The Weekly Juan #3: Coordinate Systems in Morphic 3.0

Juan Vuletich jvuletich at
Tue Oct 31 01:54:27 UTC 2006

Hi Trygve,

Trygve Reenskaug escribió:
> Juan,
> Many thanks for your open discussion. Hans-Martin has given 
> well-founded comments. There is a lot of solid experience in them that 
> I recognize and that should be useful in your work.
> I would like to add two comments based on my own experience.
> 1) I underestimated the geometric complexity of even a simple window - 
> until I drew ALL the relevant rectangles in a scrollable text editor. 
> (I believe I have the report somewhere, but can't find it). There were 
> the outer view rectangles outside and inside the border. Then the 
> scrollbar with all its details. The text viewport outside and inside 
> the border, and the text view itself. The problem was that every 
> rectangle was handled and even named somewhere in the code. Hap-hazard 
> naming lead to an abundance of synonyms and homonyms which made it 
> hard to find and fix the many bugs. Much grief would have been avoided 
> if naming had been taken seriously from the very start. The complexity 
> of the ST80 code is partly caused by naming problems but mainly, I 
> believe, by complexity inherent in the task.
Ok. Anyway, I'm not rewriting PluggableMorphs, at least not right now...
> 2) An off-by-one error is very visible on the screen and very 
> irritating. We once used floats for certain coordinates. Accumulating 
> errors caused problems even after we changed to double precision 
> floats. On the other hand, I believe Display PostScript  (and PS 
> itself) uses float coordinates? If so, it could be worth while to look 
> into how users of DPS protect against cumulating errors.
> (See )
Well, Floats accumulate a lot less error than integers...
> Cheers
> --Trygve
Juan Vuletich

More information about the Squeak-dev mailing list