Bitmaps, colorspaces, etc.
Bitmaps, colorspaces, etc.
- Subject: Bitmaps, colorspaces, etc.
- From: Graham Cox <email@hidden>
- Date: Mon, 21 Dec 2015 12:15:09 +1100
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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