You could use a cli php script and the libgd support to average out all
the pixels. And produce an average of a sample of the pixels.
Shouldn't be too difficult to code if processing power isn't an issue..
Simple to code as well. I did something similar a while ago for a
simple 'machine vision' experiment..
imagecolorat() will return the RGB value of a single pixel, and you can
load the image into memory with imagecreatefromjpeg() or similar...
I use php/libgd for quite a bit of image work, and it handles all my
thumbnailing, watermarking and proof printing requirements just fine,
although on 16 and 8 Mp images it's a bit rough on processor load for
the initial scaling process, so I scale all the images down using
imagemagick and a n execute() first...
Cheers, Me.
Andrew Errington wrote:
Hi,
Can anyone comment knowledgably on determining the light level in a JPG
image snapped by a webcam?
I have the simple requirement of taking hourly snapshots from my webcam in
the back garden and posting them on my webpage. At the moment I have a
cron job, which I tweak during the year to adjust when it takes a picture
(i.e from 8 to 5 in winter to 5 till 10 in summer), but really it should be
automated. I could rig up a light sensor, and the PC could query it before
taking a webcam image, but really, the webcam itself is a light sensor
(duh!).
I am thinking of the imagemagick 'identify' command, and maybe extracting
the mean value of the red, green and blue channels and comparing them to a
hard limit. Does this idea have merit, or is there a better tool?
I could also just look at the file size: at night the images are mostly
black, which compresses to a very small file size.
Comments are welcome,
Thanks,
Andrew
