GLua code for drawing circles and arcs with thickness

circles_and_arcs.lua

-- Circles and Arcs (CC0 Public Domain, free to use in whatever you want)
-- Created by: Zachary Blystone ( [email protected] )

-- Vertex cache to prevent table allocations during draw operations
local vertCache = {}
for i=1, 256 do vertCache[i] = { x = 0, y = 0, u = 0, v = 0, } end

-- Localized functions
local math_rad = math.rad
local math_pi = math.pi
local math_cos = math.cos
local math_sin = math.sin
local math_min = math.min
local math_max = math.max
local surface_DrawPoly = surface.DrawPoly

-- surface.DrawCircle
--   centerX, centerY           the center of the circle
--   radius                     the size of the circle
--   angle [0]                  the rotation of the circle (noticable if you have a small number of vertices)
--   numVerts [50]              the number of vertices (corners) on the circle

function surface.DrawCircle(centerX,centerY,radius,angle,numVerts)

	numVerts = math_min(numVerts or 50, 256)

	local temp = nil

	-- Setup angles and rotation
	local increment = 2 * math_pi / numVerts
	local angle = math_rad(angle or 0)
	local cos = math_cos(increment)
	local sin = math_sin(increment)
	local x = math_cos(angle)
	local y = math_sin(angle)

	for i=1, numVerts do

		-- Set the vertex
		local v = vertCache[i]
		v.x = centerX + x * radius
		v.y = centerY + y * radius
		v.u = (x + 1) * .5
		v.v = (y + 1) * .5

		-- Rotate the current frame by increment
		temp = x * cos - y * sin
		y = x * sin + y * cos
		x = temp

	end

	-- Put a 'notch' in the vertex cache so only these vertices are drawn
	temp = vertCache[numVerts+1]
	vertCache[numVerts+1] = nil
	surface_DrawPoly( vertCache )

	-- Restore vertex cache
	vertCache[numVerts+1] = temp

end

-- surface.DrawArc
--   centerX, centerY           the center of the arc
--   radius                     the inner boundary of the arc
--   thickness [10]             defines how thick the arc is
--   startAngle [0]             the angle the arc should start from
--   endAngle [360]             the angle the arc should end up at
--   numSegments [50]           the number of segments (corners) the arc should have

function surface.DrawArc(centerX,centerY,radius,thickness,startAngle,endAngle,numSegments)

	numSegments = math_max(numSegments or 50, 1)

	local temp = nil

	-- Setup angles and rotation
	local angle = math_rad(startAngle or 0)
	local sweep = math_rad(endAngle or 360) - angle
	local increment = sweep / numSegments
	local cos = math_cos(increment)
	local sin = math_sin(increment)
	local x = math_cos(angle)
	local y = math_sin(angle)
	local invSegments = 1 / numSegments

	-- Gather vertices from cache
	local v0 = vertCache[1]
	local v1 = vertCache[2]
	local v2 = vertCache[3]
	local v3 = vertCache[4]

	-- Negative thickness pushes the radius inward
	if thickness < 0 then
		radius = radius + thickness
		thickness = -thickness
	end

	local outer = radius + (thickness or 10)

	-- Put a 'notch' in the cache so only these vertices are drawn
	local prevCache = vertCache[5]
	vertCache[5] = nil

	-- Each segment is 4 vertices
	for i=1, numSegments do

		-- Set vertices for this segment
		v0.x = centerX + x * outer
		v0.y = centerY + y * outer
		v0.u = (i-1) * invSegments
		v0.v = 0
		v3.x = centerX + x * radius
		v3.y = centerY + y * radius
		v3.u = (i-1) * invSegments
		v3.v = 1

		-- Rotate the current frame by increment
		temp = x * cos - y * sin
		y = x * sin + y * cos
		x = temp

		-- Set vertices for the next segment
		v1.x = centerX + x * outer
		v1.y = centerY + y * outer
		v1.u = i * invSegments
		v1.v = 0
		v2.x = centerX + x * radius
		v2.y = centerY + y * radius
		v2.u = i * invSegments
		v2.v = 1

		surface_DrawPoly( vertCache )

	end

	-- Restore vertex cache
	vertCache[5] = prevCache

end