# Re: Fast Algorithm to Determine Average Brightness of an Image

```@BR

I adjusted your function a litte bit (and it works now here).
There is a new parameter for testing the upper/lower half
of the image (this param = empty tests the full image).
The mouseUp is my suggestion for a first testing. On base of
these two values you could develop a decision rule for a
dark/light setting.
[This may be better than simply averaging "sky" and "stones".
For example my first trial for a rule would be
if 0.62*upperBrightness + 0.38*lowerBrightness > 127 then
set totalBrightnes to "light"
else set totalBrightness to "dark"]```
```
on mouseUp
set the paintcompression to RLE
set topleft of img 1 to the bottomleft of me
put the millisecs into m1
put avgBrightness2(the long id of img 1,"upper") into b
set backcolor of grc 1 to (b,b,b)
put b into c
put avgBrightness2(the long id of img 1,"lower") into b
set backcolor of grc 2 to (b,b,b)
put comma & b after c
put c & ": " & (the millisecs - m1) & " ms" into fld 1
end mouseUp

## The adjusted function:

-- theImage = the long id of an img
-- thePart= "upper" or "lower" or empty (or anything else)
function avgBrightness2 theImage, thePart
lock screen; lock messages
put "tmp.42.42" into ii
put the imageData of theImage into iData0
put the width of img 1 into tOrigWidth
if thePart is "upper" then
put (the height of img 1) div 2 into tOrigHeight
put byte 1 to 4*tOrigHeight*tOrigWidth of iData0 into iData0
else if thePart is "lower" then
put (the height of img 1) div 2 into tOrigHeight
put byte -4*tOrigHeight*tOrigWidth to -1 of iData0 into iData0
else
put the height of img 1 into tOrigHeight
end if
repeat while there is an img ii
delete img ii
end repeat
set the paintcompression to RLE -- important for speed
create img ii
set resizeQuality of img ii to "normal" -- "good", "best"
set width of img ii to tOrigWidth   -- may be important*
set height of img ii to tOrigHeight -- may be important*
set imagedata of img ii to iData0
put 8 into newBase -- choose newBase in range 1-64 (watch speed!)
if tOrigWidth > tOrigHeight then
put newBase into newHeight
put trunc(newBase*tOrigWidth/tOrigHeight) into newWidth
else
put newBase into newWidth
put trunc(newBase*tOrigHeight/tOrigWidth) into newHeight
end if
set width of img ii to newWidth
set height of img ii to newHeight
put the imagedata of img ii into iData
delete img ii
put 0 into vRed; put 0 into vGreen; put 0 into vBlue
repeat with i=1 to newHeight
put (i-1)*4 into i0
repeat with j=1 to newWidth
add byteToNum(byte i0+4*j-2 of iData) to vRed
add byteToNum(byte i0+4*j-1 of iData) to vGreen
add byteToNum(byte i0+4*j   of iData) to vBlue
end repeat
end repeat
-- return round((0.1*vRed+0.6*vGreen+0.3*vBlue)/newWidth/newHeight)
return round(avg(vRed, vGreen, vRed)/newWidth/newHeight)
unlock screen; unlock messages
end avgBrightness2

* Your experience of "deleting" or not img "tmp" was an effect of the
resizeQuality setting because you didn't set the tmp-image to the
original's size. This differs not this much from setting the size, but
it differs by a 'jumping' amount for different original's size and
different resizeQuality.

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```