Fast Algorithm to Determine Average Brightness of an Image

hh hh at hyperhh.de
Tue Sep 20 22:14:07 CEST 2016


> So is there a fast, efficient way to analyze a 1200 X 800 px
> image to get this average brightness value.

You could try the following function. It is "dirty" (the definition
of 'brightness' is also unsharp!), but it is very fast.

local ii="tmp-1.414214"

-- uses the average color intensity of the pixel
-- resulting from scaling down to a 1x1 image
function avgBrightness theImage
  lock screen; lock messages
  if there is no img ii then create img ii
  set resizeQuality of img ii to "best"
  set width of img ii to the width of img 1
  set height of img ii to the height of img 1
  set imagedata of img ii to the imagedata of theImage
  set width of img ii to 1
  set height of img ii to 1
  put the imagedata of img ii into iData
  delete img ii
  return avg (byteToNum(byte 2 of iData), \
        byteToNum(byte 3 of iData), \
        byteToNum(byte 4 of iData))
  unlock screen; unlock messages
end avgBrightness




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