Computer Graphics with Photoshop - boundary treatment when extracting objects in front of blue screen?

Author

boundary treatment when extracting objects in front of blue screen?

peter

2007-01-27, 10:17 pm

If I have a person shot against a blue screen, how can I extract the person
to put in front of other background?

The simplest solution would be to use magic wand to select the blue
background, inverse, then copy the person.

However, there is a problem at the boundaries. Let's say this person is
wearing white clothing. At the boundary of the white clothing and the blue
background, some pixels are a mixture of white and blue color. If these
pixels are included, there will be a faint ring of blue color around the
person. If these pixels are excluded, let's say by contracting the
selection, then part of the person is lost. Even if I feather the selection,
this problem still exists.

What is the solution?

ronviers@gmail.com

2007-01-27, 10:17 pm

peter wrote:
> If I have a person shot against a blue screen, how can I extract the person
> to put in front of other background?
>
> The simplest solution would be to use magic wand to select the blue
> background, inverse, then copy the person.
>
> However, there is a problem at the boundaries. Let's say this person is
> wearing white clothing. At the boundary of the white clothing and the blue
> background, some pixels are a mixture of white and blue color. If these
> pixels are included, there will be a faint ring of blue color around the
> person. If these pixels are excluded, let's say by contracting the
> selection, then part of the person is lost. Even if I feather the selection,
> this problem still exists.
>
> What is the solution?

There are a lot of techniques but they each depend on the details of
the images. Can you post links to the images?

Mike Russell

2007-01-27, 10:17 pm

> peter wrote:[color=darkred]

This is a well known problem with blue screening. If you have control
during the shoot, use a high angled backlight with a warm gel, which will
generate a cleaner mask as well as getting rid of the fringe.

If you are dealing with images after the fact, there are a variety of ways
to deal with it, as Ron says. You can hit the blue areas with the sponge in
desaturate mode, or knock back the blue with curves, Hue Sat, select the
area using the matte and use defringe, or a variety of other tools in
Photoshop.
--
Mike Russell
www.curvemeister.com/forum/

ronviers@gmail.com

2007-01-27, 10:17 pm

Mike Russell wrote:
> use a high angled backlight with a warm gel, which will
> generate a cleaner mask as well as getting rid of the fringe.

> Mike Russell
> www.curvemeister.com/forum/

Hi Mike,
Why does this work? If I assume you mean the light source should be
centered with relation to the subject this would effectively create a
gradient on the backdrop with maximum contrast along the edge of the
subject while shifting the blue background towards neutral. This would
complicate exposure but if detail is maintained in the subject then I
can imagine how there would appear to be an almost white glow emanating
from behind the subject. Is that the idea?
My experience with backlights and gobos is that they complicate the
exposure and screw with white balance.

Thanks,
Ron

peter

2007-01-27, 10:17 pm

"Mike Russell" <RE-MOVEmike@Curvemeister.comRE-MOVE> wrote in message
news:sE2rh.67776$wP1.35512@newssvr14.news.prodigy.net...
>
> This is a well known problem with blue screening. If you have control
> during the shoot, use a high angled backlight with a warm gel, which will
> generate a cleaner mask as well as getting rid of the fringe.

Could you explain what "high angled backlight" is?

> If you are dealing with images after the fact, there are a variety of ways
> to deal with it, as Ron says. You can hit the blue areas with the sponge
> in desaturate mode, or knock back the blue with curves, Hue Sat, select
> the area using the matte and use defringe, or a variety of other tools in
> Photoshop.

Thanks for the pointer to Defringe. I didn't know about it before.

I experimented with it a little. It is a good idea, but the implentation
could use a little more intelligence. Right now it can blur the edge after
defringe. And if the fringe includes too much background color (1 extra
pixel width), defringe would totally mess up.

John

2007-01-27, 10:17 pm

<ronviers@XXXXXXXXXX> wrote in message
news:1168952195.618795.87550@38g2000cwa.googlegroups.com...
>
> Mike Russell wrote:
>
>
> Hi Mike,
> Why does this work?

I am not Mike, but I am a photographer. Picture the extra light coming from
behind the subject, out of view of the camera lens, and therefore it is not
illuminating the blue background. With a warming filter, it creates a clean
color contrast at the edges of the subject that would otherwise be
commingled (via hue and luminosity) with the blue background.

ronviers@gmail.com

2007-01-27, 10:17 pm

John wrote:

> I am not Mike, but I am a photographer. Picture the extra light coming from
> behind the subject, out of view of the camera lens, and therefore it is not
> illuminating the blue background. With a warming filter, it creates a clean
> color contrast at the edges of the subject that would otherwise be
> commingled (via hue and luminosity) with the blue background.

Hi John,
Thanks for the additional information. This is totally different from
what I was thinking. So let me get this straight, the role of the
backlight is to eliminate the shadows from the subject. The colored
fringe is a result of a penumbra created from the backlight's
interaction with the subject illumination. The role of the gel is to
remove the penumbra by shifting the balance of the foreground and
background illumination to either behind or to the edge of the subject.
Is this correct? Does the gel need to be warming or could it be ND?

Thanks,
Ron