Re: uneven brightness correction


<alexxx.magni@xxxxxxxxx> wrote in message
news:1152266621.939423.243080@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hello again,
thank you everybody for your help.

Roger, 'flat fielding' must be in astronomy what in microscopy is
termed 'shading correction', and I agree that this is THE preliminary
step. In fact, the starting images were taken with shading correction
on. The uneven brightness, unfortunately, is possibly a remaining
artifact.
Seriously, it should correct it completely.
What you are descibing, is what in astronomy, is commonly referred to as
'vignetting', and a flat field will correct this totally. However it is
not something that can be done as a 'turned on' operation as your software
supports, but something needing a fairly major pre-processing step.
Typically, ten or more flat field images need to be taken, to minimise the
effect of noise in the imaging system, of a white surface or an artificial
flat field can be generated, by using software, and manually selecting
points of equal final intensity (but this is a less accurate system).
Unfortunately the images you have posted, are not openable in Adobe, or
any of the other image packages that I have.

aruzinsky, "dodge&burn" is undoubtly an interesting step to make, but I
hoped to find something at least semi-automatic: I have a lot of
images!

Steven: RoboRealm looks very promising, but I was unable to install it:
I had problems with the required FreeImage program: I cannot compile
the source (don't have a C compiler handy), while I don't know where to
put the DLL files of the alternate package.

everything considered, I still think that the most promising way is
that of using a gradient or similar on some "alpha" channel, and to
base the brightness correction on that. Unfortunately I don't know
almost anything concerning how to use alpha channels...
A flatfield, applies an inverse alpha gradient. Where it is brightest, you
have to dim the final image the most.

Best Wishes


.

Relevant Pages

  • Re: New telescope technology
    ... really AO, because the actual light for correction, comes sufficiently far ... I wouldn't say there will never be such an amateur system, ... With this, perhaps 300 images are taken in rapid succession, ... On 'multiple scopes', many published images, are the result of combining ...
    (uk.sci.astronomy)
  • Re: Problem with digital imaging on scope
    ... Is this Leitz eyepiece correction explained somewhere online? ... "Leitz Plano Objectives 'Plan' New series of ... Periplan GF or GW eyepieces for getting a flat field, ...
    (sci.techniques.microscopy)
  • Re: how far is the horizon and OT photography
    ... the curve to straight lines is barrel distortion and causes ... use the lens uncorrected. ... distortion correction software. ... If you're going for full frame and>20MP images you're probably also ...
    (uk.rec.walking)
  • Re: Which hurts images more, shift lenses or correcting perspective in software?
    ... bought a TSE lens and a shift adapter for my MF lenses. ... But if you do the vertical convergence correction thing with perfect ... of the resolution you paid for, and can barely make web-quality images. ...
    (rec.photo.digital.slr-systems)
  • Re: Innocent Download of kp
    ... CORRECTION TO MY PREVIOUS POST ... If the production of child abuse images is illegal (which I ... there's no connection between what I said & your conclusion that the people who sell images of abuse would *have* to sell it to an increasing customer base. ...
    (uk.legal)
Quantcast