GFILL_AREA(3NCARG)

🎀 GFILL_AREA 🎀

okay bestie, welcome to gfill_area from the NCAR Graphics manual, section 3NCARG 💖 this is the output primitive that fills polygonal areas, and honestly? it's giving main character energy for your graphics. sit down, hydrate, let's go ✨

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✨ NAME ✨

gfill_area — the output primitive for filling polygonal areas 🫶 like, it takes a polygon and it fills it. iconic behavior.

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🌸 SYNOPSIS 🌸

#include <ncarg/gks.h>

void gfill_area(const Gpoint_list *point_list);

that's it. one function, one pointer, infinite vibes 💅

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🦋 DESCRIPTION 🦋

so the function takes a Gpoint_list, and that struct is doing a lot of heavy lifting. here's the tea on its fields:

• point_list.num_points — (Gint, Input) the number of points in the polygon you want filled. must be greater than two. like, that's non-negotiable bestie, a polygon needs at least three points, that's just geometry 😭
• point_list.points — (Gpoint *, Input) the X and Y world coordinates of the polygon. your actual shape lives here.

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💅 USAGE 💅

okay so here's how the fill area actually gets defined. the region you're filling is delimited by a sequence of straight line segments connecting the successive points, going like:

(point_list.points[0].x, point_list.points[0].y),
...
(point_list.points[point_list.num_points-1].x,
 point_list.points[point_list.num_points-1].y)

and here's a lil detail that matters: the last point in the polygon gets connected back to the first point with a straight line segment — but only in the case that it isn't already the same as the first point. so it auto-closes your shape for you. queen behavior 👑

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🌈 the "what even is inside" situation 🌈

now. when you've got a polygon whose edges cross each other, it becomes ambiguous as to what actually counts as the "inside" of the polygon. messy. chaotic. and GKS has a whole algorithm for this, so here it is:

for a given point, draw a ray starting at that point and going to infinity in any direction. then count the intersections:

• if the number of intersections between the ray and the polygon is odd → the point is within the polygon 💖
• otherwise → the point is outside 🙈
• if the straight line passes through a polygon vertex tangentially, the intersection count is not affected (so don't stress about that edge case)

and if a point is within the polygon, it gets included in the area to be filled. simple as that, no cap.

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🎨 interior styles 🎨

several interior styles can be selected — hollow, solid, and various hatch patterns 🌸 see the man pages for gset_fill_int_style and gset_fill_style_ind for all of that.

⚠️ note well, bestie: by default in GKS, the interior fill style is hollow, aka no fill. so if you call gfill_area and you don't get a filled interior like you expected, you will probably need to call gset_fill_int_style to set the fill style to something other than "hollow". this is the #1 gotcha and it gets everybody, fr fr 😭💀

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🔥 ACCESS 🔥

to use the GKS C-binding routines, you need to load the ncarg_gks and ncarg_c libraries. load them. don't skip them. that's the whole thing 🫶

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⭐ SEE ALSO ⭐

Online:

• gset_fill_int_style(3NCARG)
• gset_fill_style_ind(3NCARG)
• gset_colr_rep(3NCARG)
• gset_fill_colr_ind(3NCARG)
• ginq_fill_int_style(3NCARG)
• ginq_fill_style_ind(3NCARG)
• gks(3NCARG)
• ncarg_gks_cbind(3NCARG)

Hardcopy:

• User's Guide for NCAR GKS-0A Graphics
• NCAR Graphics Fundamentals, UNIX Version

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👑 COPYRIGHT 👑

Copyright (C) 1987-2009 University Corporation for Atmospheric Research The use of this Software is governed by a License Agreement.

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and that's the whole page, bestie 💜 this one dropped on UNIX back in March 1993 as part of the NCAR GRAPHICS collection — yes she's an elder but she still eats. go fill some polygons. slay. periodt. 🎀✨