[feat]IBA:demosaic add X-Trans demosaicing (#4579)

Implements X-Trans pattern demosaicing using a simple bilinear
algorithm. Adds the Xtrans pattern layout to the ImageBuf attributes in
the raw reader plugin.

I have implemented unit tests in imagebufalgo_test.cpp which cover all
permutations of the Bayer and X-Trans layouts, as the X-Trans pattern is
a bit too complicated to generate using oiiotool.

---------

Signed-off-by: Anton Dukhovnikov <antond@wetafx.co.nz>

Author: antond-weta

src/doc/oiiotool.rst

@@ -4212,13 +4212,17 @@ current top image.
     Optional appended modifiers include:
 
       `pattern=` *name*
-        sensor pattern. Currently supported patterns: "bayer"
+        sensor pattern. Currently supported patterns: "bayer", "xtrans".
       `layout=` *name*
-        photosite order of the specified pattern. For layouts of the Bayer
-        pattern supported: "RGGB", "GRBG", "GBRG", "BGGR".
+        photosite order of the specified pattern. The default value is "RGGB"
+        for Bayer, and "GRBGBR BGGRGG RGGBGG GBRGRB RGGBGG BGGRGG" for X-Trans.
       `algorithm=` *name*
-        the name of the algorithm to use: "linear"(simple bilinear demosaicing),
-        "MHC"(Malvar-He-Cutler algorithm)
+        the name of the algorithm to use.
+        The Bayer-pattern algorithms:
+        - "linear"(simple bilinear demosaicing),
+        - "MHC"(Malvar-He-Cutler algorithm).
+        The X-Trans-pattern algorithms:
+        - "linear"(simple bilinear demosaicing).
       `white-balance=` *v1,v2,v3...*
         optional white balance weights, can contain either three (R,G,B) or four
         (R,G1,B,G2) values. The order of the white balance multipliers is as

src/doc/pythonbindings.rst

@@ -3701,9 +3701,10 @@ Color manipulation
                 bool ImageBufAlgo.demosaic (dst, src, pattern="", algorithm="", layout="", white_balance=py::none(), roi=ROI.All, nthreads=0)
     Demosaic a raw digital camera image.
 
-    `demosaic` can currently process Bayer pattern images (pattern="bayer")
+    `demosaic` can currently process Bayer-pattern images (pattern="bayer")
     using two algorithms: "linear" (simple bilinear demosaicing), and "MHC"
-    (Malvar-He-Cutler algorithm). The optional white_balance parameter can take
+    (Malvar-He-Cutler algorithm); or X-Trans-pattern images (pattern="xtrans")
+    using "linear" algorithm. The optional white_balance parameter can take
     a tuple of three (R,G,B), or four (R,G1,B,G2) values. The order of the
     white balance multipliers is as specified, it does not depend on the matrix
     layout.

src/include/OpenImageIO/imagebufalgo.h

@@ -2164,7 +2164,9 @@ bool OIIO_API repremult (ImageBuf &dst, const ImageBuf &src,
 ///
 ///   - "pattern" : string (default: "bayer")
 ///
-///     The type of image sensor color filter array. Currently only the Bayer-pattern images are supported.
+///     The type of image sensor color filter array. Currently suported patterns:
+///     - `bayer` - Bayer-pattern image.
+///     - `xtrans` - X-Trans-pattern image.
 ///
 ///   - "algorithm" : string (default: "linear")
 ///
@@ -2174,11 +2176,14 @@ bool OIIO_API repremult (ImageBuf &dst, const ImageBuf &src,
 ///     - `MHC` - Malvar-He-Cutler linear demosaicing algorithm. Slower than `linear`, but produces 
 ///       significantly better results.
 ///
-///   - "layout" : string (default: "RGGB")
+///     The following algorithms are supported for X-Trans-pattern images:
+///     - `linear` - simple linear demosaicing. Fast, but can produce artefacts along sharp edges.
+///
+///   - "layout" : string (default: "RGGB" for Bayer, "GRBGBR BGGRGG RGGBGG GBRGRB RGGBGG BGGRGG" for X-Trans)
 ///
 ///     The order the color filter array elements are arranged in, pattern-specific.
 ///
-///   - "white-balance" : float[3] or float[4] (default: {1.0, 1.0, 1.0, 1.0})
+///   - "white-balance" : float[3] or float[4], (default: {1.0, 1.0, 1.0, 1.0})
 ///
 ///     Optional white-balancing weights. Can contain either three (R,G,B), or four (R,G1,B,G2) values.
 ///     The order of the white balance multipliers does not depend on the matrix layout.