Martin Brown wrote:
On 19/09/2012 22:44, Alan Browne wrote:
On 2012.09.19 03:13 , Martin Brown wrote:
On 18/09/2012 23:52, Alan Browne wrote:

Raise doubts. It's claim is to look at how images are made (signature)
by the camera. If there is a doubt it will be raised. A change to an
image in PS would not pass that.

Only a cack handed amateur would do it that way. It really isn't that
difficult to transplant an arbitrary JPEG stream into a given cameras
signed envelope. Anyone that relies on this tool is an idiot.

Turn off your assumptions. By signature they are looking at what are
essentially artifacts of how various cameras generate their output.

Perhaps "fingerprint" would be a better term.

Certainly if you are working in their firm's marketing department.

There are a handful of independent JPEG implementations - most follow
the original spec closely enough that there is little or no distinction
between them (apart from in PSPro 8 which contained gross errors). It
has to be like that or you would see much worse artefacts if some JPEG
encoders made significant mistakes (as in fact happened with PsP 8).

The only real variation is the exact choice of quantisation table and
Photoshop is distinctive there, but most of the rest use a scaled
version of the canonical JPEG standard Qtables from the original spec.

So there's no variation in noise between camera types, sensor
technology and pixel sizes? There's no different JPEG denoising
between cameras?

-Wolfgang