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.. On 22 April 2014 14:05, Igor Stasenko <siguc...@gmail.com> wrote: > > > > On 14 April 2014 13:53, Juraj Kubelka <juraj.kube...@gmail.com> wrote: > >> >> El 14-04-2014, a las 8:21, Juraj Kubelka <juraj.kube...@gmail.com> >> escribió: >> >> Thank you Stef, >> >> the example helped me a lot to understand how to play with Athens. >> >> The solution is not what I am looking for. If someone starts to play with >> alpha canal, s/he will get unexpected results. >> >> I have finally found an example in C language. It look like this: >> -=-=-=-=- >> cairo_set_line_width (cr, 0.1); >> cairo_save (cr); >> cairo_scale (cr, 1, 0.2); >> cairo_arc (cr, 1.5, 1.5, 1, 0, 2 * M_PI); >> cairo_restore (cr); >> cairo_set_source_rgba (cr, 0, 1, 0, 0.50); >> cairo_stroke (cr); >> -=-=-=-=- >> <tips-ellipse.png> >> >> I do not know how to write the same instructions with Athens. >> >> First, they draw one arc (cairo_arc (cr, 1.5, 1.5, 1, 0, 2 * M_PI);), I >> draw 4 arcs: >> -=-=-=-=- >> canvas >> createPath: [ :builder | >> builder >> absolute; >> moveTo: 0 @ 0.5; >> ccwArcTo: 0.5 @ 0.0 angle: 90 degreesToRadians; >> ccwArcTo: 0.0 @ -0.5 angle: 90 degreesToRadians; >> ccwArcTo: -0.5 @ 0.0 angle: 90 degreesToRadians; >> ccwArcTo: 0 @ 0.5 angle: 90 degreesToRadians ]. >> -=-=-=-=- >> >> >> OK, now I can draw ellipse with just one arc: >> -=-=-=- >> canvas >> createPath: [ :builder | >> builder >> absolute; >> moveTo: 0.5 @ 0; >> arcCenterX: 0 centerY: 0 radius: 0.5 startAngle: 0 endAngle: Float twoPi >> -=-=-=- >> >> Why is this method in private protocol? Why I should not use it? >> >> > because its private.. and implementation specific. > other implementations may not have this method at all, and your code will > simply throw DNU. > And that's why you got the horizontal line btw.. because of using private > protocol bypassing coordinate tracking. > > >> Why I have to use “moveTo:”? In C-example they do not use it. But I have >> to, otherwise there is an extra line on the canvas: >> >> Well, I see in AthensCairoPathBuilder>>createPath:, there is call of >> "self moveToX: 0 Y: 0.”. If I remove it, I do not have to write “moveTo: >> 0.5@0”. So, I understand why I have to write it in my code. The question >> is what is the purpose of "self moveToX: 0 Y: 0.”? I guess it simplify some >> situations, am I right? Which one? >> > > the initial moveTo: command lets you set the origin (starting point) of > your path. > if you don't start path with moveTo: command, it is implicitly set to 0@0. > > >> >> There is actual Athens example: >> -=-=-=- >> AthensSceneView new scene: [ :can | >> | path | >> path := can createPath: [ :builder | >> builder >> absolute; >> moveTo: 0.5 @ 0; >> arcCenterX: 0 centerY: 0 radius: 0.5 startAngle: 0 endAngle: Float twoPi >> ]. >> can pathTransform restoreAfter: [ >> can pathTransform scaleBy: 200 @ 50. >> >> (can setStrokePaint: Color red) width: 0.1. >> can drawShape: path. >> ]. >> ] ; >> openInWindow >> -=-=-=- >> >> Thank you, >> Juraj >> >> >> Second, they set line width, before any other action (like cairo_save). I >> do: >> -=-=-=-=- >> canvas pathTransform scaleBy: 200 @ 50. >> (can setStrokePaint: Color red) *width: 0.1*. >> can drawShape: path. >> -=-=-=-=- >> >> So, my result in Athens is like this: >> <Captura de pantalla 2014-04-14 a la(s) 8.18.38.png> >> >> Any idea? The example is explained >> here<http://cairographics.org/tutorial/#L2linewidth>, >> C source code is here <http://cairographics.org/tutorial/tips-ellipse.c> (by >> clicking on the image in the >> page<http://cairographics.org/tutorial/#L2linewidth> >> ). >> >> Thank you, >> Jura >> >> El 12-04-2014, a las 4:01, Pharo4Stef <pharo4s...@free.fr> escribió: >> >> AthensSceneView new >> scene: [ :can | >> | path | >> path := can >> createPath: [ :builder | >> builder >> absolute; >> moveTo: 0 @ 0.5; >> ccwArcTo: 0.5 @ 0.0 angle: 90 degreesToRadians; >> ccwArcTo: 0.0 @ -0.5 angle: 90 degreesToRadians; >> ccwArcTo: -0.5 @ 0.0 angle: 90 degreesToRadians; >> ccwArcTo: 0 @ 0.5 angle: 90 degreesToRadians ]. >> >> can pathTransform >> restoreAfter: [ >> can pathTransform scaleBy: 200 . >> can >> setPaint: Color red ; >> drawShape: path. >> "(athensCanvas setStrokePaint: strokePaint) >> width: (self strokeWidth / self scale) asFloat." >> can drawShape: path.]. >> can pathTransform >> restoreAfter: [ >> can pathTransform scaleBy: 190 . >> can >> setPaint: Color blue ; >> drawShape: path. >> "(athensCanvas setStrokePaint: strokePaint) >> width: (self strokeWidth / self scale) asFloat." >> can drawShape: path >> >> ] >> ] ; >> openInWindow >> >> >> <Screen Shot 2014-04-12 at 09.01.09.pdf> >> >> >> >> > > > -- > Best regards, > Igor Stasenko. > -- Best regards, Igor Stasenko.
