Filled circular sector

no-reply@allegro.cc (Polybios) — Thu, 02 Dec 2010 03:43:57 +0000

Hi guys,

how do I draw a filled circular sector aka pieslice with Allegro 4.9/5.0?

I suppose, I need some combination of al_calculate_arc and the low level primitives routines... But how exactly should I do this?

Any help is highly appreciated!

no-reply@allegro.cc (Arthur Kalliokoski) — Thu, 02 Dec 2010 03:45:30 +0000

Draw a line from center of circle to each end of the arc and floodfill?

no-reply@allegro.cc (Polybios) — Thu, 02 Dec 2010 03:54:31 +0000

That was my first thought, too, but there is no floodfill in Allegro 4.9...!

All the pixel-reading, which a floodfill would have to do, would be really slow on textures, I guess.

I should add, I need it to be quite ... fast!

no-reply@allegro.cc (SiegeLord) — Thu, 02 Dec 2010 03:59:18 +0000

Polybios said:

I suppose, I need some combination of al_calculate_arc and the low level primitives routines... But how exactly should I do this?

That is the proper way... it'd be something like this (created by mushing al_draw_filled_ellipse and al_draw_arc)(untested):

#SelectExpand
  1void al_draw_filled_pieslice(float cx, float cy, float r, float start_theta,
 float delta_theta, ALLEGRO_COLOR color)
  3{
 ALLEGRO_VERTEX vertex_cache[ALLEGRO_VERTEX_CACHE_SIZE];
 int num_segments, ii;
 
 num_segments = fabs(delta_theta / (2 * ALLEGRO_PI) * ALLEGRO_PRIM_QUALITY * sqrtf(r));
  8
 if (num_segments < 2)
    return;
 
 if (num_segments >= ALLEGRO_VERTEX_CACHE_SIZE) {
    num_segments = ALLEGRO_VERTEX_CACHE_SIZE - 1;
 }
    
 al_calculate_arc(&(vertex_cache[1].x), sizeof(ALLEGRO_VERTEX), cx, cy, r, r, start_theta, delta_theta, 0, num_segments);
 vertex_cache[0].x = cx; vertex_cache[0].y = cy;
 
 for (ii = 0; ii < num_segments + 1; ii++) {
    vertex_cache[ii].color = color;
    vertex_cache[ii].z = 0;
 }
 
 al_draw_prim(vertex_cache, 0, 0, 0, num_segments + 1, ALLEGRO_PRIM_TRIANGLE_FAN);
 25}

You can also hack around it by using a thick outlined arc.

EDIT: Fixed the code a bit... still untested though.

no-reply@allegro.cc (Polybios) — Thu, 02 Dec 2010 04:22:30 +0000

Oh thank you so much!

It seems that there is always a vertex drawn which is at 0,0; that means the pieslice is crudely extended to the left-upper corner of the screen.

Maybe you meant to pass "&(vertex_cache[1].x)" to al_calculate_arc?

no-reply@allegro.cc (SiegeLord) — Thu, 02 Dec 2010 04:23:48 +0000

Yes. Fixed now.

no-reply@allegro.cc (Kibiz0r) — Thu, 02 Dec 2010 05:58:03 +0000

I recall there was a request for this forever.5 ago, people started talking about what to call it, then the thread died.

no-reply@allegro.cc (jmasterx) — Thu, 02 Dec 2010 06:45:55 +0000

al_draw_sector
al_draw_pie_sector
al_draw_circle_sector

are these good names?

no-reply@allegro.cc (Arthur Kalliokoski) — Thu, 02 Dec 2010 06:49:19 +0000

I'd call it al_pie_slice().

no-reply@allegro.cc (SiegeLord) — Thu, 02 Dec 2010 07:27:17 +0000

Kibiz0r said:

I recall there was a request for this forever.5 ago, people started talking about what to call it, then the thread died.

The implementation for the outlined pieslice/sector/w/e is very non-trivial, so I haven't had the time to implement it.

no-reply@allegro.cc (Steve Terry) — Thu, 02 Dec 2010 07:43:54 +0000

mmmmmmmmmmmmmmmmmm pie

no-reply@allegro.cc (Polybios) — Thu, 02 Dec 2010 14:22:21 +0000

Thanks again, SiegeLord!

One question though: Could you (or anyone else) explain why you take the square root of the radius there in order to calculate the number of segments?

no-reply@allegro.cc (SiegeLord) — Thu, 02 Dec 2010 21:15:58 +0000

It's... complicated. I don't remember exactly how I came up with it. The last thing I remember is that it is an approximation to this formula:
N = \frac{2 \pi}{\arccos (1 - 0.25 / r)}" />
Which calculates the number of segments by limiting the length of the KB segment in the diagram on this page.

no-reply@allegro.cc (Kibiz0r) — Fri, 03 Dec 2010 04:31:06 +0000

I'd call it al_pacman().