Re: Mission impossible
Re: Mission impossible
- Subject: Re: Mission impossible
- From: Marc Levine <email@hidden>
- Date: Sun, 30 Jan 2011 19:23:38 -0500
Hi Roger,
I could be wrong here (and I would invite other perspectives). My impression is that Photoshop sends color (Lab/XYX) to the PCS (not device values). The OS the creates the device values by transforming the PCS color values into device values using the display profile. It is my impression that there is ALWAYS a display ICC profile loaded in the system, whether you created it or just inherited if from the default system setup.
-I'm OK with "a", although it could be either RGB or CMYK
-I agree with "b" (same disclaimer)
-For "c", I would say it's up to the host (OS) to convert the PCS value (Lab/XYZ) to device values for the display (RGB) using the display profile.
It's my impression that Phoroshop, for the most part, doesn't really need to know anything about the display profile. It just converts the device values of the active document (RGB, CMYK) to PCS values using either the assigned profile or the workspace (if no profile is assigned). The OS would then pick up those PCS values and convert them for display.
In the logic you have outlined, the display profile would serve as a color bottleneck. Let's say that your RGB colorspace was wayyyy bigger than your display (for example, ProPhoto). If you converted from ProPhoto to ProPhoto (or some other space that was bigger than your display), then you would effectively clip the colors of your image to the display colorspace. That would be bad.
My understanding is that, when you convert from one colorspace to another, Photoshop simply takes the document PCS colors (determined by the assign/workspace profile, the same ones that the OS shuttles off to the display) and converts them into the new colorspace, creating new device values. Photoshop then assigns the new profile (the one you converted to) to the new image and uses THAT profile to send new color values to the PCS (which get routed to the display, just like they did before the conversion).
Whew!
M
On Jan 30, 2011, at 5:31 PM, Roger Breton wrote:
> Marc,
>
> Please correct me if I am wrong but I've always been under the (wrong?)
> impression that Photoshop wrote to the video card directly? On Windows or
> MacOSX. To me, the relation between Photoshop and the host OS was as follows
> :
>
> a) Load some RGB image from disk;
> b) Assign or read its embedded RGB ICC profile;
>
> You're saying that, from this point on, because of the implementation of
> color management at the OS level, providing that there exists some ICC
> profile at the system level describing the monitor's characterization, all
> Photoshop does is write the device values into the video RAM? And it's up to
> the host CMM to pick up the tab at that point to :
>
> c) Convert from the device RGB value to the actual monitor profile using the
> CMM?
>
> I must not be getting this chain of conversion right, somehow.
>
> I realize this is about separation of duty. That much I know we'll agree on.
>
> I figure, since Photoshop *knows* about the active monitor profile, I always
> thought, naively perhaps, that Photoshop always matched (converted) from the
> Document profile to the Monitor profile, and wrote the converted values in
> the video RAM for display.
>
> I can see Photoshop making a system call to carry this conversion but I fail
> to see how Photoshop could this differently.
>
> Kindest regards / Roger
>
> -----Original Message-----
> From: colorsync-users-bounces+graxx=email@hidden
> [mailto:colorsync-users-bounces+graxx=email@hidden] On
> Behalf Of Marc Levine
> Sent: January-29-11 3:29 PM
> To: email@hidden
> Cc: Breton Roger
> Subject: Re: Mission impossible
>
>
> On Jan 29, 2011, at 3:02 PM, email@hidden wrote:
>
>> there is no way to be able to coerce Photoshop into choosing a monitor
>> profile of our choosing, in lieu of whatever can be found in either
>> the Registry, on Windows, I believe, or the System Profile, on Mac OSX.
>
> Hi Roger,
> I believe the problem is that it's not Photoshop's duty to send color from
> the PCS to the display. It's the OS that does that. You can dictate which
> colors Photoshop sends to the PCS by switching either the workspace or the
> assigned profile. You can even ask Photoshop to "dial back" the colors by
> reducing the "monitor saturation". However, once those color values get to
> the PCS, Photoshop says good-bye.
>
> The only workaround I could see if setting up some time in the lab where you
> could calibrate in admin mode and save the profiles. You could then toggle
> them around in class. If you like, so could still walk the students through
> the steps (like you do today). In general, I would recommend keeping the
> concept of monitor calibration/profiling and photoshop color management
> distinctly separate. The last thing you would want is for students to get
> the impression that they should assign their monitor profile to an image.
>
> Photoshop is smarter than the a-ver-age bear (sometimes, too smart), but I
> don't believe it can get around that basic color linkage between the display
> and the OS.
>
> Marc _______________________________________________
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