Hi Niranjan,
hopefully someone else will provide a more enlightened answer. I
tried the method you suggested on a couple of images. With a 'normal' image
it blew away the whites by increasing contrast. With an extremely hazy
landscape image taken with a long lens in a dusty desert it did a nice job
of restoring the contrast of the image. However, the haze removal module
did as good or better job with this hazy image when I used a very high
strength value.
The user manual for DT has this to say about the blend mode of multiple:
multiplyMultiply the pixel values of the input and output together. When
blending in display-referred color spaces, pixel values are between 0 and
1.0, the final output will be clamped and will always be darker. When
blending in the “RGB (scene)” color space, this value is further multiplied
by a value proportional to the “blend fulcrum”. In this case, values may be
greater than 1.0 and therefore brighten the base image. This may have other
side-effects, such as updating the white point in the filmic module.
Multiply blending simulates an optical variable density filter, where the
density is defined by the output of the module. It has many applications,
from blooming and local contrast enhancements (when used with a blur or
low-pass filter) to dodging/burning and global contrast enhancements (when
used with exposure). The fulcrum sets the output intensity threshold
between darkening and brightening (any RGB value below fulcrum will darken).
On Tue, 17 Aug 2021 at 16:49, Niranjan Rao <[email protected]> wrote:
> Greetings,
>
> On darktable 3.6.0
>
>
> I saw this in one of the videos on YouTube and trying to understand what
> exactly is happening. The technique used was increase the exposure to
> about 2.5 or 3 and then change the blend mode to multiplication.
>
> Quality of the image seems to be improving a lot and lot more details
> not visible in default setting of exposure seems to be coming up.
> Notably haziness of the image reduces dramatically.
>
> Multiplication mode is listed under darken modes so it makes sense to
> increase the exposure before darkening to keep the brightness of image
> about same. But I'm not able to figure out why haziness of the image
> gets cleared up.
>
>
> Any helpful hints will be really appreciated.
>
>
> Regards,
>
>
> Niranjan
>
>
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--
Dr Terry Pinfold
Cytometry & Histology Lab Manager
Lecturer in Flow Cytometry
University of Tasmania
17 Liverpool St, Hobart, 7000
Ph 6226 4846 or 0408 699053
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