How to perform Image Subtraction?

K
Posted By
Kewb
Dec 5, 2008
Views
412
Replies
10
Status
Closed
I am using Adobe photoshop CS3. I have two images that are identical aside from one image has a fingerprint mark. i want to remove one image from another to give me just the fingerprint mark. Is this possible?

Any direction or help will be greatly appreciated, thanks in advance.

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DM
dave_milbut
Dec 5, 2008
put them in layers and change the top layer mode to difference.
K
Kewb
Dec 5, 2008
Thank you for your fast reply. i will look into that.

Much appreciated.
DM
dave_milbut
Dec 5, 2008
no prob. 🙂
O
OldBob
Dec 5, 2008
Research for a future CSI episode?
P
PECourtejoie
Dec 5, 2008
Or to have a mask ready, use image>calculations
GH
Gernot_Hoffmann
Dec 5, 2008
Yes, Image > Calculations > Subtract.
Here one can choose Offset (128px) and Scale (e.g. 1).
‘Subtract’ shows the signed difference with respect
to Offset.
‘Difference’ shows only one half of the signed
difference with respect to black (more black is ignored).

Best regards –Gernot Hoffmann
GH
Gernot_Hoffmann
Dec 5, 2008
The fingerprint mark is perhaps not visible
in the medium gray Subtract result. Then use
a steep curve, centered at 128.
G.H.
GH
Gernot_Hoffmann
Dec 6, 2008
The explanation of ‘Difference’ was wrong.
Help text:
"Difference: Looks at the color information in each channel and subtracts either the blend color from
the base color or the base color from the blend color,
depending on which has the greater brightness value.
Blending with white inverts the base color values;
blending with black produces no change."
G.H.
S
Silkrooster
Dec 7, 2008
Thats different that what I was thinking. I would assume white blending with black would show a change. Apparently the base has priority. Huh. Go figure.
GA
George_Austin
Dec 7, 2008
"…I would assume white blending with black would show a change. Apparently the base has priority. Huh. Go figure…"

The Difference blend produces the absolute value of the differnce between top and bottom values. Black’s value is ZERO. Go figure.

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