[solved] Trouble with TLazCanvas.Arc
lazarus / 39-Graphics

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Valdas · 2023-03-16 18:23

Hello,

I' trying to adopt an example from the this post to produce an Arc.
 Instead of
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
cnv.Ellipse(0, 0, BITMAP_SIZE, BITMAP_SIZE); I'm using
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i cnv.Arc(0,
0, 20, 20, 0, 180 * 16); But I'm not getting the Arc, only a green
field.
 Am I missing something?

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wp · 2023-03-16 19:12

Can you show your code so that I can reproduce it?

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wp · 2023-03-16 19:53

I see: TLazCanvas does not implement its Arc method, it inherites the
one from TFpCustomCanvas, and this does - nothing...

Here is a quick-and-dirty procedure which you can call instead. It
draws the arc as short line segments; their angular length is 5 degree
- this is not optimized: for very small arcs you can increase the
value because many points will be overlapping; and for large arcs you
must decrease the value if you can distinguish the individual
straight-line segments in the curve. (Ideally the procedure should be
tuned to determine the correct value by itself).
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i uses 
Math;procedure Arc(ACanvas: TLazCanvas; ALeft, ATop, ARight, ABottom:
Integer;  AStartDeg, ASweepDeg: Double);var  a, b: Double;  phi,
phi_end, dphi: Double;  sinphi, cosphi: Double;  x, y: Integer; 
Center: TPoint;  done: Boolean = false;begin  a := (ARight - ALeft) /
2;  // major ellipse axis  b := (ABottom - ATop) / 2;  // minor
ellipse axis  Center := Point((ALeft + ARight) div 2, (ATop + ABottom)
div 2);  dphi := DegToRad(5);  // 5-degree steps, not optimized  if
ASweepDeg < 0 then dphi := -dphi;  phi := DegToRad(AStartDeg); 
phi_end := DegToRad(AStartDeg + ASweepDeg);  sincos(phi, sinphi,
cosphi);  x := Center.X + round(cosphi * a);  y := Center.Y -
round(sinphi * b);  ACanvas.MoveTo(x, y);  repeat    phi := phi +
dphi;    if ((ASweepDeg > 0) and (phi > phi_end)) or ((ASweepDeg < 0)
and (phi < phi_end)) then    begin      phi := phi_end;      done :=
true;    end;    sincos(phi, sinphi, cosphi);    x := Center.X +
round(cosphi * a);    y := Center.Y - round(sinphi * b);   
ACanvas.LineTo(x, y);  until done;end; 
ALeft, ATop, ARight, ABottom define the rectangle enclosing the *full*
ellipse from which the arc is cut. AStartDeg is the start angle of the
arc, in degrees, measured from the x axis in counter-clockwise
direction. ASweepDeg is the "length" of the arg given in degrees also.
If positive the arc runs in counter-clockwise direction, otherwise in
clockwise direction. The following sample draws a rounded rectangle:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   Arc(cnv,
0, 0, 30, 30, 180, -90);  cnv.Line(15, 0, img.Width-15, 0);  Arc(cnv,
img.Width-30, 0, img.Width-1, 30, 0, 90);  cnv.Line(img.Width-1, 15,
img.Width-1, img.Height-15);  Arc(cnv, img.Width-30, img.Height-30,
img.Width-1, img.Height-1, 0, -90);  cnv.Line(img.Width-15,
img.Height-1, 15, img.Height-1);  Arc(cnv, 0, img.Height-30, 30,
img.Height-1, -90, -90);  cnv.Line(0, img.Height-15, 0, 15);

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Valdas · 2023-03-17 05:59

Thank you!