RGB relative dimming from KNX

[alexll]

To dim a color your need to convert RGB to HSV, change V up/down and then convert back to RGB.
Here are the functions that you can use.

Code:

function rgbtohsv(r, g, b)
  local max, min, d, h, s, v

  max = math.max(r, g, b)
  min = math.min(r, g, b)
  d = max - min
  s = max == 0 and 0 or (d / max)
  v = max / 255

  if max == min then
    h = 0
  elseif max == r then
    h = (g - b) + d * (g < b and 6 or 0)
    h = h / (6 * d)
  elseif max == g then
    h = (b - r) + d * 2
    h = h / (6 * d)
  elseif max == b then
    h = (r - g) + d * 4
    h = h / (6 * d)
  end

  return h, s, v
end

function hsvtorgb(h, s, v)
  local r, g, b, i, f, p, q, t, z

  i = math.floor(h * 6)
  f = h * 6 - i
  p = v * (1 - s)
  q = v * (1 - f * s)
  t = v * (1 - (1 - f) * s)
  z = i % 6

  if z == 0 then
    r = v
    g = t
    b = p
  elseif z == 1 then
    r = q
    g = v
    b = p
  elseif z == 2 then
    r = p
    g = v
    b = t
  elseif z == 3 then
    r = p
    g = q
    b = v
  elseif z == 4 then
    r = t
    g = p
    b = v
  elseif z == 5 then
    r = v
    g = p
    b = q
  end

  r = math.round(r * 255)
  g = math.round(g * 255)
  b = math.round(b * 255)

  return r, g, b
end

function rgbstep(r, g, b, up, step)
  local h, s, v = rgbtohsv(r, g, b)
  local v2 = up and math.min(1, v + step) or math.max(0, v - step)
 
  if v == v2 then
    return
  end

  return hsvtorgb(h, s, v2)
end

rgbstep(r, g, b, up, step) can perform up/down step of current RGB value.
arguments:

• r, g, b - color components in 0..255 range
  
• up - true = step up / false = step down
  
• step - step size, V range is 0..1 so the step should be something like 0.1 or 0.05 depending on how many total steps you want
  

rgbstep will return new RGB values but it might also return nil when further step is not possible.

Code:

-- values from split RGB value
r = 127
g = 255
b = 10
-- step down
up = false
-- maximum 20 steps between min and max
step = 0.05

r, g, b = rgbstep(r, g, b, up, step)
if r then
  -- write new values
else
  -- stop dimming
end

Thank you very much! So my only doubt is which group addresses I need to define