sRGB: A Standard Color Space

As a result of Internet bandwidth considerations, Hewlett-Packard and Microsoft have proposed the adoption of a standard predefined color space known as sRGB (IEC 61966-2-1), so as to allow accurate color mapping with very little data overhead.

A help-file version of a white paper discussing the technical details of sRGB, sRGB.hlp, is available in the \Help folder of the WCS 1.0 Programmer's Reference.

Different file formats may use or add a flag to specify that the image is in sRGB color space. In the Windows device-independent bitmap (DIB) format, setting the bV5CSType member of the BITMAPV5HEADER structure to LCS_sRGB specifies that the DIB colors are in the sRGB color space.

WCS 1.0 provides native support for sRGB. There are two ways to use WCS 1.0 for rendering an image defined in the sRGB color space:

To render an image inside the device context

1. Create a device context (DC) on the display device.
2. Set color management using the SetICMMode function.
3. Use the SetDIBitsToDevice function to transfer the DIB into the DC. As long as the bV5CSMType member of the DIBs BITMAPV5HEADER structure is set to LCS_sRGB, the system will perform the appropriate color management.

To render an image outside the device context

1. Create a transform using CreateColorTransformW. The lcsCSType member of the LOGCOLORSPACE structure pointed to by the pLogColorSpace parameter should be set to LCS_sRGB. The hDestProfile parameter indicates the display device's color space.
2. Use the created color transform to color match the image before displaying it on the device.

WCS 1.0 Defaults for Input Color Space and Output Profile

When no input color space is specified, by default WCS 1.0 uses the sRGB color space as the input color space for color mapping.

When no output profile is specified, but a default device is specified, WCS 1.0 selects a default output profile. If the default device does not have an associated profile, WCS 1.0 uses the sRGB color space as the output profile.

The following table shows the resulting color transforms when a default device is not available.

	Output Profile Specified	Output Profile Not Specified
Input Color Space Specified	Transform uses the specified profiles.	Transform converts from known input color space to sRGB.
Input Color Space Not Specified	Transform converts from sRGB to known output profile.	Transform from sRGB to sRGB is assumed; nothing is done.

 

sRGB and Embedded Profiles

Beginning with ICM version 2.0, applications that utilize WCS can embed profiles in images. Embedded profiles assist users' applications in maintaining a consistent color appearance even if images are transmitted across the Internet.

Images that use the sRGB color space do not need an embedded color profile. Because they have no embedded profile, sRGB-based images are smaller and more readily transferable across data channels with limited bandwidth.

Applications should set the LCS_sRGB flag in the image's bitmap header to indicate that the image uses the sRGB color space. For details, see Windows Bitmap Header Structures and LOGCOLORSPACE.