reply to post by FlySolo
The very NATURE of digital photography makes it IMPOSSIBLE to
absolutely PROVE or DISPROVE the authenticity of any single photo.
In the past, I could examine film emulsions, do a gas chromatographic
analysis on the chemicals used in the development process and even do
radiographic dating on the plastics and chemicals used in order
to obtain probable places, dates and manufacturers of the films
and then match specific camera models based upon film sprocket
"teething" patterns (similar to ballistic analysis) in order
to obtain a reasonably sure opinion as to the authenticity of a photo.
Nowadays, I use software-based forensic tools that analyze
the results of specific image processing algorithms to match against
KNOWN image processing programs so that I can find out if any
Digital Signal Processing or Layer Management was completed
on a digital photo.
Since most CMOS cameras uses a Beyer image sensor pattern
to obtain RGB colour information, I can see if that Beyer
or other interleaved or parallel quadrants image sensor pattern
has been disrupted by pixel-based editing such as addition or erasure.
I can also do an image sampling analysis to see if certain
artificial numeric patterns have been injected into an image.
These usually have to do with mathematical concepts such
as Fourier transforms, inverse-squared, square-root,
and uniform numeric distributions that would NOT normally be
found within an unretouched digital photograph!
I can also match gamma-correction curves to specific camera models,
image sensors and image retouching applications and of course match
the JPEG/Wavelet/Fractal artifacting to specific VERSIONS of
individual image compression algorithms when can THEN be matched
to specific software to indicate to me which photos are fake or "Real".
I can also analyze lightwave paths and shadow falloffs to see if
they are artificial or natural (i.e. check for common logarithmic values)
HOWEVER, someone with sufficient EXPERTISE (such as ME!...or...
someone high up at the NRO - National Reconnaissance Office)
could use high end CCD (Charge Coupled Device) linear RGB colour
image sensors using uncompressed capture and using a CUSTOM pixel
processing program to FAKE an image by countering the VERY image
analysis techniques I usually use to catch the Photoshoppers.
All I can do is provide a PROFESSIONAL OPINION which
CAN STILL BE WRONG because SOMEONE MOGHT BE WILLING ENOUGH
or or has enough TIME to make that fake image good enough
to PASS even the HIGH END image analysis tools I use!
I can only give a PROBABILITY of "Real-ness" and
NOT ANY ABSOLUTE YES OR NO answer
as to the authenticity of a photo.
And to also point out, it is STILL POSSIBLE to use LIVE STAGE EFFECTS
to fake a scene ENOUGH that it requires NO DIGITAL RETOUCHING !!!
Thus the Digital Photo itself may be "Real", but the subject matter
may NOT be!