Hi,
trying to get to know babl (wrt the LCH layer modes), quite a few
questions came up that made me wish for a babl 'Conversion implentor's
guide'. (If such a document exists just disregard the rest of this mail
-- but please show me where).
So maybe the best I could do for starters is write such guide -- as long
as I still remember what the questions are of someone newly confronted
with babl. But I'll need a little help with the answers... ;o)
First, two general questions about babl:
- babl decides which conversion to use, based on speed and accuracy. Is
that a build-time or run-time decision?
- What is the reference for judging accuracy?
Adding conversions to babl:
I haven't gone deep into the babl source yet (and hope I won't have to),
so the following is a mixture of my best guesses and some open questions.
I'll be glad for 'yes/no' confirmations and of course any additional
info/answers:
- Create source file in subdirectory 'extensions' and add it to
extensions/Makefile.am. The rest is automagic.
- Source must have 'int init (void)' function which announces/registers
the new conversions:
- Register color model and its components using
babl_component_new()
[ what about "luma", "chroma", "alpha"? ]
babl_model_new()
[ lots of parameter possibilities... ]
[ naive-CMYK e.g. registers "CMYK" but not "RGBA". I assume RGBA is
a core-format that can always be relied on to exist? Is there a
list of such formats?
PS: babl-core.c suggests that RGBA (+ its components) is the only
hardcoded model, along with the PAD component and the double
type.
PPS: babl-base.c has additional types and models:
types: float/u8/u16/u32
models: (plus respective components Ra/R'/R'a etc.)
RGB: RGB/RaGaBaA/R'G'B'/R'G'B'A/R'aG'aB'aA
Gray: Y/YA/YaA/Y'/Y'A/Y'aA
YCbCr: YCbCr/YCbCrA... now it gets confusing. Where
the hell does "y'" come from?
PPPS: Maybe I should just compile w/o extensions and see what
comes out?
]
- Register the new provided conversions using babl_conversion_new().
[ What are "linear", "plane", "planar" ? ]
- Default data type of components is 'double'. Others can be
registered using babl_format_new().
- Specifying additional formats with babl_format_new() doesn't mean
you have to provide conversions for those. You can leave it to babl
to find a way.
[ Or maybe not? naive-CMYK registers a "CMYK float" format without
defining any conversions. BablFishPath.html shows that format w/o
any conversions to or from it. ]
- The components of one format don't all have to have the same data
type.
- New data types (e.g. integer with certain ranges) can be added using
babl_type_new().
That's it for now.
Regards
Rupert
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