Re: Blue/Green Screens


samarsh@...
 

Andrew Engelhardt wrote:

Hi all, just wondering if anyone has any experience shooting or dealing with images that have been shot against a blue screen? >
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Hello Andrew, sorry - no experience. Try this link:

http://macworld.zdnet.com/1998/05/create/4317.html

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We have to try to facilitate a high volume of single product images with clipping paths in a fairly fast turn-around time and we're looking at how to speed up the process of getting paths on everything. We're hoping that the blue or green screen background might help either Photoshop or another program like Mask Pro with creating a path quickly and accurately. These are files that will be re-purposed for a bunch of different uses from all kinds of printing conditions to web use, thus the paths on just about everything. The only problem we can think of so far is when the colour of the product is close to that of the background. Any input or advice would be greatly appreciated. >
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There seems to be a common misconception about cross application
transparency - and transparency in general.

There are three common areas for transparency:

i) Photoshop layer compositing

ii) Clipping Paths for PostScript 'Print' Output (Adobe blur the lines
with PDF however, this is both monitor and print...but it works on non
PS devices - I don't understand how PDF 'masks' to QuickDraw or PCL,
Quartz seems obvious for PDF however)

iii) Web and Multi Media (monitor) based transparency output

Most users knew the three 'general rules' as above.

Now APS6 changes things by using vector based layer shapes for layer
transparency within Photoshop - while saving a clipping path in a EPS or
TIFF for transparency outside of Photoshop is another thing (I presume -
I currently use version 5.5, so only have past experience to base things
on).

Does Photoshop 6 use the term CLIPPING MASK or CLIP PATH for these
vector shape transparency effects? I hope they make some sort of naming
difference between them. It's like users who say crop when they mean
clip. It leads to confusion.

Traditionally, for PostScript output and transparency - there were only
two methods available:

i) Clip Paths on a contone image (Greyscale/RGB/CMYK)

ii) Bitmap mode TIFF or EPS (1 bit file)

Both these methods only gave 1 bit transparency. you could not have
varying tones of opacity like in Photoshop - it was all or nothing.

At this point, the only professional *print* tool that I know of that
can use a raster (pixel) alpha mask for transparency is Adobe
Illustrator 9 (or Acrobat 5 perhaps?). This is from a layer mask in a
native layered PSD file from Photoshop.

The good news is that the transparency can now vary from 0-255 levels,
instead of only 0 or 255.

I understand FreeHand and Canvas had transparency before Illustrator -
but it was not useful for this 'deep etching' purpose, AFAIK.

Now for the 'bad news'. This raster transparency for print has not been
proven to me. The manual shows it - but I have not had the time to test
this, let alone trust it to a live job going to film - or even worse,
digital output at a publication which we have no quality control over.

Who knows how this 'magic' variable raster mask based transparency will
behave if saved as EPS and imported into layout on *another* background
for separations? Will all RIPs behave the same? I don't even know if it
works within Illustrator 9.0.1 (Don't let me get started about AI9 - and
I use it on the PC, which seems to have had a better ride than the Mac).
I will shortly try the 9.0.2 update...

As for third party masking like Knockout and MaskPro - as far as I know,
these only perform raster based alpha masking...which is perhaps the
opposite to a clip path.

Most of these developers do not claim to create PostScript clip paths -
or cross application transparency for print.

They simply promise to deliver an alpha channel. It is up to the user to
understand all the issues of transparency and output.

As far as I know, this transparency is all for layer compositing (where
a merged flat image is finally delivered) or web/multi media monitor
output, once agian a final flat image with an alpha channel.

Now if I have misunderstood, and you know about the limitations of
transparency for different applications, output and media - then you
must have been referring to using a selection loaded off one of these
alpha channel masks to turn into a path, for use as a PostScript Clip
Path.

This is something that I never suggest doing (if quality is a concern).
Autotracing in this method does have it's creative uses - just not for
etching an image from it's background.

I do not know of any quick way to etch an image with a precise vector
path. Even human operators can make a bad clip and expose some
background or clip too much of the image.

As for transparency for web or multi media - that is another subject,
possibly off topic or off list.

Waiting to see what the regulars have to say (go easy).

So, to conclude:

At this point in time, it seems like you still need a Photoshop jockey
to manually etch out the tedious clip path, if quality and accuracy are
a concern (as I sincerely hope they are).

There are other thoughts on actually using clip paths that I can offer,
but these are best left for later.

At the moment you have to figure out what type of transparency you
really need.

Test, verify - then retest. Don't believe the hype.

Sincerely,

Stephen Marsh.

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