Notes on photo sphere/panoramic EXIF metadata
- Google's EXIF/XMP metadata tag specification for panoramic images
- Panorama images with sky are to have a negative value for the CroppedAreaTopPixels tag. (The panoramic viewer of Facebook ignores the value altogether, though, and assumes the horizon in the middle of the picture.)
- If a camera or software normally produces panoramic images with cylindrical projection, then doing a vertical capture actually produces a transverse cylindrical image. There is no standardized value for the EXIF ProjectionType field to cover this. (One option is to reproject them to equirectangular — which, again, is not very well supported.)
- Facebook's document on what tags to set
- The Facebook web interface insists that FullPanoHeightPixels be half the size of FullPanoWidthPixels. Else you get “no new photos were uploaded”.
/ Tags: Photo.
Type-checking casts in (GNU) C
This is word to let the world know that I have found a way to
make the C++ new-style casts, specifically static_cast<>
and const_cast<>, in (GNU) C (that's right, not C++).
I had seen annotation-only “casts” in C projects (and not
just my own) before. Just grepping -Pin '\w+_cast\(' in all
.tar.gz/.tar.bz2 source tarballs that are contained
within a bunch of .src.rpms ([gklmnop]*.src.rpm from
OpenSUSE was my run) of the distro already reveal quite a bit. It looks
like this:
/* The below macro is intended to be used for type casts. By using this
macro, type casts can be easily located in the source code with
tools like "grep". */
#define JAS_CAST(t, e) ((t) (e))
/* C++ style type casts */
#define const_cast(m_type, m_expr) ((m_type) (m_expr))
(These two examples from libjasper and Midnight Commander (mc))
Essentially, these developers just use the plain old cast, and it does not do much except the annotation. In fact, it can easily lead to warnings not appear, because it is essentially just the old reinterpreting cast. That is to say:
/* Former code */
static void foo(const char *y)
{
char *x = const_cast(char *, y);
}
This can easily fall apart when forgetting to change half of the code (which is bound to happen at some point in larger codebases):
/* Former code */
static void foo(const int *y)
{
char *x = const_cast(char *, y);
}
You do not get any warning for the apparent type mismatch. While that sucks, it is justified, because you actually used what is a reinterpret_cast, not an actual const_cast with a #define like that above. Comes along GNU C.
The presence of the
ARRAY_SIZE/BUILD_BUG_ON macros in the Linux
kernel source inspired
me to try something similarly freaky with GCC's extensions, for fun and
profit.
The result are some macros that actually do
type-checking instead of an annotation that has a reinterpret-pitfall
in it. I am not aware of this macro setup having been done before, as
it is pretty impossible to grep or google for such without turning up
lots of C++ code. It is further my assumption that since the GNU C
compiler itself only carries annotating “CONST_CAST”
macros, that my approach is indeed novel.
The reference implementation for the discovered
macros is present in libHX starting from 2.0. Linux distro packages
will be doing an update soon. Interested parties are encouraged to
use of libHX/defs.h.
Using these new macros, const_cast1(char *,
y); (note the '1' in “const_cast1”) will not allow y
to be something other than char * or const
char *!
Posted 2008-12-24 11:12 / Tags: Compilers. / link