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| Computers Forum Index » Computer - Graphics - Applications (Photoshop) » How to brighten the sides of an image?... |
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| iratecat... |
 Posted: Wed Aug 19, 2009 9:46 pm |
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Guest
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Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
Thank you! |
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| Mike Russell... |
| Posted: Thu Aug 20, 2009 10:37 am |
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On Wed, 19 Aug 2009 14:46:17 -0700 (PDT), iratecat wrote:
Quote: Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
The operation is called flat-fielding and it is commonly discussed in
astronomical groups. Photoshop will not do this operation directly, since
accurate flat-fielding requires arithmetically dividing channel values.
I suggest you try using the free Gimp utility for this task, since it does
support the all-important "divide" layer mode.
In addition to your normal image, take a reference image of a clear area of
slide, de-focusing if necessary to remove significant detail from the
reference image. Stack this as a layer on top of the image you want to
fix, and set its layer mode to "divide". This will have several effects on
the image - the field will be pure white, with no color cast, and the
lighting will be perfectly even, with no vignetting or doming.
For increasing contrast, you may be content with Levels for a while. I
suggest that you consider using curves instead of levels, since curves
provide more precise control over the contrast of individual areas. For
sharpening, unsharp mask is probably the preferred method. If the image is
reasonably sharp to begin with, keep the radius at about .5, and the amount
at 100 percent or more. Both of these operations may be done in either
Photoshop or Gimp.
I can only make an educated guess why this important function is missing:
during the early years of Photoshop, divides were very expensive
computationally. During the mid to late 80's per-pixel divide was
considered to be a show stopper, spoken of in whispers in the image
processing community.
--
Mike Russell - http://www.curvemeister.com |
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| Bob Williams... |
| Posted: Thu Aug 20, 2009 10:41 am |
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iratecat wrote:
Quote: Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
Thank you!
Have you tried the DODGE tool?
OR
Use the circular selection tool...Feather around 25 -50 pixels
Select the bright area.....SELECT > Inverse
IMAGE > Adjust > Levels
Brighten the darker area to suit your taste.
Bob Williams |
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| John Stafford... |
| Posted: Thu Aug 20, 2009 11:10 am |
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Bob Williams wrote:
Quote:
Have you tried the DODGE tool?
OR
Use the circular selection tool...Feather around 25 -50 pixels
Select the bright area.....SELECT > Inverse
IMAGE > Adjust > Levels
Brighten the darker area to suit your taste.
Bob Williams
It is answers like this that make me wish there were a THINK TOOL. Bob,
it is likely the darkened area is a graduated darkness. |
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| Morrie Gasser... |
| Posted: Thu Aug 20, 2009 9:22 pm |
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Guest
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If this is similar to lens vignetting, then photoshop has a lens correction
filter that compensates for this. You adjust the amount of correction and
the portion of the image affected.
A much more complicated alternative, but maybe giving you better results, is
to use a gradient mask. One way to make the mask is to take a picture
through the microscope of a plain white field, and then invert the image so
that black becomes white, etc. Therefore the center will be black and it
will taper off to gray near the edges. Then create an adjustment layer on
top of the image you want to fix, using the levels control, brightening the
whole image so that the edges look as bright as the center of the original
image. Apply the mask to the adjustment layer, which will cause variable
brightening from center to edge. You might have to adjust the transparency
of the mask.
"iratecat" <irate.cat at (no spam) gmail.com> wrote in message
news:cc7556b2-d860-419d-957d-af104b8bd91f at (no spam) a37g2000prf.googlegroups.com...
Quote: Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
Thank you! |
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| Doug McDonald... |
| Posted: Fri Aug 21, 2009 3:20 am |
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Guest
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iratecat wrote:
Quote: Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
Thank you!
Easy.
Go into Quick mask mode and install a gradient mask with a radial gradient.
You will have to guess exactly where to start it.
Then exit quick mask mode and use curves or levels to adjust the outer part.
Doug McDonald |
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| Morrie Gasser... |
| Posted: Mon Aug 24, 2009 3:48 pm |
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Guest
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I've tired using radial masks for this type of thing, but the problem I've
found is that the light fall-off (at least when it's due to vignetting) is
not linear, so no matter where I start the mask I can't quite adjust it to
match my image. This is easily visible if the darkening is in a uniform
area like the sky. Still, it could be a good-enough approximation of this
purpose.
"Doug McDonald" <mcdonald at (no spam) NoSpAmscs.uiuc.edu> wrote in message
news:h6klo2$r03$1 at (no spam) news.acm.uiuc.edu...
Quote: iratecat wrote:
Hello,
I have a series of images, taken with a microscope / camera system,
that I am trying to convert to binary. The problem is, the microscope
lights unevenly, so the center of each image is brighter than the
sides, and this is throwing off the threshold. Is there a way to
selectively brighten the sides of the image, while leaving the center
alone? Color fidelity is not an issue, obviously, but contrast in
edges is important. Simply increasing contrast just merges the entire
sides into big black blobs.
I have access to versions 6.0 and CS, and maybe others (haven't
looked).
Thank you!
Easy.
Go into Quick mask mode and install a gradient mask with a radial
gradient.
You will have to guess exactly where to start it.
Then exit quick mask mode and use curves or levels to adjust the outer
part.
Doug McDonald |
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