On 22 Apr 2014, at 2:23 , Igor Stasenko <siguc...@gmail.com> wrote: > as for why there's 4 arc segments instead of one, its because > of bad approximation, when drawing more that 90 degree arcs. > > also, in athens, arc segment is defined with following inputs: > - end point of previous segment (implicit) > - angle > - direction (clockwise/counterclockwise) > - end point > > the radius, therefore calculated automatically, because with given set of > parameters there's only one way to draw an arc connecting given points. > > Now if you put angle of 360 degrees, you cannot draw arc without specifying > radius, > because your end points will coincide, which means there's infinite number of > ways to draw full circle passing through a single point, with any radius. > > cairo using different inputs for specifying arc segments.. > - center, radius, start angle, end angle > > the problem with such parametrization is that it is completely separate from > rest of commands (line/move/bezier etc).. and you will be very lucky if your > arc will be connected with rest of your path.. because arc's starting point > depends on start angle, instead of last point of previous path segment. > > this was the main reason to use more appropriate parametrization to get rid > of inconsistency.. while losing ability to draw full circle with single > command..
AFAICT, still doesn’t explain how to draw an ellipse with constant stroke path width, which was the original question :) One way is to not use a transformed circle path altogether, but draw the actual ellipsis path using cubic beziers: http://www.charlespetzold.com/blog/2012/12/Bezier-Circles-and-Bezier-Ellipses.html ellipsisOfExtent := [:builder :anExtent | | halfX halfY | halfX := anExtent x / 2. halfY := anExtent y / 2. “We expect relative builder, and start the ellipsis at anExtent x / 2 @ 0" builder curveVia: 0@(halfY negated * 0.55) and: (0.45 * halfX)@halfY negated to: halfX@ halfY negated; curveVia: halfX* 0.55 @ 0 and: halfX@ (0.45 * halfY) to: halfX @ halfY; curveVia: 0 @ (halfY * 0.55 ) and: (0.45 * halfX negated @ halfY) to: halfX negated @ halfY; curveVia: (halfX negated * 0.55) @ 0 and: halfX negated @ (halfY negated * 0.45) to: halfX negated @ halfY negated; close]. AthensSceneView new scene: [ :can | | path | path := can createPath: [ :builder | builder moveTo: 10@60. ellipsisOfExtent value: builder value: 200@100 ]. (can setStrokePaint: Color red) width: 8 asFloat. can drawShape: path ] ; openInWindow Cheers, Henry
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