How to assess rendering quality?

Hi list,

I'm doing a comparative study of OpenSource cartographic servers 
(Mapserver, Geoserver and Mapnik). Beside raw performance and features, 
I'd like to assess the rendering quality, say how pretty produced maps 
are. Precisely, I'm interested in the quality of the drawing work, my 
point is not about symbology, nor styling of maps.

I have some problems to find a set of objective criteria I could 
benchmark my servers against. So far, I have already identified the 
following:
- sharpness of details
- smoothness of lines
- uniformity of colors

I'm open to any comments. Do you think these criteria are consistent 
regarding the purpose of my study? Does anyone have other criteria to 
suggest?

Regards

-- 
Gilles Bassiere
MAKINA CORPUS
30 rue des Jeuneurs
FR-75011 PARIS
http://www.makina-corpus.com