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Subject: re: [cgmo-webcgm] The sRGB color space


Benoit

As I understand it, after reading the sRGB description at
http://www.w3.org/Graphics/Color/sRGB, for a CGM viewer to 
correctly render a CGM in which the color model = sRGB, a 
ICC color profile is required. This color prfile enables Windows 
to convert sRGB to RGB for the target device. 

I don't see where the WebCGM profile provides a way to specify 
a color profile, so I believe we have an incomplete specification
as it stands. 

So on that note, I agree that we should deprecate sRGB.

Regards, 
Don.

  > Hi cgmo-webcgm,

  >   There were talks about sRGB and RGB during our last FTF meeting. I
  >   asked a friend (no affiliation) of mine a few questions on the
  >   matter: 

  >   Question: WebCGM 1.0 allows for RGB and sRGB color spaces, there are
  >   talks of deprecating the sRGB color space. I'm wondering if
  >   implementations are not using sRGB without knowing it?
  >   
  >   Answer: "RGB isn't a color space. It's just raw data that probably
  >   just gets sent to the display hardware directly, which may or may
  >   not have a color space close to sRGB (depends on whether it's a CRT,
  >   what correction curves the video card is using, etc.). So yes, using
  >   raw RGB is sort of like using sRGB, as long as you're displaying the
  >   image on a CRT and the video card's gamma correction is set close to
  >   1.

  >   It may make sense to deprecate sRGB (or color management in general)
  >   for the types of applications CGM has mostly been used for, but if
  >   CGM docs are ever going to be printed in color, or expected to match
  >   CSS colors on a web page, for example, it would be a good idea to
  >   encourage the use of sRGB. Color calibration may also become more
  >   important as more people use non-CRT displays.    

  >   Unless sRGB is the default color space, CGM implementations are
  >   probably NOT using it, just uncalibrated RGB, which isn't the same
  >   thing." 

  >   Question: A PNG image is expressed in sRGB right? I know it doesn't
  >   support embedded ICC profiles so it has to be sRGB. Right? If I take
  >   a PNG and display it on screen with the traditional
  >   createDIBSection, BitBlt methods, am I not in fact assuming that the
  >   source in is sRGB values?

  >   Answer: "PNG data isn't sRGB in general (here's the spec:
  >   http://www.w3.org/TR/REC-png-multi.html). By default the colors are
  >   uncalibrated, but there can be optional gamma and color correction
  >   data. I don't know whether that data is sufficient to calibrate it
  >   as sRGB. A lot of apps probably ignore or mishandle the calibration
  >   data if it's present - for example, you might want to read this
  >   article (http://hsivonen.iki.fi/png-gamma/) that I came across a few
  >   days ago."

  >   Question: If you create a bitmap and set the RGBQUAD to be
  >   (0,0,255), aren't those values expected to be in sRGB.

  >   Answer: "I had a quick look at the color management docs for Windows.
  >   Search for the page with title "Using GDI Functions With ICM". Some
  >   functions use CMM, others don't. E.g.  

  >   SetDIBitsToDevice
  >   Color management is performed. If the specified BITMAPINFO structure
  >   is not version 4 or version 5, the color profile of the current
  >   device context is used as the source color space profile. If it
  >   doesn't have one, the sRGB color space is used. If the specified
  >   BITMAPINFO structure is version 4 or version 5, the color space
  >   profile associated with the bitmap is used as the source color
  >   space.
  >   
  >   The DC has to be enabled for CMM (I don't know how exactly - I've
  >   never read any of these docs). Most apps probably don't use it, in
  >   which case all bitmaps RGB values are device-dependent (i.e.
  >   uncalibrated).   

  >   Note that in the description of SetDIBitsToDevice it says the
  >   bitmap's color space is considered to be the same as the DC's for
  >   version <= 3, which means no color correction will be done, so it's
  >   equivalent to having CMM turned off. In other words the RGB values
  >   are equivalent to uncalibrated, NOT sRGB.

  >   In summary, it looks like colors are not assumed to be sRGB when
  >   unspecified - they're treated as uncalibrated."

  >   Regards,
  >   
  > -- 
  >  Benoit                 mailto:benoit@itedo.com


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