Re: plotted circle is regular decagon on opengl graphics

[Daniel J Sebald]

On 08/11/2016 03:12 AM, Andreas Weber wrote:
> Hi Tatsuro,
>
> Am 11.08.2016 um 08:17 schrieb Tatsuro MATSUOKA:
> > Graphics tool kit: qt and fltk
> > plot (1:1, 'o', 'markersize', 24)
> > print -depsc circle.eps
> > plotted circle is regular decagon (ugly).
>
> The code to draw the markers with OpenGL is in gl-render.cc
> opengl_renderer::make_marker_list
>
>      case 'o':
>        {
>          double ang_step = M_PI / 5;
>
>          glBegin ((filled ? GL_POLYGON : GL_LINE_LOOP));
>          for (double ang = 0; ang < (2*M_PI); ang += ang_step)
>            glVertex2d (sz*cos (ang)/2, sz*sin (ang)/2);
>          glEnd ();
>        }
>
> As you can see the decagon is hardcoded. AFAIK there is no "draw circle"
> command in OpenGL so the only way to get a smoother circle would be to
> increase the numer of segments.

I'm surprised there is no type of primitive for arcs in OpenGL (or its 
utility library GLU) so that hardware/CPU optimization on graphics card 
drivers could be used.

The performance cost is the cos/sin computations, not the underlying 
driver.  The driver essentially sweeps out the same arc no matter how 
fine the angular resolution is (i.e., it may draw more polygon lines, 
but the individual polygon lines are shorter so don't need as many 
computations to complete).

Rik's changeset works, e.g., test:

graphics_toolkit ('qt')
plot(rand(6,1), rand(6,1), 'or')
set(get(gca,'children'),'MarkerSize',100)
set(get(gca,'children'),'MarkerFaceColor','green')

Attached is another algorithm using Pythagorean theorem that seems to 
work too.

Dan

octave-nontrig_circle_symbol-djs2016aug11.patch
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