> Rafe thanks - I do this sort of thing regularly (shows I am not good at
> taking flat, well-lit shots!). The problem I was discussing arises when
> you get blooming from one scanner exposure to another - then it becomes
> difficult if not impossible to combine them satisfactorily using these
> techniques. The difficulty is that the blooming extends over a small
> "dark" area of the hi-exposure scan that is therefore not covered
> satisfactorily by that scan, but it is also not covered satisfactorily by
> the low- exposure scan (still too dark). You get horrible edges, whether
> you use the manual masking that you describe, or a kind of semi-automatic
> masking such as using one of the image exposures (inverted) as the mask.
> (the last technique was described here some time ago, but I have found
> doing it manually to be better in general).
>
> It is only a problem where your image has very high contrast at an edge,
> not when you get more gradual changes. It is possible that if I were more
>
> careful I might be able to do it better, so that I was using the low
> exposure pass for the edges of the "dark side", which you probably would
> not notice. This I gather is what Dean is doing in his software -----
> which I think would make a great photoshop plug-in.
>
> Dean wrote:
> >Some CCDs feature anti-blooming so that this does not happen. Anyone know
> >if any of the linear CCDs used in scanners have this? A way around this
> >problem is to throw away any pixels above a certain value PLUS its
> >neighbors. These pixels get their values from a lower exposure pass. I
> >have implemented this type of multi exposure for a completely different
> type
> >of application and it works fairly well.
>
> >Afraid this won't be of any use to you - the combining of multiple
> exposure
> >levels was done in a custom application written in C++ running under
> Unix.
> >It was just one small part of a much larger program that I helped write.
>
>
>
Let me describe some of what the software did and how it might relate to
combining images taken with different exposure levels. The software was
using multiple exposure levels to obtain information from scenes containing
extremely high dynamic ranges. In order to capture the entire range, we
would take 3 or 4 sets of images, each with 4 times the exposure of the
previous set. The limitation was haze from the bright reflections swamping
out the dark regions and blooming causing entire regions of the CCD to be
unusable. In order to eliminate this, we would physically block the light
from the very brightest regions while taking the longer exposure images.
Our base image would be the darkest image - the brightest spot would be just
below saturation on the camera. We would then take the next brightest image
and create a mask for all pixels above some threshold level (below
saturation). Because the blooming causes light to spill over from the
blooming pixel to the adjacent pixels, we would perform a dilation operation
(Photoshop grow region?) on the mask. Typically we would increase the size
of the mask by two pixels in all directions. This mask would then be used
to combine the information from the two images. Repeat for each additional
exposure level.
We were not after the image itself, but rather other information within the
set of images. I expect that the hardest part for obtaining a good image
would be matching the intensity levels from one image to the next. You
wanted to increase the exposure time by exactly two stops but the shutter
was may be off by 10-20% If I were writing a program or plugin to do the
combining, I would define some overlap region where I expect pixels from
both images to be reasonably good and to perform a regression on these
pixels to match levels from the two images.
> ----
> Dean Shough
> [EMAIL PROTECTED]
>
