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Channel Mixer and Lab L Channel

Channel Mixer and Lab L Channel

2005-11-10 by Steve Kale

I am puzzled by something I stumbled across yesterday.  I was working up an image 
converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted to sharpen the 
image and tried a variation of the Smart Sharpen/Reduce Noise Sharpen action I use - 
specifically I copied the image, converted it to Lab Colour Mode (flattening in the process) 
and only sharpened the L channel.  I made a mistake while doing this and brought just the 
L channel of the copy over to the original as a layer.  This revealed something I found 
surprising:  the Lab Colour copy ( a flattened greyscale image) had luminance information 
in the a and b channels.  That is the luminance of the L channel alone was different from 
the luminance when all three (L, a and b) are displayed.  This would seem to fly in the face 
of what I thought the a and b channels represented.  

I did another quick test.  I took the RGB original with the Channel Mixer layer and flattened 
it, leaving a greyscale image in RGB.  Converted it to Lab.  If I uncheck either one of the a 
and b channels the image stays looking the same.  But if I uncheck both the a and b 
channels (leaving just the Lightness channel displayed) the luminance of the image 
changes significantly.

I must be missing something basic but I don't quite get it.... Ideas?

Steve

PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)

RE: [Digital BW] Channel Mixer and Lab L Channel

2005-11-10 by John Moody

Steve,
I believe when you turn off the a and b channel, PS displays the image based
on your working gray space.  The same thing probably happened when you
brought the L channel over, it got converted to your working gray.
Try setting your working gray to Lab grayscale and see if the results are
closer to what you were expecting.

Best regards,
John Moody
Show quoted textHide quoted text
-----Original Message-----
From: DigitalBlackandWhiteThePrint@yahoogroups.com
[mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Steve Kale
Sent: Thursday, November 10, 2005 6:00 AM
To: DigitalBlackandWhiteThePrint@yahoogroups.com
Subject: [Digital BW] Channel Mixer and Lab L Channel

I am puzzled by something I stumbled across yesterday.  I was working up an
image
converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted to
sharpen the
image and tried a variation of the Smart Sharpen/Reduce Noise Sharpen action
I use -
specifically I copied the image, converted it to Lab Colour Mode (flattening
in the process)
and only sharpened the L channel.  I made a mistake while doing this and
brought just the
L channel of the copy over to the original as a layer.  This revealed
something I found
surprising:  the Lab Colour copy ( a flattened greyscale image) had
luminance information
in the a and b channels.  That is the luminance of the L channel alone was
different from
the luminance when all three (L, a and b) are displayed.  This would seem to
fly in the face
of what I thought the a and b channels represented.

I did another quick test.  I took the RGB original with the Channel Mixer
layer and flattened
it, leaving a greyscale image in RGB.  Converted it to Lab.  If I uncheck
either one of the a
and b channels the image stays looking the same.  But if I uncheck both the
a and b
channels (leaving just the Lightness channel displayed) the luminance of the
image
changes significantly.

I must be missing something basic but I don't quite get it.... Ideas?

Steve

PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)






[Non-text portions of this message have been removed]

Re: [Digital BW] Channel Mixer and Lab L Channel

2005-11-10 by Steve Kale

I think you are right.  With QTR Gray Lab there is no luminance shift.
Seems odd though.  I would expect the single channel to still be displayed
in the document's (colour) space or at the very least converted to the grey
workspace rather than assigned this different space.  If I drag a layer from
one document to the another in a different colour space I believe the layer
is converted to the new space.  At any rate I found the gamma change from
the assignment of Gray Gamma 2.2 rather than Lab to be quite beneficial -
much more "pop".
Show quoted textHide quoted text
> From: John Moody <moodymz3@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Thu, 10 Nov 2005 08:53:45 -0500
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: RE: [Digital BW] Channel Mixer and Lab L Channel
> 
> Steve,
> I believe when you turn off the a and b channel, PS displays the image based
> on your working gray space.  The same thing probably happened when you
> brought the L channel over, it got converted to your working gray.
> Try setting your working gray to Lab grayscale and see if the results are
> closer to what you were expecting.
> 
> Best regards,
> John Moody
> 
> -----Original Message-----
> From: DigitalBlackandWhiteThePrint@yahoogroups.com
> [mailto:DigitalBlackandWhiteThePrint@yahoogroups.com]On Behalf Of Steve Kale
> Sent: Thursday, November 10, 2005 6:00 AM
> To: DigitalBlackandWhiteThePrint@yahoogroups.com
> Subject: [Digital BW] Channel Mixer and Lab L Channel
> 
> I am puzzled by something I stumbled across yesterday.  I was working up an
> image
> converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted to
> sharpen the
> image and tried a variation of the Smart Sharpen/Reduce Noise Sharpen action
> I use -
> specifically I copied the image, converted it to Lab Colour Mode (flattening
> in the process)
> and only sharpened the L channel.  I made a mistake while doing this and
> brought just the
> L channel of the copy over to the original as a layer.  This revealed
> something I found
> surprising:  the Lab Colour copy ( a flattened greyscale image) had
> luminance information
> in the a and b channels.  That is the luminance of the L channel alone was
> different from
> the luminance when all three (L, a and b) are displayed.  This would seem to
> fly in the face
> of what I thought the a and b channels represented.
> 
> I did another quick test.  I took the RGB original with the Channel Mixer
> layer and flattened
> it, leaving a greyscale image in RGB.  Converted it to Lab.  If I uncheck
> either one of the a
> and b channels the image stays looking the same.  But if I uncheck both the
> a and b
> channels (leaving just the Lightness channel displayed) the luminance of the
> image
> changes significantly.
> 
> I must be missing something basic but I don't quite get it.... Ideas?
> 
> Steve
> 
> PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)

Re: Channel Mixer and Lab L Channel

2005-11-11 by pavvelc

--- In DigitalBlackandWhiteThePrint@yahoogroups.com, "Steve Kale"
<stevekale@b...> wrote:
>
> I am puzzled by something I stumbled across yesterday.  I was
working up an image 
> converted to B&W with Channel Mixer (60% Green, 40% Blue).  I wanted
to sharpen the 
> image and tried a variation of the Smart Sharpen/Reduce Noise
Sharpen action I use - 
> specifically I copied the image, converted it to Lab Colour Mode
(flattening in the process) 
> and only sharpened the L channel.  I made a mistake while doing this
and brought just the 
> L channel of the copy over to the original as a layer.  This
revealed something I found 
> surprising:  the Lab Colour copy ( a flattened greyscale image) had
luminance information 
> in the a and b channels.  That is the luminance of the L channel
alone was different from 
> the luminance when all three (L, a and b) are displayed.  This would
seem to fly in the face 
> of what I thought the a and b channels represented.  
> 
> I did another quick test.  I took the RGB original with the Channel
Mixer layer and flattened 
> it, leaving a greyscale image in RGB.  Converted it to Lab.  If I
uncheck either one of the a 
> and b channels the image stays looking the same.  But if I uncheck
both the a and b 
> channels (leaving just the Lightness channel displayed) the
luminance of the image 
> changes significantly.
> 
> I must be missing something basic but I don't quite get it.... Ideas?
> 
> Steve
> 
> PS:  I am using PS CS2.  (My RGB space is ProPhoto RGB.)
>

L will always be lighter than grayscale. LUTs are set up this way. You
can check with a color picker. In a space with gamma 2.2 midtone
128,128,128 translates into L 54 while in gamma 1.8 into L 61 if i
remember correctly. So using L for masks or sharpening doesn't give
exactly the same results as using grayscale or reverting sharpening to
luminosity in RGB. Regards,

Pawel

Re: [Digital BW] Re: Channel Mixer and Lab L Channel

2005-11-11 by Steve Kale

Well yes but I was surprised that there was a profile assignment going on
when you simply uncheck the a and b channels for view.  In the least I would
expect a profile conversion.
Show quoted textHide quoted text
> From: pavvelc <pawel_czapiewski@...>

> 
> L will always be lighter than grayscale. LUTs are set up this way. You
> can check with a color picker. In a space with gamma 2.2 midtone
> 128,128,128 translates into L 54 while in gamma 1.8 into L 61 if i
> remember correctly. So using L for masks or sharpening doesn't give
> exactly the same results as using grayscale or reverting sharpening to
> luminosity in RGB. Regards,
> 
> Pawel

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