I’m trying to understand how properly to use colorspaces with bitmaps.
In my app’s Export function, we’d like to offer the ability to choose a
colorspace for the exported image, in whatever format the user chose. My
thoughts were to use NSBitmapImageRep to capture the image, then use its
-representationUsingType:properties: method to get the image data in whatever
format before writing it to disk. So far so good.
NSBitmapImageRep accepts a ‘colorspaceName’ parameter when it’s created, and
these can be one of the following:
• NSCalibratedWhiteColorSpace
• NSCalibratedBlackColorSpace
• NSCalibratedRGBColorSpace
• NSDeviceWhiteColorSpace
• NSDeviceBlackColorSpace
• NSDeviceRGBColorSpace
• NSDeviceCMYKColorSpace
• NSNamedColorSpace
• NSCustomColorSpace
Which are all strings, and seem to represent a very generic non-specific kind
of colorspace.
These don’t match with the colorspaces known to the system as returned by
+[NSColorSpace availableColorSpacesWithModel:], which returns an array of
NSColorSpace objects, each of which is able to return a -localizedName
property, which are very specific, and do not relate to the above names at all.
How do I bridge this impedance mismatch? If the user wants to Export using a
specific, named colorspace, as chosen for example from a list of the
colorspaces returned by the +availableColorSpaces method, how should I go about
creating the appropriate bitmap? I see that NSBitmapImageRep has methods such
as -bitmapImageRepByRetaggingWithColorspace: and
-bitmapImageRepByConvertingToColorSpace:renderingIntent:
What does ‘retagging’ mean? Presumably converting duplicates the bitmap, so
could be a memory strain. This seems unnecessary if I could create the bitmap
with a specific colorspace in the first place.
Also, when creating a bitmap, what does using ‘NSNamedColorspace’ do? Which
named colorspace? What about ‘NSCustomColorSpace’? These seem only to indicate
what general kind of colorspace to expect, not a specific one.
The problem here is that I can find no documentation that goes anywhere near
explaining this. Maybe NSBitmapImageRep is too high level and I need to drop
down to CGImageRef? That class does seem to take a specific CGColorSpace
object, though how I get there from +[NSColorSpace availableColorSpecs] is
unclear.
Am I even asking sensible questions? This sort of impedance mismatch usually
suggests a conceptual misunderstanding somewhere, but without a clear
explanation of how colorspaces and bitmaps are used, I can’t see where I may
have gone wrong.
Anyone able to illuminate?
—Graham
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