Re: method to reveal photoshop manipulations

On 26 sep, 15:22, "Eric Miller" <millerericnos...@xxxxxxxxxxxxx>
wrote:
"sobriquet" <dohduh...@xxxxxxxxx> wrote in message

news:1190755485.102473.27070@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

On 25 sep, 22:59, Eric Miller <millereric_nosp...@xxxxxxxxxxxxx>
wrote:
The cyclist is part of both pictures, so it must be part of the
original.

Not true. Because the photos were taken from the same position, the
cyclist could have been added to either photo.

Logic dictates that if one of the two pictures is the original, and
the (same) cyclist is part of both images, it dictates that the
cyclist must have been part of the original image.

That is a truism and it is logical. If a=b and b=c then a=c. The problem
with stating that truism here is that it has nothing to do with the
conclusion that you intend it support. The question is not whether the
cyclist was part of both images; the question is which of the two images was
manipulated.

The question which of the two images was manipulated translates to the
question on which side of the cyclist the white van was in the
original picture. The white van (and it's associated shadows,
reflections and headlights shining on the road) is the only element in
the pictures that is relevant to the question which of the two
pictures has been manipulated and the cyclist isn't (because the
cyclist is part of the original image).


Only the elements
that change between the images as you alternate between the original
and the modified version are potentially not part of the original
image.

Ah, finally, the logical fallacy that is hanging you up here: begging the
question. This begs the question about what has changed between the images,
which is the ultimate question.

No. You can subtract x.jpg from y.jpg in photoshop and that will show
you exactly what has been changed. But it doesn't answer which of the
two pictures is the original and which is the modified picture.
But to answer the question which of the pictures has been modified,
you must examine the changing elements and ignore the identical
elements in both pictures.

I take your word that the changes are as you
describe, but logic does not dictate that result. The cyclist was in the
position portrayed in both your images at one time and an image was
recorded. That cyclist, in that position, could have been added to either
photo.

No, because adding something that is already there doesn't change
anything. The cyclist is part of the original picture, adding it once
more to the modified picture doesn't change anything about that fact.

That modification could have been done to make it appear that the
cyclist had not moved when, in fact, he had.

It would be impossible for the cyclist to move without changing. If
the cyclist had moved and was pasted in to make it seem it hadn't
moved, you could still use subtraction and levels in photoshop to show
the differences between the moved cyclist and the cyclist in the
original position.

In fact, you could have
modified the second image to keep the van and cyclist in the same position.
That fact partly addresses your question regarding the ability to
objectively determine if changes have been made. If a person could not
modify the exif data, but could modify the images, he could modify a
sequence of images taken minutes apart to appear as if nothing happened
when, in fact, a bank robbery (or any other event) took place.

But the exif data and the content of the image can both be edited
independently. I might as well have copied the exif info of the
original picture to the modified picture instead of stripping the exif
data from both the original and the modified picture.
So basically, the exif data has nothing to do with the question of
whether or not the actual content of the image has been manipulated.




The white bus has been edited out from one position and has been
edited in at an adjacent position.

Perhaps, but the "white bus" is from two separate images. This is
apparent by the fact that the occupants of the white bus are in
different positions in both images.

One of the white vans was part of the original picture and the other
has been copied and pasted from a nearly identical picture (where the
van and it's occupants were in a different position).
But as far as I can see there is no infallible argument that implies
which of the two vans has been edited out or was part of the original
picture.

Infallible arguments are few and far between, even when they support a
proposition that is later established as fact.

Well ok, infallible is perhaps too much to ask for, but a plausible
argument maybe. :-)


For instance, some people might falsely assume that since the
van is approaching the photographer, the picture of the van that is
nearest must be the one that has been edited in.

That would be a silly assumption. But many, if not most, assumptions are
silly.





The question is whether the white
bus was on the left or right of the cyclist in the original picture.

But that is a different question than the one that you originally posed and

No, because an answer to that question is exactly an answer to the
question which of the two picture has been modified.

resolves, as a given, that the cyclist has not been moved.

Eric Millerwww.dyesscreek.com

That the cyclist hasn't been moved or modified can be seen visually.
Just subtract x.jpg from y.jpg in photoshop and you will find that the
cyclist is completely black and even the levels command will not be
able to reveal any differences between the cyclist in both pictures,
so that demonstrates it's exactly identical and hence part of the
original picture, regardless of whether it has been added again later
on which wouldn't have changed anything.
It's a bit like having a picture filled with black and filling the
picture with black once again and asking whether or not the picture
has been modified. Since it is still identical to the first picture,
it has not been modified.

.

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