[Drawkit] span vs. span unit

[Michael Caron]

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Thanks, Graham, that explains things very well.

Michael

On Mar 29, 2009, at 6:46 PM, Graham Cox wrote:

>
> On 29/03/2009, at 5:32 AM, Michael Caron wrote:
>
>> Hi Graham,
>>
>> In DKGridLayer.m, the use of the term "span" seems confusing to me.
>>
>> First, the conversionFactor argument says "distance in points  
>> represented by a single span unit", and then there's a "span"  
>> argument that says "the span distance in grid coordinates".
>>
>> Is a "span unit" like a pixel (given that you've marked 72 as the  
>> english conversion factor, and 72 pixels/inch is a standard  
>> resolution) or is a "span unit" one tick on the grid ruler?
>>
>> I don't really understand "the span distance in grid coordinates".  
>> What does that mean? Does that mean the distance between ruler  
>> ticks on the grid in ticks?
>>
>
> Hi Michael,
>
> I agree it's a bit confusing. Partly it stems from the evolution of  
> the DKGridLayer - maybe some comments need to be revised.
>
> I'll try and explain as best I can. The important point is that the  
> underlying coordinate system used by Quartz is fixed such that 1pt =  
> 1/72nd of an inch. So everything else ultimately has to end up  
> expressing itself in these terms so we can draw the grid and rulers  
> using Quartz.
>
> The grid 'span' is the basis for where it lays down the various grid  
> lines. The span is broken down into finer 'divisions', the smallest  
> drawn grid square, and also multiplied up to a certain number of  
> 'majors' which are the bolder lines drawn every so often.
>
> The conversion factor sets up the scaling between a single span and  
> the underlying quartz coordinates, so for example if you want a span  
> of 1 inch, the conversion factor is 72.0, meaning each span line is  
> drawn every 72 points, or 1 inch apart. A span of 1 centimetre has a  
> conversion factor of 28.346 because 1cm = 28.346 points (you can  
> arrive at this figure by dividing 72 by 2.54, the number of  
> centimetres in 1 inch). So each span line is drawn 1cm apart.
>
> The confusion probably comes in because there is also the 'span'  
> parameter in:
>
> - (void)					setDistanceForUnitSpan:(float) conversionFactor
> 							drawingUnits:(NSString*) units
> 							span:(float) span
> 							divisions:(unsigned) divs
> 							majors:(unsigned) majors
> 							rulerSteps:(unsigned) steps;
>
> This parameter is a multiplier - for the situation outlined above,  
> the multiplier is 1.0, meaning that the span lines are drawn for  
> every span. If the multiplier was 2.0, the spans would be drawn  
> every 2 spans, and so on. The use of a multiplier allows you to pass  
> in the same conversion factor for the underlying coordinate system  
> and calculate the interval for you. It also permits non-integer  
> multipliers if you wanted. However, I anticipate that almost all of  
> the time you'd simply pass 1 here. The multiplier is handy if you  
> wanted to work at very small or very large scales, such as microns  
> or huge values used in mapping to ensure a sensible density of grid  
> lines at those scales, but for typical ordinary uses, there's not  
> much reason to use it.
>
> The rulers are also affected. Because of the way NSRulerViews work,  
> we need to supply a name and various factors ahead of time so that  
> the grid and rulers can be set up to agree with each other. The main  
> interval on the ruler (those marked 1, 2, 3, 4 etc) are established  
> by multiplying the span by the conversion factor. The rulerSteps are  
> just a hint as to how those intervals are split up - it's just  
> passed along to NSRulerView as the "step-up cycle" parameter.
>
> I hope that clarifies things a bit - if not, just say!
>
> --Graham
>
>
>
>
>
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Michael
- ------------------------------------------------
michael.r.caron at gmail.com
http://techrad.wordpress.com
http://mrcaron.wordpress.com

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