Chroma-Key and DSLR image editing

Archived from groups: rec.photo.digital.slr-systems (More info?)

"Alan Browne" <alan.browne@freelunchVideotron.ca> wrote in message
news 4jt8v$7va$1@inews.gazeta.pl...
>
> Having spent over two hours today retouching images from a local fashion
> show with an atrocious background, I'm determined to avoid that in the
> future as well as be able to slip in some more interesting BG's.
>
> Has anyones used Chroma-key BG's (blue, green, red) and done post prod
> work this way in PS? Love to hear about your experience.
>
> In particular I'm concerned with how evenly lit the Chroma key BG has to
> be lit behind the subject.
>
> Cheers,
> Alan
> --
Chroma key has to be the exact colour value across the entire field. I
played with it back in 1998-99 with video. Any shadow or hotspot will not
drop out, much like the Photoshop magic wand tool when you get other values.

Archived from groups: rec.photo.digital.slr-systems (More info?)

It's not that difficult to do In Photoshop if the background colour
doesn't clash TO much with the foreground. It doesn't even have to be
perfectly evenly lit either. A simple colour selection (Select/Color
Range) to get the initial selection works fine. You might need to play
with the Fuzziness bar and maybe add some more colours to the selection
if the background isn't that evenly lit (Shift/Click on colour).

Once you have the selection, you can fine tune it by switching to Quick
Mask mode. From that selection it's easy to drop in another background.
Good luck!

Archived from groups: rec.photo.digital.slr-systems (More info?)

"bH" <hein.dejager@gmail.com> wrote in message
news:1114512498.408024.323430@z14g2000cwz.googlegroups.com...
> It's not that difficult to do In Photoshop if the background colour
> doesn't clash TO much with the foreground. It doesn't even have to be
> perfectly evenly lit either. A simple colour selection (Select/Color
> Range) to get the initial selection works fine. You might need to play
> with the Fuzziness bar and maybe add some more colours to the selection
> if the background isn't that evenly lit (Shift/Click on colour).
>
> Once you have the selection, you can fine tune it by switching to Quick
> Mask mode. From that selection it's easy to drop in another background.
> Good luck!

Great and how would you like to do that to 200+ shots against the clock?

Archived from groups: rec.photo.digital.slr-systems (More info?)

"canongirly" <me@me.com> wrote in message
news 4lipa$rjr$1@news8.svr.pol.co.uk...
>
> "bH" <hein.dejager@gmail.com> wrote in message
> news:1114512498.408024.323430@z14g2000cwz.googlegroups.com...
>> It's not that difficult to do In Photoshop if the background colour
>> doesn't clash TO much with the foreground. It doesn't even have to be
>> perfectly evenly lit either. A simple colour selection (Select/Color
>> Range) to get the initial selection works fine. You might need to play
>> with the Fuzziness bar and maybe add some more colours to the selection
>> if the background isn't that evenly lit (Shift/Click on colour).
>>
>> Once you have the selection, you can fine tune it by switching to Quick
>> Mask mode. From that selection it's easy to drop in another background.
>> Good luck!
>
> Great and how would you like to do that to 200+ shots against the clock?

Trained monkey's will probably help here.

Archived from groups: rec.photo.digital.slr-systems (More info?)

Alan Browne wrote:
>
> Having spent over two hours today retouching images from a local fashion
> show with an atrocious background, I'm determined to avoid that in the
> future as well as be able to slip in some more interesting BG's.
>
> Has anyones used Chroma-key BG's (blue, green, red) and done post prod
> work this way in PS? Love to hear about your experience.
>
> In particular I'm concerned with how evenly lit the Chroma key BG has to
> be lit behind the subject.
>
> Cheers,
> Alan

It's not as easy as it sounds. I have a green Chroma-key background that
I use for portraits. I like to cut it out and create my own digital
backgrounds. Here are the problems I have regularly encountered:

You have to make sure the subject is far enough from the background so
that no reflected green light bounces off the background onto the
subject. This can give the sides and edges of your subject a green tint.
That is hell to correct. Oddly the places you need this background the
most never seem to have enough room to do this.

Blond hair is no fun. It's light and wispy and lets lots of the
background through. The worst is that it partially lets the green
through. So, you have to have to get rid of part of the green without
getting rid of the hair. Dark hair isn't nearly the same problem.

I've tried a ton of ways to select the green for deletion. The best for
me is to make a mask starting with a channel. I seem to pick the "a"
channel of LAB most of the time. I then manually edit the mask. I crank
up the contrast to separate the subject and background as much as
possible. The key is to watch the hair parts so you get as much
transparency as possible and still get the green. You will probably have
to try a number of times to get it the best you can.

Usually that doesn't get rid of all the green in the hair. I can either
brush on the mask to make it more transparent or I can brush to tint the
hair back to it original color. Neither is perfect and both are often used.

I usually have to edit the mask to clean up the black and the white. The
LAB "a" isn't perfect. If you don't clean it to pure black and white
(except for the edges) you will have a lot of partially transparent
spots in you picture. Part of the reason for this is that the green
usually isn't evenly lit. If it's far enough away to not bounce light,
it probably isn't inside your lighting setup. Actually, it shouldn't be
or you will get green bounce.

Another reason for this is that people's clothes aren't as monochromatic
as your eye sees them. You tell them to not wear green and they don't.
However, Photoshop sees a touch of green in brown, yellows, and other
colors. This is a big reason for making the mask rather than other
selection tools. [The first time I got this, I put my background back
there and got a bunch of spots of that color popping through. Funny only
for a few seconds.]

When I get it, it looks pretty good. My clients have been happy with the
results so far. I do get a bride that has had that during the engagement
portrait and wants it at her wedding. I have talked them out of it. I
can't imagine the trouble of cutting the green coming through a bridal
veil. White dresses should pick up even faint green bounce.

I don't use my green any more than I really have to. It's a lot of work
to do all of this. I never use this technique on a whole batch of
pictures. It would take forever. When you need it, it is very nice to have.

I have never found anyone with a better way of doing it. Well, I have
heard there are some very high end pro tools that make it much easier.
You will pay through the nose for these, but if you will do a lot of
these they may be worth it.

If you are shooting a bunch of fashion shots I would seriously look at
paying the money. A green background will be better than nothing, but
you will still spend way more time on each picture than the high-speed
turn-around the fashion industry demands. I wouldn't count on Photoshop
being the answer here.

The other problem with fashion is that you can't control what colors the
clothes are. You are likely to get color the same as your background.

Clyde

Archived from groups: rec.photo.digital.slr-systems (More info?)

Clyde wrote:

> Alan Browne wrote:
>> In particular I'm concerned with how evenly lit the Chroma key BG has
>> to be lit behind the subject.
>
> It's not as easy as it sounds. I have a green Chroma-key background that
> I use for portraits. I like to cut it out and create my own digital
> backgrounds. Here are the problems I have regularly encountered:
>
> You have to make sure the subject is far enough from the background so
> that no reflected green light bounces off the background onto the
> subject. This can give the sides and edges of your subject a green tint.
> That is hell to correct. Oddly the places you need this background the
> most never seem to have enough room to do this.

The place that gave me the most fits would have been difficult to light
the BG evenly in the first place. OTOH, it was a fashion show and I had
fairly flat lighting on the subjects so I think that would have
clobbered the blue/green back reflection.


>
> Blond hair is no fun. It's light and wispy and lets lots of the
> background through. The worst is that it partially lets the green
> through. So, you have to have to get rid of part of the green without
> getting rid of the hair. Dark hair isn't nearly the same problem.

I had that same problem in any case. The scene had a dark brown beam
across the back that in some shots crossed the head line. PS'ing it was
an iron clad b*****.

>
> I've tried a ton of ways to select the green for deletion. The best for
> me is to make a mask starting with a channel. I seem to pick the "a"
> channel of LAB most of the time. I then manually edit the mask. I crank
> up the contrast to separate the subject and background as much as
> possible. The key is to watch the hair parts so you get as much
> transparency as possible and still get the green. You will probably have
> to try a number of times to get it the best you can.

Ugh.

>
> Usually that doesn't get rid of all the green in the hair. I can either
> brush on the mask to make it more transparent or I can brush to tint the
> hair back to it original color. Neither is perfect and both are often used.

Hmm. I'm less and less enthralled.

<snipped>
> I don't use my green any more than I really have to. It's a lot of work
> to do all of this. I never use this technique on a whole batch of
> pictures. It would take forever. When you need it, it is very nice to have.

I think I'm now convinced that it will be cheaper (in time) to rent a
backdrop next time I'm stuck with a bad scene. The next show I'm doing
(same client) will be outdoor, and hopefully I can control the scene
better by allowing elements in it that are not distracting... that are
natural for the event.


> The other problem with fashion is that you can't control what colors the
> clothes are. You are likely to get color the same as your background.

Yep. And esp. this year cool greens and blues seem to be in everything.

Thanks for much for your detailled reply.

Cheers,
Alan


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Archived from groups: rec.photo.digital.slr-systems (More info?)

On Sat, 30 Apr 2005 10:39:08 -0500, Clyde <clyde@world.comedy> wrote:

>Alan Browne wrote:
>>
>> Having spent over two hours today retouching images from a local fashion
>> show with an atrocious background, I'm determined to avoid that in the
>> future as well as be able to slip in some more interesting BG's.
>>
>> Has anyones used Chroma-key BG's (blue, green, red) and done post prod
>> work this way in PS? Love to hear about your experience.
>>
>> In particular I'm concerned with how evenly lit the Chroma key BG has to
>> be lit behind the subject.
>>
>> Cheers,
>> Alan
>

The CS version of Photoshop (and presumably the new CS2 version) has
an "Extract" feature which does an outstanding job of preserving the
fine lines of hair and other wispies against the chroma background.
In fact, I have had great results using a green screen made from some
yard goods bought at the local super center.

I use this approach to take shots of students at school events (proms,
awards nights, etc) and then drop in a school logo as the background.
I'm a long way from being a high-end photographer, but the results are
pretty stunning.

Using the feature in PS couldn't be easier: you just highlight the
area of the picture that is the boundary between the subject and the
BG, then highlight the area you want to preserve. PS does the work.

Come to think of it, you might want to give this whole process a try
with your "atrocious background" from your fashion show, and compare
PS results with your hand-retouched version; I'd be interested to know
how the two compare.

HTH

Archived from groups: rec.photo.digital.slr-systems (More info?)

Bud Evers wrote:

> Using the feature in PS couldn't be easier: you just highlight the
> area of the picture that is the boundary between the subject and the
> BG, then highlight the area you want to preserve. PS does the work.
>
> Come to think of it, you might want to give this whole process a try
> with your "atrocious background" from your fashion show, and compare
> PS results with your hand-retouched version; I'd be interested to know
> how the two compare.

First off I have Elements 3.0, and using things like the magic wand with
the threshold carefully set cannot do a good job of separarion of the
model from even a somewhat even BG. Where the model has shaddow tone,
those tones get selected into the background. Two models had pretty
loose hair, and getting this separated was a b****.

I don't want to spend the dough on CS, if I can avoid it.

I photographed the store's logo (a very nice relief, coarse gold/black
on bold red) with the intention of putting behind the models. But
separating the models from that BG was too tedious. So on some images
the logo appears smaller and in the corner. I delivered the poster
Friday to the store (24 x 36) and the proprietor was very happy with it
as well as the other material. (As the poster is a mosaic of 30 someodd
phots, the res is fantastic).

I just have to find a way to spend less time with each project. I
can't afford it. I'd rather control the scene than work PS.

Cheers,
Alan

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-- r.p.e.35mm user resource: http://www.aliasimages.com/rpe35mmur.htm
-- r.p.d.slr-systems: http://www.aliasimages.com/rpdslrsysur.htm
-- [SI] gallery & rulz: http://www.pbase.com/shootin
-- e-meil: Remove FreeLunch.