Standard "unit" for -evaluate Gaussian-noise[SOLVED]

[Toki]

Hi everybody,

I'm using -evaluate Gaussian-noise to add noise to some images like this:

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convert ./woman_blonde.tif -evaluate Gaussian-noise 5 ./woman_blonde_50.tif 

And this works fine, it adds the noise and I can control the amount of noise added.

Problem is I'm unsure what exactly the 5 in

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-evaluate Gaussian-noise 5

does.
What is 5? Does it mean 5%?

In gimp you can add Gaussian blur, they control the intensity of it by having a standard deviation that ranges from 0 - 50. does ImageMagick use something similar?

Thanks,
Yathindu

[fmw42]

It is a gain or attenuation factor for noise amplitude. It is the same as using -attenuate 5 +noise Gaussian.

http://www.imagemagick.org/script/comma ... p#evaluate

http://www.imagemagick.org/script/comma ... #attenuate

http://www.imagemagick.org/script/comma ... .php#noise
http://www.imagemagick.org/Usage/canvas/#random

So 1 is the equivalent of 100% in GIMP, I would presume. So if you want to halve the amplitude in IM it would 0.5 and in GIMP it would be 50%, I presume. But it is just a relative factor. Base amplitude in each may not be equivalent. So you may not get the exact same amplitude of noise in the corresponding cases.

Also note there is in IM -seed, which sets the seed for the randomness. If you want a reproducible result, you should set some seed value otherwise, you will get a different seed value by default if it is left off.

see

http://www.imagemagick.org/script/comma ... s.php#seed

[Toki]

Thanks for the quick reply. I didn't know about seed.

Problem is when I try:

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convert ./foo.tif -evaluate Gaussian-noise 1 ./fooNoise.tif

The generated image has no noise visible at all.
foo.tif -
fooNoise.tif -

However, if I use

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convert ./foo.tif -evaluate Gaussian-noise 2 ./fooNoise.tif

:
fooNoise.tif -
Now there a little bit of noise visible.

It doesn't seem that 1 is the maximum noise. If it matters foo.tif has a pixel depth of 1.

[fmw42]

It doesn't seem that 1 is the maximum noise. If it matters foo.tif has a pixel depth of 1.

It is not. You can increase or decrease the value. "Maximum noise" must be an error or it has been changed since that documentation.

[snibgo]

"1" certainly isn't the maximum. For Gaussian noise, I use the rule of thumb that 100 totally obscures the image. So it's a bit like a percentage, although visually it isn't linear.

[Toki]

Thanks for the clarification guys!