darkening around the edges

[Brian Cluff]

Here's some specifics of the blend modes.
https://en.wikipedia.org/wiki/Blend_modes
http://www.linuxtopia.org/online_books/graphics_tools/gimp_advanced_guide/gimp_guide_node55_002.html

For your example pictures, screen will over brighten the darker areas of 
your picture, so dodge produces a better result.  In a different 
picture, screen could be the better choice.

Brian

On 03/23/2016 12:56 PM, Michael wrote:
> please, what is the difference between the two modes?
>
> On Wed, Mar 23, 2016 at 2:35 PM, Brian Cluff <brian at snaptek.com
> <mailto:brian at snaptek.com>> wrote:
>
>     Oops, use the Dodge layer mode instead of screen.  You'll get better
>     results.
>
>     Brian
>
>
>     On 03/23/2016 11:22 AM, Brian Cluff wrote:
>
>           > On 03/23/2016 04:49 AM, Michael wrote:
>           > Thanks Brian. I am putty in your hands!
>
>         I get the bounces from the PLUG mailing list, so I got your
>         images even
>         though they were too big for the queue... nobody else did
>         though, so my
>         answer will be the only one based off your image.
>
>         Anyway, looking at your pictures, it appears that the darkened edges
>         aren't due to fading over time rather they are caused by what
>         appear to
>         be a flash going off that lit the center of the image more than the
>         edges. This can also be caused of lower quality lenses giving you
>         darkened edges in what is called a vignette.
>
>         If you want to try and reduce/eliminate it, in gimp you would add a
>         layer above your picture, but before you do that, if you have the
>         option, go to Image -> Precision and set it to 16 ot 32 bit, so
>         that you
>         have more color data to work with.  Unfortunately you'll only
>         have that
>         option if you are running the development version of GIMP, but
>         it will
>         still work OK if you can't do that setting. Next use the
>         gradient tool,
>         changing the gradient type to radial, and make a gradient that
>         goes from
>         black in the center fading to white at the corners. (draw the
>         gradient
>         from the center to one of the corners).
>
>         Then set your new gradient layer's mode to "screen".
>
>         At this point the effect will probably be too strong, so you
>         will need
>         to play with the opacity till it looks better and you can also
>         play with
>         the color curves moving the line on the lower left back and
>         forth across
>         the bottom to adjust the shape of the gradient till it better
>         matches
>         the vignette in your image.  You can change the gradients layer mode
>         back to normal to get an idea of what the color curve adjustment is
>         doing to the gradient.
>
>         Brian Cluff
>
>         On 03/22/2016 06:42 PM, Brian Cluff wrote:
>
>             Can you send an example so that I can see what you mean by
>             "darkened
>             around the edges"?
>
>             I'll see if I can easily correct it and then send
>             instructions on what I
>             did.
>
>             Brian Cluff
>
>             On 03/22/2016 04:18 PM, Michael wrote:
>
>                 how does one use a transparent layer? is it only for
>                 drawing things on?
>                 I have some old pictures that have darkened around the
>                 edges. How I
>                 think I can fix it is to duplicate the layer then
>                 lighten the exposure
>                 value on the lower image, then paint black and uncover
>                 the lighter image
>                 over the section that has darkened. How do I do this
>                 with gradients and
>                 Darktable?
>
>                 --
>                 :-)~MIKE~(-:
>
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