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Originally Posted by Don Hoey
Referring to a comment by Christine on file size, I most certainly cannot understand this. BLT image I have posted before file size 144.1kb. Wanting to leave it filed where it is I opened it and did ' save a copy as '. New file size 419.2kb. Wierd. Perhaps need techie input on this .... ROB
Don
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Don
A couple of things are happening here. Your file contains image data and what is called meta data. Lets deal with the easy bit first. Meta data is mainly text that contains such things as your camera Exif info, the time and date when the file was edited, the names of the programs that have edited the file and so on. In recent versions of programs data is included to provide evidence ( for legal purposes) that an image has been tampered with by editing. So each time you save a bit more info is tagged on.
The other bit of file size change relates to the jpeg encoding and decoding. A lot of current technology is based on mathematics developed 200 yrs ago. A group of mathematicians in france ( namely fourier and laplace ) developed the concept of transforms. Without getting too involved with the complexities lets convert something that is difficult to work with into something that is simpler to work with.
e.g if we take a heap of iron ore and process it ( transform ) into uniform steel rods. We can work with the steel rods. Then if nature takes it's course the rods will revert back ( transform ) to the heap of iron ore.
In the case of jpeg the concept is somewhat more difficult to grasp if your are not from these disciplines. Fourier came up with the concept that any waveform ( think of a sound wave of music ) can be made of a number of component sine waves of different frequencies and amplitudes.
So the theory is a sound can be dismantled and reassembled using these components.
In the case of a picture we are converting data from a space domain into a time domain. If that statement has just made your brain blow a fuse no worries it's not an easy subject.
The point is that calculations are performed on the image to encoded and decode the data. The result will not be an exact fit into one of the 256 binary values in the image. This is one of the minor drawbacks of the digital world is the result has to be rounded up or down to fit.
e.g 106.86568 will be rounded to 107
this rounding produces what is know as digital noise.
This effect is one of the factors that contribute to image noise.
The other being produced by the camera sensor due to heat (thermal shot noise) and you could include gamma rays from outter space !!
This low level of noise added to the image effectively increases the number variations in image data and thereby reduces the compression efficiency.
The highest efficiency is achived if the whole image contained the same data value. A very boring image at that
One of the major benefits of image noise reduction programs is the smaller file sizes produced.
I hope that was not too heavy for you folks but I did try to make it simple.