photoshop parameters not good on graphicsmagick - image-processing

Question

I'm trying to translate a photoshop setting for sharpening images to graphicsmagick. Therefore I found this helpful article:
https://redskiesatnight.com/2005/04/06/sharpening-using-image-magick/
The problem is that if I use to photoshop equivalent values explained in the article in graphicsmagick the images are not so sharp and clear like on photoshop.
For example I use this settings on photoshop:
Strength: 500%
Radius: 2.0 Pixel
Threshold: 8
In the article the parameters are explained like this:
The radius parameter
The radius parameter specifies (official documentation)
“the radius of the Gaussian, in pixels, not counting the center pixel”
Unsharp masking, like many other image-processing filters, is a
convolution kernel operation. The filter processes the image pixel by
pixel. For each pixel it examines a block of pixels surrounding it
(the kernel) and does some calculations on them to render the output
pixel value. The radius parameter determines which pixels surrounding
the center pixel will be considered in the convolution kernel: (think
of a circle) the larger the radius, the more pixels that will need to
be processed for each individual pixel.
Image Magick’s radius is similar to the same parameter in Photoshop,
GIMP and other image editors. In practical terms, it affects the size
of the “halos” created to increase contrast along the edges in the
image, increasing acutance and thus apparent sharpness.
How do you know how big of a radius to use? It depends on your output
target resolution, for one thing. It also depends on your personal
preferences, as well as the specific needs of the image at hand. As
far as the resolution issue goes, the GIMP User Manual recommends that
unsharp mask radius be set as follows:
radius = (output ppi / 30) * 0.2 Which is very similar to another commonly found rule of thumb:
radius = output ppi / 150 So for a monitor with 72 PPI resolution, you’d use a radius of approximately 0.5; if your targeting a printer
at 300 PPI you’d use a value of 2.0. Use these as a starting point;
different images have different sharpening requirements, and
individual preference is also a consideration. [Aside: there are a few
postings around the net (including some referenced in this article)
that suggest that Image Magick accepts, but does not honor, fractional
radii; that is, if you specify a radius of 0.5 or 1.2 it is rounded,
or defaults to an integer, or is silently ignored, etc. This is not
true, at least as of version 5.4.7, which is the one that I am using
as I write this article. You can easily see for yourself by doing
something like the following:
$ convert -unsharp 1.2x1.2+5+0 test.tif testo1.tif $ convert -unsharp
1.4x1.4+5+0 test.tif testo2.tif $ composite -compose difference test01.tif testo2.tif diff.tif $ display diff.tif you can also load
them into the GIMP or Photoshop into different layers and change the
blend mode to “Difference”; the resulting image is not black (you may
need to look closely for a 0.2 difference in radius). No, this
mistaken impression likely comes from the fact that there is a
relationship between the radius and sigma parameters, and if you do
not specify sigma properly in relation to the radius, the radius may
indeed be changed, or at least not work as expected. Read on for more
on this.]
Please note that the default radius (if you do not specify anything)
is 0, a special value which tells the unsharp mask algorithm to
“select an appropriate value for the radius”!.
The sigma parameter
The sigma parameter specifies (official documentation)
“the standard deviation of the Gaussian, in pixels”
This is the most confusing parameter of the four, probably because it
is “invisible” in other implementations of unsharp masking, and it is
most sparsely documented. The best explanation I have found for it
came from a google search that unearthed an archived mailing list
thread which had the following snippet:
Comparing the results of
convert -unsharp 1.2x1+4+0 test test1.2x1+4+0
and
convert -unsharp 30x1+4+0 test test30x1+4+0
results in no significant differences but the latter takes approx. 50 times
longer to complete.
That is not surprising. A radius of 30 involves on the order of 61x61
input pixels in the convolution of each output pixel. A radius of 1.2
involves 3x3 or 5x5 pixels.
Please can anybody give me any hints, what 'sigma' means?
It describes the relative weight of pixels as a function of their
distance from the center of the convolution kernel. For small sigma,
the outer pixels have little weight. Another important clue comes from
the documentation for the -unsharp option to convert (emphasis mine):
The -unsharp option sharpens an image. We convolve the image with a
Gaussian operator of the given radius and standard deviation (sigma).
For reasonable results, radius should be larger than sigma. Use a
radius of 0 to have the method select a suitable radius.
Combining the two clues provides some good insight: sigma is a
parameter that gives you some control over how the strength (given by
the amount parameter) of the sharpening is “graduated” or lessened as
you radiate away from a given pixel at the center of the convolution
matrix to the limit defined by the radius. My testing confirms this
inferred conclusion, namely that a bigger sigma causes more pronounced
sharpening for a given radius. That is why the poster in the mailing
list question (above) did not see any significant difference in the
sharpening even though he was using an amount of 400% (!!) and a
threshold of 0%; with a sigma of only 1.0, the strength of the filter
falls off too rapidly to be noticed despite the large difference in
radius between the two invocations. This is also why the man page says
“for reasonable results, radius should be larger than sigma”; if it is
not, then the sigma parameter does not have a graduated effect, as
designed, to “soften” the halos toward their edges; instead it simply
applies the amount evenly to the edge of the radius (which may be what
you want in some circumstances). A general rule of thumb for choosing
sigma might be:
if radius < 1, then sigma = radius else sigma = sqrt(radius) Summary:
choose your radius first, then choose a sigma smaller than or equal to
that. Experimentation will yield the best results. Please note that
the default sigma (if you do not specify anything) is 1.0. This is the
main culprit for why most people don’t see as much effect with Image
Magick’s unsharp mask operator as they do with other implementations
of unsharp mask if they are using a larger radius: unless you bump up
this parameter you are not getting the full benefit of the larger
radius!
[Aside: you might be wondering what happens if sigma is specifed
larger than the radius. The answer, as the documentation states, is
that the result may not be “reasonable”. In my testing, the usual
result is that the sharpening is extended at the specified amount to
the edge of the specified radius, and larger values of sigma have
little if any effect. In some cases (e.g. for radius < 0) specifying a
larger sigma increased the effective radius (e.g. to 1); this may be
the result of a “sanity check” on the parameters in the code. In any
case, keep in mind that the algorithm is designed for sigma to be less
than or equal to the radius, and results may be unexpected if used
otherwise.]
The amount parameter
The amount parameter specifies (official documentation)
“the percentage of the difference between the original and the blur
image that is added back into the original”
The amount parameter can be thought of as the “strength” of the
sharpening: higher values increase contrast in the halos more than
lower ones. Very large values may cause highlights on halos to blow
out or shadows on them to block up. If this happens, consider using a
lower amount with a higher radius instead.
amount should be specified as a decimal value indicating the
percentage. So, for example, if in Photoshop you would use an amount
of 170 (170%), in Image Magick you would use 1.7.
Please note that the default amount (if you do not specify anything)
is 1.0 (i.e. 100%).
The threshold parameter
The threshold parameter specifies (official documentation)
“as a fraction of MaxRGB, needed to apply the difference amount”
The threshold specifies a minimum amount of difference between the
center pixel vs. sourrounding pixels in the convolution kernel
necessary to apply the local contrast enhancement. Increasing this
value causes the algorithm to become less sensitive to differences
that may define edges. Specifying a positive threshold is often used
to avoid sharpening smooth areas that may contain noise (e.g. an area
of blue sky). If you have a noisy image, strongly consider raising the
threshold, or using some kind of smart sharpening technique instead.
The threshold parameter should be specified as a decimal value
indicating this percentage. This is different than GIMP or Photoshop,
which both specify the threshold in actual pixel levels between 0 and
the maximum (for 8-bit images, 255).
Please note that the default threshold (if you do not specify
anything) is 0.05 (i.e. 5%; this corresponds to a threshold of .05 *
255 = 12-13 in Photoshop). Photoshop uses a default threshold of 0
(i.e. no threshold) and the unsharp masking is applied evenly
throughout the image. If that is what you want you will need to
specify a 0.0 value for Image Magick’s threshold. This is undoubtedly
another source of confusion regarding Image Magick’s sharpening
algorithm.
So I did it like than and come up to this command:
gm convert file1.jpg -unsharp 2x1.41+5+0.03 file1_2x1.41+5+0.03.jpg
But like I said the images does not get that much sharpen like in photoshop. We also experimented with a lot of other values but without good images. So is it possible to do photoshop sharpening stuff with graphicsmagick? Or is it just a not good library? The main problem of just using photoshop for sharpenings is that we want to improve the images on our linux server and photoshop is not really good running on linux.