The sRGB color space

[Benoit Bezaire <benoit@itedo.com>]

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