OpenCV Brightness & Contrast like in Gimp

Question

I want adjust the contrast in Python's OpenCV like in Gimp. I want do this:

In Gimp it is simple. I just use Colors->Brightness & Contrast, and set contrast to 127. But I don't know how to do this in Python and I cannot find any solution to this problem.

Answer 1

Check here. The formula is:

new_value = (old_value - 0.5) × contrast + 0.5 + brightness

In python you can easily apply these as matrix operations. Please note that pixel values here are in [0,1], so 0.5 is equivalent to 127 when the range is in [0,255].

In the same link is reported also the code, with pixel-wise operations (I copy just for completeness):

function transform(value, brightness, contrast)
  return (value-0.5)*contrast+0.5+brightness
end

function transform_rgb(r,g,b, brightness, contrast)
  return transform(r, brightness, contrast), transform(g, brightness, contrast), transform(b, brightness, contrast)
end

function bcontrast_get_rgb(x,y,brightness, contrast)
   r,g,b=get_rgb(x,y)
   return transform_rgb(r,g,b, brightness, contrast)
end

function bcontrast(brightness, contrast)
    for y=0, height-1 do
      for x=0, width-1 do
        set_rgb(x,y, bcontrast_get_rgb(x,y,brightness,contrast))
      end
    end
    flush ()
end

bcontrast(0.25, 2.0)