6 levels at roughly evenly spaced gray colors of 0, 51, 102, 153, 204, and 255.