Technically OT: Photocompositing Technique?

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C J Southern - 30 Jul 2006 00:02 GMT

Often when I'm replacing blue-screen backgrounds with other (eg mountain)
scenery I used to have a problem where the 2 images just didn't look right -
there was too much of an abrupt transition between, say, the head of the
subject and the (new) background behind.

One day I sat down and spent a couple of hours solid trying to get the
images to blend in a more photorealistic way. I tried different blending
modes - different shadow effects - you name it, I tried it.

In the end I came up with something that actually seemed to work quite
well - just thought I'd take a moment to share it with you good folks in th
hope that others with better techniques can also chip in?

--

I started out with a couple shot against a blue screen in the studio, and an
Alaskan mountain scene.

First step, erase the blue background and toss the remainder (head +
shoulders) & the mountain onto seperate layers

2. Set the magic wand to a tolerance of 255 (non-contiguous) and click on
the couple - giving a selection only around the periphery of the couple

3. Choose SELECT -> MODIFY -> CONTRACT and contract the selection by
around 2 pixels (actual amount will vary between photos depending on
relative size of object and resolution)

4. Choose SELECT -> INVERSE (so at this point we've essentially only got
a few pixels around the periphery of the couple unmasked.

5. Choose FILTER -> BLUR -> GAUSSIAN BLUR and pick a number that works
for you (try about 4 to start with)

Job done.

Best I've been able to come up with so far - anyone got anything better?

Cheers,

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Mike Warren - 30 Jul 2006 00:13 GMT

> Often when I'm replacing blue-screen backgrounds with other (eg
> mountain) scenery I used to have a problem where the 2 images just
[quoted text clipped - 8 lines]
> well - just thought I'd take a moment to share it with you good folks
> in th hope that others with better techniques can also chip in?

I would use a layer mask made from the blue channel. Apply blur and
use levels to adjust the transition. This is very quick and can be effective
if the background is significantly different.

Don't you find the blue bleeds into the subject a bit though?

-Mike

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C J Southern - 30 Jul 2006 00:27 GMT

Thanks for that - I'll give it a try.

> Don't you find the blue bleeds into the subject a bit though?

Do you mean the blur?

Technically it does, but in reality it's one of those things where it's so
subtle the eye can't resolve it. I wanted to simulate the fact that when you
have one object in front of another, what the left eye sees is slightly
different to what the right eye sees - with the difference being in this
transitional area.

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Mike Warren - 30 Jul 2006 00:33 GMT

> "Mike Warren" <miwa-not-this-bit@or-this-csas.net.au> wrote in message
>> Don't you find the blue bleeds into the subject a bit though?
>
> Do you mean the blur?

No. Strongly coloured backgrounds tend to reflect on the skin of people
and other objects in the picture.

Here is a DPR thread where I showed someone how to fix the problem.

http://forums.dpreview.com/forums/read.asp?forum=1006&message=16305400

This person used a green screen.

-Mike

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Ming-Ho Chi - 30 Jul 2006 02:13 GMT

>> "Mike Warren" <miwa-not-this-bit@or-this-csas.net.au> wrote in message
>>> Don't you find the blue bleeds into the subject a bit though?
[quoted text clipped - 11 lines]
>
> -Mike

Penny just dropped - I thought you were talking about the finished
composite, not the chromakey extraction.

What you're talking about is colour spill - it occurs if you have the
subject too close to the backdrop, or if the backdrop isn't evenly (and
usually seperately) lit.

If you're not already familiar with it, take a look at primatte from
www.DigitalAnarchy.com

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C J Southern - 30 Jul 2006 02:44 GMT

>> "Mike Warren" <miwa-not-this-bit@or-this-csas.net.au> wrote in message
>>> Don't you find the blue bleeds into the subject a bit though?
[quoted text clipped - 7 lines]
>
> http://forums.dpreview.com/forums/read.asp?forum=1006&message=16305400

I did a lot of reading on the topic and ended up investing in a green and a
blue screen, especially designed to be wrinkle free and highly saturated,
which was the first step.

We got better results if we moved the subject 2 meters or more in front, but
are hoping for even better results with seperate backdrop lighting. I
suspect the one remaining issue is where there's fine detail and the
de-mosaicing / anti-aliasing has combined hair pixels and blue-screen pixels
to produce a visable colour that shouldn't be there, but can't be seen by
the extraction software.

It's a work in progress :(

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Paul Mitchum - 30 Jul 2006 18:01 GMT

> > Often when I'm replacing blue-screen backgrounds with other (eg
> > mountain) scenery I used to have a problem where the 2 images just
[quoted text clipped - 12 lines]
> use levels to adjust the transition. This is very quick and can be effective
> if the background is significantly different.

Another way: Put the blue-screen image on a layer, and the background
image behind it on another layer. Select the blue-screen layer, and do
this menu: Layer -> Layer Style -> Blending Options... (You can also get
to it from the drop-down menu on the Layers palette, or by
right-clicking on the layer in the palette).

Now pay attention to 'Advanced Blending' and 'Blend If..' Start with the
blue channel. Do what comes naturally. :-) Slide the 'This Layer...'
slider.

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