The color curve function is based on the classic curve image filtering operation. It works by remapping one or more color channels (R, G, B, A, or RGB together) by applying a function over the range [0-255]. A table can also be passed in, containing a series of key points to be interpolated.

curve ({ points, interpolation = 'linear', channel = 'rgb', src = input, dst = output })

The points argument is a function or table taking input values from 0 to 255 and mapping those to the same range 0 to 255. The old string-based syntax is still supported but not recommended.

The easiest way to specify a curve is to create a sparse table with keys and values from 0 to 255 (yes, the table starts at 0 and not 1 as is usual in Lua):

-- color invert
p = {}
p[0] = 255
p[255] = 0
curve ({ points = p })

This can of course be inlined:

curve ({ points = { [0] = 255, [255] = 0 } })

Another solution is to pass in a function f(x) that will output values in the 0-255 range. For example:

p = function(x)
 return 255 * math.sin(x / 255 * math.pi)
end
curve ({ points = p })

filter_curve.lua

a = buffer ('alpha')
b = buffer ('alpha')
blur { 6, dst = a }
 
c = {}
c[0] = 0
c[50] = 255
c[100] = 0
c[150] = 255
c[200] = 0
c[255] = 255
 
curve { src = a, points = c, dst = b }
blend {src = b }

Input	Color curve function	Output

Color curves can thus be used to draw contours of text characters:

contour.lua

a = buffer ('alpha')
blur ({ 4, dst = a })
 
p = {}
p[0] = 0
p[20] = 0
p[60] = 255
p[160] = 255
p[200] = 0
p[255] = 0
 
curve ({ points = p, src = a, dst = a })
blend ({ src = a, color = 'white' })